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The thermal decomposition of methyl disulfide Coope, John Arthur Robert 1952

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(  THE THERMAL DECOMPOSITION OF METHYL DISULFIDE by JOHN ARTHUR ROBERT COOPE  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of Chemistry  We accept t h i s thesis as conforming t o the standard required from candidates f o r the degree of Master o f Science.  Members of the Department of Chemistry  THE UNIVERSITY OF BRITISH COLUMBIA October, 1 9 5 2  ABSTRACT  This report describes a k i n e t i c decomposition o f methyl d i s u l f i d e .  study o f t h e thermal The study was made as  a c o n t r i b u t i o n t o the chemistry o f o r g a n i c s u l f u r  compounds.  The homogeneous decomposition was studied by the s t a t i c method i n an a l l - g l a s s apparatus.  I t was f o l l o w e d by ob-  s e r v a t i o n o f t h e accompanying p r e s s u r e changes and by anal y s i s o f t h e r e a c t i o n mixture a t v a r i o u s stages o f t h e r e action.  S e v e r a l new a n a l y t i c a l methods were developed f o r  this analysis.  The experimental c o n d i t i o n s were v a r i e d by  v a r y i n g the temperature  o f t h e decomposition, the p r e s s u r e  o f the r e a c t a n t , and t h e s u r f a c e t o volume r a t i o o f t h e r e a c t i o n v e s s e l , and by adding v a r i o u s substances t o t h e r e a c t i n g system. The experimental r e s u l t s show t h a t t h e r e a c t i o n i s composite:  V.  (1)  Initially  a f r a c t i o n of the reactant  disulfide  adsorbs on the s u r f a c e o f t h e pyrex r e a c t i o n v e s s e l a t a r a t e which agrees w i t h t h e t h e o r e t i c a l r a t e f o r a c o l l i s i o n mechanism. (2)  There f o l l o w s an i n d u c t i o n p e r i o d d u r i n g which  the d i s u l f i d e does n o t decompose. (3)  The f i r s t decomposition i s i n t o methyl mercap-  t a n and a n o n - v o l a t i l e product b e l i e v e d t o be a t h i o f o r maldehyde polymer.  The suggested o v e r a l l r e a c t i o n s  CH3-S-S-CH3  -» CH^SH  n CH »S  ->  2  (4)  +  are:  CH =S 2  (-CH -S-) 2  n  A l a t e r , competing, process produces hydrogen  s u l f i d e , ethylene,  f r e e s u l f u r and p o l y s u l f i d e s .  It i s  p o s s i b l e t h a t the. carbon d i s u l f i d e formed i s produced by t h i s r e a c t i o n , and i t i s probable t h a t the t h i o a l d e h y d e polymer i s decomposed. may  The o v e r a l l unbalanced  reactions  be w r i t t e n :  GH" SSCH 3  3  +  ((CH S) ?) 2  n  -*  HS 2  .+ C g H ^ . + (CS ?) 2  CHo-S-S-CHo  3  3  (5)  *  S  v  — •  CH--S  3  x  S *.  * (?)  -CH,  x  3  Secondary r e a c t i o n s continue l o n g a f t e r t h e d i -  s u l f i d e has been used up.  The major process appears t o be  the degradation o f t h e p o s t u l a t e d  polythioformaldehyde t o  hydrogen s u l f i d e and p o s s i b l y t o carbon d i s u l f i d e and ethylene.  Degradation o f t h e p o l y s u l f i d e s appears t o occur  a l s o , and t h e r e may w e l l be f u r t h e r secondary r e a c t i o n s .  vi. The nature of the v a r i o u s component  processes have  been d i s c u s s e d i n some d e t a i l , and l i n e s o f f u r t h e r r e search have been suggested. The s t r u c t u r e  o f d i s u l f i d e s and p o l y s u l f i d e s  and  the bonding o f organic s u l f u r compounds have been d i s cussed.  ACKNOWLEDGMENTS  I am g r e a t l y i n d e b t e d t o Dr. W. A. Bryce who d i r e c t e d t h i s work. I am a l s o indebted t o Mr. G. D. Maingot f o r h i s a s s i s t a n c e lopment of.the  i n t h e deve-  a n a l y t i c a l methods.  TABLE OF CONTENTS ABSTRACT  PAGE iv  CHAPTER I.  INTRODUCTION  1  Basis and scope of the investigation  1  The structure of methyl disulfide  4  The preliminary investigation of Patrick  5  Survey of the literature  7  II. EXPERIMENTAL WORK  17  Reagents  17  Description of the apparatus  IB  Description of a typical experiment  28  The pressure-time curve for a typical experiment  32  The reproducibility of pressure-time curves  34  The i n i t i a l pressure decrease in the reaction  36  The dependence of the rate of pressure change on the i n i t i a l pressure of methyl disulfide  41  The effect of packing the reaction vessel  44  The dependence of the rate of pressure change on temperature  45  The analysis of the reaction mixture  49  The direct method for the determination of mercaptan 55. The direct method for the determination of hydrogen sulfide 62  1 1 .  PAGE The method for the determination of hydrogen sulfide plus mercaptan .  68  The determination of hydrogen sulfide i n the reaction  72  The determination of mercaptan in the reaction  77  The determination of free sulfur in the reaction  81  The method for the determination of disulfide  83  The determination of disulfide i n the reaction  89  Analysis for thiophenes  93  Analysis for carbon disulfide  94  Analysis for hydrocarbons  94  The involatile products of the reaction  96  Correlation of the analytical results for disulfide, hydrogen sulfide, and mercaptan^  99  The stoichiometry of the reaction  102  The effect of mercaptan on the rate of reaction 105 The effect of hydrogen sulfide on the rate of reaction  106  The effect of hydrogen sulfide on the f i n a l pressure  110  The effect of the reaction mixture on the rate of reaction  110  The effect of n i t r i c oxide on the rate of reaction  113  The thermal decomposition of ethyl disulfide  114  Summary of the experimental results for methyl disulfide Summary of the experimental results for ethyl disulfide  , 117 123  iii. PAGE III.  124  DISCUSSION Description  124  o f the r e a c t i o n '  144  Suggestions f o r f u r t h e r work  149  Mechanisms o f the r e a c t i o n  APPENDIX I .  The s t r u c t u r e o f d i s u l f i d e s and polysulfides  154  APPENDIX I I . The bonding and bond p r o p e r t i e s of o r g a n i c s u l f u r compounds  162  BIBLIOGRAPHY  176  CHAPTER  I  INTRODUCTION B a s i s and scope o f the i n v e s t i g a t i o n The thermal decomposition r e a c t i o n s o f o r g a n i c s u l f u r compounds a r e worthy o f e x t e n s i v e i n v e s t i g a t i o n .  The study  of these r e a c t i o n s would i n c r e a s e our understanding o f the g e n e r a l chemical b e h a v i o r o f compounds c o n t a i n i n g s u l f u r . I t would add t o our knowledge o f the nature o f these compounds, o f t h e i r atomic s t r u c t u r e and t h e i r bond e n e r g i e s . I t would l e a d t o added i n s i g h t i n t o the processes by which, thermal decomposition r e a c t i o n s o c c u r . In s p i t e o f t h e i r i n t e r e s t , these r e a c t i o n s have r e c e i v e d l i t t l e i n v e s t i g a t i o n o f a fundamental n a t u r e .  Know-  ledge o f the o v e r a l l nature o f the decompositions o f mercaptans, s u l f i d e s , d i s u l f i d e s , p o l y s u l f i d e s , t h i o a l d e h y d e s , or o f o t h e r compounds i s o n l y fragmentary, and knowledge o f the fundamental processes which occur i n these decompositions i s almost non-  2. existent.  I t was t o add t o t h i s sparse knowledge t h a t t h e  present i n v e s t i g a t i o n  i n t o the thermal decomposition o f  methyl d i s u l f i d e was undertaken. The decomposition r e a c t i o n s o f many s u l f u r compounds demand i n v e s t i g a t i o n .  The p a r t i c u l a r c h o i c e o f the a l k y l  d i s u l f i d e s f o r study was made f o r two reasons. preliminary investigation  Firstly, a  o f t h e thermal decompositions o f  methyl and e t h y l d i s u l f i d e s had a l r e a d y been made i n t h i s laboratory;!  i t seemed worthwhile t o continue t h i s study.  Secondly, the d i s u l f i d e s were c o n s i d e r e d p a r t i c u l a r l y i n teresting system.  compounds because they c o n t a i n the C-S-S-C bond An understanding o f t h e thermal b e h a v i o r o f t h i s  bond system would h e l p t o i l l u m i n a t e n a t u r e o f the p o l y s u l f i d e s .  the l i t t l e  understood  Of p a r t i c u l a r importance i n  t h i s r e s p e c t were t h e q u e s t i o n s o f whether or not t h e C-S or S-S bonds i n d i s u l f i d e molecules would be r u p t u r e d by heat, whether o r not t h e chain o f s u l f u r atoms would r e arrange t o a branched c o n f i g u r a t i o n , C-S(:S)-C,  and whether  or n o t the short s u l f u r chain would r e a d i l y add on more s u l f u r atoms.  I f t h e molecules d i d s p l i t  into free  radicals,  the important p o s s i b i l i t y would a r i s e o f determining t h e C-S or the S-S bond d i s s o c i a t i o n sulfides.  energies i n a s e r i e s o f d i -  Knowledge o f these e n e r g i e s would be o f c o n s i d e r a b l e  i n t e r e s t i n the f i e l d o f t h e o r e t i c a l symmetric  chemistry, owing t o the  c o n f i g u r a t i o n o f t h e d i s u l f i d e molecule.  An e x t e n s i v e and c o n t i n u i n g i n v e s t i g a t i o n  i n t o t h e thermal  decompositions  o f a l k y l d i s u l f i d e s would be concerned  with  determining f o r each d i s u l f i d e t h e f o l l o w i n g knowledge o f i t s decomposition. position:  (1) The o v e r a l l n a t u r e o f the decom-  the nature o f the products formed, t h e presence  o r absence o f i n t e r m e d i a t e s , t h e r a t e s o f t h e o v e r a l l processes and t h e s t o i c h i o m e t r y o f t h e r e a c t i o n . elementary mechanism. elementary  (2) The  r e a c t i o n s which make up t h e decomposition (3) The r a t e s and a c t i v a t i o n energies o f these processes.  The present i n v e s t i g a t i o n had,  n e c e s s a r i l y , a more l i m i t e d aim.  T h i s aim was simply t o  e s t a b l i s h t h e o v e r a l l nature o f t h e decomposition  of a  s i n g l e d i s u l f i d e , and t o o b t a i n knowledge which would  con-  t r i b u t e t o an e v e n t u a l f o r m u l a t i o n o f t h e complete r e a c t i o n mechanism. The s i m p l e s t o f t h e a l k y l d i s u l f i d e s e r i e s , methyl d i s u l f i d e , was s e l e c t e d f o r d e t a i l e d study because t h e r e s u l t s obtained were expected t o be t h e l e a s t e q u i v o c a l . A k i n e t i c approach t o t h e problem was adopted.  This  approach c o n s i s t s i n e s t a b l i s h i n g t h e i d e n t i t y o f the e l e mentary r e a c t i o n s which make up t h e decomposition  mechanism  as a whole by determining f i r s t l y , t h e nature o f t h e r e a c t i o n products and the r e a c t i o n i n t e r m e d i a t e s , and secondly, t h e r a t e s o f f o r m a t i o n o r o f decomposition  o f the r e a c t a n t , t h e  i n t e r m e d i a t e s , and t h e products, and t h e manner i n which these r a t e s depend on the v a r i o u s experimental  variables.  Such an approach normally l e a d s t o a sound f o r m u l a t i o n o f the v a r i o u s s t e p s o f t h e mechanism.  4.  In b r i e f , t h e r e f o r e , the purpose was  t o study the thermal decomposition  of t h i s  investigation  o f methyl  disulfide  w i t h a view t o determining " i t s decomposition mechanism.  It  i s hoped t h a t the knowledge gained w i l l add t o our unders t a n d i n g o f the chemical and p h y s i c a l nature o f d i s u l f i d e s , and t o our understanding o f the g e n e r a l chemical b e h a v i o r of o r g a n i c s u l f u r compounds. The  s t r u c t u r e o f methyl  disulfide.  I t i s important f i r s t t o c o n s i d e r the p h y s i c a l n a t u r e o f the methyl d i s u l f i d e molecule.  The q u e s t i o n o f the  atomic  s t r u c t u r e o f methyl d i s u l f i d e and the evidence f o r the v a l u e s o f the bond d i s s o c i a t i o n e n e r g i e s a r e d i s c u s s e d i n some d e t a i l i n the appendices.  I t i s o n l y necessary here t o present our  essential conclusions. The gas molecule o f methyl d i s u l f i d e has been shown by the e l e c t r o n d i f f r a c t i o n method t o have the s t r a i g h t c h a i n configuration:^ CH3-S-S-CH and t h e r e i s other evidence t o support the view t h a t a t r e l a t i v e l y low temperatures chain.  the s u l f u r atoms a r e arranged i n a  (See Appendix I ) .  However, the p o s s i b i l i t y  t h a t a t the e l e v a t e d temperatures experiments  remains  o f the present p y r o l y s i s  the molecule might rearrange t o the  branched  c h a i n c o n f i g u r a t i o n CH.-S(:S)-CH . The only d i r e c t evidence f o r the C-S  and S-S bond d i s s o -  c i a t i o n e n e r g i e s i n methyl d i s u l f i d e i s p r o v i d e d by the r e c e n t  electron impact values of Franklin and Lumpkin.3  These  investigators determined the value DtCH^S-SCRy  .  73.2  k.cal./mole.  and estimated the value of DfCH^-SSCH^) as of the same order, that i s , about 73  k.cal./mole.  These values are undoubtedly  too high. We have examined the question of these dissociation energies in detail i n Appendix II where i t i s suggested on semi-theoretical grounds that the values D(CH S-SCH )  m  5 5 k.cal./mole.  D(CH -SSCH )  =  64  3  3  are  3  3  probably nearer to the truth.  k.cal./mole.  The important point i s that  the S-S bond should be weaker than the C-S bond. The preliminary investigation, of Patrick. A preliminary study of the thermal decompositions of methyl and ethyl disulfides, made in this laboratory by Patrick,! has been of much help i n the present investigation. Patrick decomposed the pure gaseous disulfides i n a quartz static system at temperatures above 320° C , and followed the reaction by measurement of pressure changes, supplemented, i n the case of methyl disulfide, by some analyses for hydrogen sulfide and mercaptan.  His results may be summarized as  follows: (1)  Methyl and ethyl disulfides decompose at measurable  rates at temperatures above 300° C. with accompanying pressure increase.  In both decompositions the maximum rate of  pressure increase i s f i r s t order with respect to the i n i t i a l  6. disulfide concentration.  With respect to time, however,  the reactions exhibit no simple order: the pressure-time curves show an induction period, followed by an autoaccelerating rate up to a maximum, and a subsequent f a l l i n g off of the rate.  (It i s interesting to note that behavior of  this sort has been observed in the thermal decompositions of ethyl mercaptan,^ ethyl s u l f i d e , ^ and propyl mercaptan.^5) The total pressure increase relative to the i n i t i a l pressure of disulfide i s approximately 70% for methyl disulfide, and ranges from 100% to 150% for ethyl disulfide, depending on the  temperature. (2)  The decomposition of methyl disulfide i s essen-  t i a l l y homogeneous. (3)  The activation energies for the maximum rate of  pressure Increase are approximately 50.8 k.cal./mole for methyl disulfide and 41.1 k.cal./mole for ethyl disulfide. (4)  The decomposition of methyl disulfide produces  large amounts of hydrogen sulfide and mercaptan. (5) A small effect of n i t r i c oxide on the reaction of methyl disulfide  indicates that free radicals may be involved  in the reaction. To measure pressure, Patrick used a mercury manometer open to the reaction vessel.  Since he observed a black mer-  cury compound in the reaction products i t was clear that mercury from the manometer entered into the reaction. However, i t was not clear whether or not the mercury interfered appreciably with the normal course of the reaction.  In c o n s i d e r i n g P a t r i c k ' s work i t was  r e a l i z e d t h a t the  c o n s i d e r a b l e data t h a t he had gathered on the p r e s s u r e changes i n the r e a c t i o n would remain meaningless u n t i l  ex-  t e n s i v e a n a l y t i c a l i n v e s t i g a t i o n had shown the meaning of the p r e s s u r e changes.  P a t r i c k s r e s u l t s immediately 1  sented the f o l l o w i n g q u e s t i o n s : real?  pre-  I s the i n d u c t i o n p e r i o d  Does the decomposition o f d i s u l f i d e p a r a l l e l the  pressure increase? tity?  What o t h e r products are formed  i n quan-  Does the f o r m a t i o n o f mercaptan p a r a l l e l the f o r m a t i o n  o f hydrogen  sulfide?  These and many o t h e r q u e s t i o n s c o u l d  only be answered by a n a l y s e s .  I t was  c l e a r t h a t the  s t a n d i n g o f the thermal decomposition o f methyl would only progress through e x t e n s i v e a n a l y t i c a l  under-  disulfide investi-  gation. Survey o f the l i t e r a t u r e . A survey o f the l i t e r a t u r e d i d not r e v e a l any s t u d i e s o f the p y r o l y s i s of pure a l k y l d i s u l f i d e s . it  d i d r e v e a l some r e p o r t s o f non k i n e t i c  kinetic However,  investigations  i n t o the thermal decomposition o f d i s u l f i d e s , and many i n v a l u a b l e r e p o r t s of a r e l a t i v e n a t u r e . In an e a r l y i n v e s t i g a t i o n Otto and R o s s i n g 4 found t h a t on d i s t i l l a t i o n a t atmospheric  p r e s s u r e amyl d i s u l f i d e  245° C t o 248° C.) g r a d u a l l y decomposes w i t h the  (b.p.  splitting  o f f of s u l f u r or o f s u l f i d e s r i c h e r i n s u l f u r than the  ori-  g i n a l substance, l e a v i n g behind a t a r r y r e s i d u e o f dark colour.  C l e a r l y , the f o r m a t i o n o f p o l y s u l f i d e s i s suggested.  B e z z i ^ has r e p o r t e d t h a t o c t y l d i s u l f i d e decomposes a t 190°  C.  8.  and 15  nim. p r e s s u r e i n s t e a d of b o i l i n g .  A n t i c i p a t i n g the  r e s u l t s o f the present i n v e s t i g a t i o n we may mately the decomposition temperatures  compare a p p r o x i -  of f o u r  disulfides.  For methyl, e t h y l , amyl, and o c t y l d i s u l f i d e s the temperat u r e s r e q u i r e d f o r a p p r e c i a b l e decomposition are r e s p e c t i v e l y  of the order o f 320° C ,  300° C ,  245° C., and 190° C.  It  i s e v i d e n t t h a t the s t a b i l i t y o f d i s u l f i d e s decreases as  we  ascend the homologous s e r i e s . Faragher, M o r r e l l and Comay  and decomposed the v a p o r i z e d s o l u t i o n s a t 496° C.  i n naphtha  by a dynamic method. t h i s way  dissolved various d i s u l f i d e s  The treatment of e t h y l d i s u l f i d e i n  produced hydrogen  sulfide, free sulfur, ethyl  mer-  captan, a l k y l s u l f i d e s , and thiophene, t o g e t h e r w i t h s a t u r a t e d and u n s a t u r a t e d hydrocarbons, and the  decompositions  o f o t h e r d i s u l f i d e s , n - p r o p y l , n - b u t y l , i-amyl analogous p r o d u c t s .  produced  These r e s u l t s gave some i d e a of the  products t h a t c o u l d be expected i n the present i n v e s t i g a t i o n , but i t had t o be remembered f i r s t l y t h a t the s o l v e n t probably took p a r t i n the r e a c t i o n s and secondly t h a t the  temperatures  used were so h i g h t h a t decomposition of products and other s i d e r e a c t i o n s would have occurred-. There are s e v e r a l r e p o r t s of the p y r o l y s e s o f a r y l d i s u l f i d e s . ^»  8  may  The work o f Schonberg,  be mentioned  because  Mustafa, and  Askar?  i t l e a d s t o the c o n c l u s i o n t h a t  t h i s compound s p l i t s t o form f r e e a r y l t h i a l r a d i c a l s ,  PhS-.  These a u t h o r s suggest the f o l l o w i n g mechanism t o account f o r  9. the decomposition products: PhS-SPh  >  2 PhS.  2 Lecher^ has reported that phenyl disulfide reacts with triphenyl methyl radicals: PhS-SPh  +  2 PhoC*  »  2  PhS-CPh.  The resultant compound reacts with oxygen to regenerate the disulfide: 2 PhS-CPh  +  0  2  *  PhS-SPh f  P h o C O O C P hl  3 This behavior strongly suggests the presence of free PhSradicals. The fact that diphenyl disulfide splits at the S-S bond does not lead us to expect that alkyl disulfides w i l l necessarily do likewise; the effect of the two phenyl groups i s to decrease markedly the dissociation energy of the S-S bond in the a l l y l position. The decomposition of ethyl disulfide in kerosene i n the presence of hydrogen has been reported.^ After exposure to 230° G. and 30 atmospheres for two hours, 75.3% of the d i sulfide was decomposed and 13% of the total sulfur was l e f t as e,thyl mercaptan. Two other papers may perhaps contain some data on the thermal behavior of disulfides, but these have not been available. » 11  12  A number of reports of certain other reactions of sulfur  10. compounds were very u s e f u l ,  both i n i n d i c a t i n g  some of  the p e c u l i a r i t i e s that might be expected i n the thermal decompositions o f d i s u l f i d e s , and i n d e s c r i b i n g h a v i o r o f the products o f the decompositions. teresting reports referred 1  1  t  h  e  the thermal  2  2Zf  the a d d i t i o n  decomposition  decomposition o f mercaptans on  metal and metal s u l f i d e s u r f a c e s , 2 1 , 2 2 mercaptans, -*>  These i n -  t o the thermal decomposition  of m e r c a p t a n s , ^ , 1 4 , 1 5 , 1 6 , 1 7 , 1 8 , 1 9 of monosulfides, ^>14,20  the be-  ^  p h o t o l y s i s of  e  o f hydrogen s u l f i d e ,  mercaptans',  and d i s u l f i d e s t o double bonds, 5>26,27,28,29,30,31 the 2  reaction  between methane and s u l f u r , 3 , 3 3 2  a n c  j ^  e  r e a  of p a r a f f i n hydrocarbons w i t h s u l f u r vapour.34,35 worthwhile t o mention particular  certain  ction  Iti s  o f these papers that are o f  interest.  One o f the very few k i n e t i c s t u d i e s of the thermal decomposition of organic s u l f u r compounds i s the study by Trenner and T a y l o r - ^ > 14  0  e t h y l mercaptan and e t h y l  f t h e thermal decompositions o f sulfide.  Trenner and T a y l o r de-  composed the pure gaseous compounds a t temperatures  ranging  from 380° C. t o 410° C. and observed the pressure changes during t h e r e a c t i o n . p e r i o d s and y i e l d e d  Both decompositions  showed i n d u c t i o n  pressure-time curves o f s i m i l a r shape  t o those observed f o r the decomposition o f d i s u l f i d e s . prior addition  The  of hydrogen s u l f i d e had no e f f e c t on the  mercaptan r e a c t i o n ,  but i t completely e l i m i n a t e d the i n -  d u c t i o n p e r i o d o f the s u l f i d e decomposition.  The a c t i v a t i o n  energies o f the s u l f i d e and mercaptan decompositions were  11.  both about 40 k.cal./mole, a value approximately the same as that found by Patrick- - for the decomposition of 1  ethyl disulfide.  Some evidence was obtained for the exis-  tence of an equilibrium.  It i s unfortunate that Trenner  and Taylor did not show the meaning of the pressure changes by analytical investigation. Trenner and Taylor supposed that the decompositions of ethyl mercaptan and ethyl sulfide involved the same fundamental processes, and proposed a single mechanism of consecutive molecular reactions to explain both decompositions: 2 C H SH 2  HS  *  2  n  * H S | (C H ) S  5  £  (C H ) S 2  ( 2 5>2 2 2 c  H  s  H  5  2  5  2  4 "(C H ) S H «  2  2  —-> 2 C H  n  5  2  2  2  f 2 H S + poly,  . sulfides  The processes of formation and decomposition of the unstable intermediate "(C H^) S H " were supposed to have activation 2  2  2  2  energies of 40 k.cal./mole and to be the rate controlling steps.  Some evidence was obtained for the existence of this  intermediate.  In connection with this mechanism i t may be  noted both that octyl mercaptan gives octyl sulfide on decomposition,'^ and that sulfide formation i s observed i n the decomposition of mercaptans on metal and metal sulfide sur- „ 21,22 faces. ' Q  a o  An analytical investigation of the thermal decomposition of ethyl mercaptan and ethyl sulfide i s desirable. Darwent and Sehon^ have recently applied the powerful toluene carrier gas technique to the study of the pyrolysis of mercaptans.  They found that methyl mercaptan decomposes  12. by dissociation into CH^* and *SE radicals; the elementary dissociation reaction has an activation energy of 67 £ 2 k.cal./mole.  Ethyl disulfide, however, decomposes primarily  by a molecular mechanism into;hydrogen sulfide and ethylene. The photolyses of mercaptans involve the reactions of free RS« radicals.  Skerret and Thompson ^ have photolysed 2  methyl mercaptan with light from amercury arc and concluded that the overall process i s mainly: 2 CH SH  ~ h v ~ * CH SSGH  + H  with a concurrent but much less important reaction: —b x — C H , + S. 4  CH.SH 3  The quantum yield i s 1.7. The primary process was suggested to be the dissociation: CH^SH  —by—*  CH^S. + .H  Meissner and Thompson ^ found that the photolysis of ethyl 2  mercaptan i s similar, but the quantum yield i s 10 and traces of unsaturated hydrocarbons are produced.  The photolysis  of propyl mercaptan ^ produces appreciable amounts of un2  saturated hydrocarbons.  These results may indicate d i f f e r -  ences in the behavior of the three RS« free radicals. The reports of the addition of mercaptans,  hydrogen  sulfide, and disulfides to olefins require consideration. Jones and Reid '* have found that the reactions 2  HS  + CH = CH  2  2  C H SH 2  5  CH  2=  CH  C H SH  2  2  5  »-:(C H) S  2  2  occur appreciably at 450° C. The reactions R=R»  + HS 2  .  R"SH  2  13. are d e s c r i b e d  i n a patent by K e y s s n e r . ^  The  2  photoadditions  o f hydrogen s u l f i d e and mercaptans to o l e f i n s have been described radical  by Vaughan and  Rust, ?> 2  2  ^ who  suggest the  free  process: HS«  +  RCH=CH  :~*  2  ICH-CH SH 2  as the f i r s t step i n the hydrogen s u l f i d e a d d i t i o n .  The  c a t a l y t i c a d d i t i o n o f mercaptans t o o l e f i n s has been i n v e s t i g a t e d by K a n e k o . 9 2  oxygen, and reaction.  Mercury, (CH^S) Hg, l i g h t  plus  2  l i g h t p l u s mercury p l u s oxygen a c c e l e r a t e Light i t s e l f i s reported  the  t o be a poor c a t a l y s t ,  but presumably t h i s depends on the wavelength used. The  f a c t t h a t these a d d i t i o n s o f hydrogen s u l f i d e or  .mercaptans t o o l e f i n s r e q u i r e l i g h t , heat, or the 1  presence  o f the paramagnetic substances, mercury and oxygen, suggests t h a t they take place by f r e e r a d i c a l mechanisms^. t h a t the formation  of s u l f i d e s involves a d d i t i o n s  t o Markownikoff's r u l e supports t h i s suggestion.  The  fact  contrary I t i s very  /'interesting t h e r e f o r e t h a t the a d d i t i o n o f d i s u l f i d e s to styrene  has been,observed.30  s u l f i d e and styrene, the  Both the aromatic phenyl d i -  the a l i p h a t i c methyl and  under the  e t h y l d i s u l f i d e s add  to  c a t a l y t i c influence of iodine, to give  products: SR  SR  In the l i g h t o f our knowledge o f phenyl d i s u l f i d e i t seems q u i t e p o s s i b l e that i t s c a t a l y z e d a d d i t i o n i n v o l v e s the  free  14. r a d i c a l s PhS*. and  I t f o l l o w s t h a t t h e a d d i t i o n s o f methyl,  e t h y l d i s u l f i d e s may i n v o l v e t h e f r e e r a d i c a l s CH^S*  and C^H^S*.  I t i s i n t e r e s t i n g t o surmise t h a t a t the  e l e v a t e d temperatures o f the present  pyrolysis investiga-  t i o n , these RS» f r e e r a d i c a l s could be produced i n t h e absence o f i o d i n e . S e v e r a l o f these a d d i t i o n r e a c t i o n s reach an e q u i l i brium.  An example i s t h e system hydrogen  sulfide-propylene-  normal p r o p y l niercaptan-isopropyl mercaptan d e s c r i b e d by B a r r and Keyes.-^  The a d d i t i o n o f hydrogen s u l f i d e t o pro-  pylene may g i v e e i t h e r the normal o r t h e i s o mercaptan, but both r e a c t i o n s reach e q u i l i b r i u m .  I n both cases the a d d i t i o n  i s favoured by low temperatures. The  e q u i l i b r i u m constants o f a number o f r e l e v a n t r e -  a c t i o n s have been c a l c u l a t e d by Barrow and P i t z e r . 3 6  At  600° K., a temperature o f the order o f those used i n t h e present  i n v e s t i g a t i o n , many o f these constants a r e o f t h e  order o f one.  Some o f them a r e reproduced i n Table I .  Table I Some e q u i l i b r i u m  constants^  Reaction HS 2  C H  f  2  C H SH 2  5  +  ...  4  C H 2  4  Log. K C H SH 2  (C H ) S  2 C H SH'  ( C  2 CH SH  ( C H  2  5  3  _o.5  5  2  5  }  3 2 ,  o  2  2 5 2 H  (#2)  S  S  V  +  *  H  2  S  1  15  I t was  r e a l i z e d t h a t i f these o r s i m i l a r  reactions  o c c u r r e d r a p i d l y amongst the p r o d u c t s o f the thermal decomposition o f d i s u l f i d e s , the r e l a t i v e amounts o f the v a r i o u s substances found i n the r e a c t i o n mixture would be the e q u i l i b r i u m amounts, r a t h e r than the q u a n t i t i e s p r o duced by the decomposition An analogous and L a y n g ! 5  who  itself.  s i t u a t i o n has been encountered by T a y l o r  found t h a t the thermal decomposition o f  normal p r o p y l mercaptan ends i n an e q u i l i b r i u m between i s o p r o p y l mercaptan, normal p r o p y l mercaptan, s u l f i d e , and p r o p y l e n e .  hydrogen  S i m i l a r l y , Trenner and  Taylor^  found t h a t the thermal decomposition o f e t h y l mercaptan probably ends i n an e q u i l i b r i u m between the r e a c t a n t mercaptan and the products hydrogen  s u l f i d e and e t h y l e n e .  A number o f c o n c l u s i o n s were drawn from t h i s t u r e survey:  I t appeared  litera-  f i r s t l y , t h a t a l a r g e number o f  products c o u l d be expected i n the thermal decomposition o f d i s u l f i d e s , " s e c o n d l y , t h a t h i g h e r d i s u l f i d e s decompose more r e a d i l y than the lower ones, t h i r d l y , t h a t aromatic d i s u l f i d e s can t h e r m a l l y d i s s o c i a t e a t the S-S bond and t h a t a l i p h a t i c d i s u l f i d e s may t h a t f r e e RS«  p o s s i b l y do the same, f o u r t h l y ,  r a d i c a l s do take p a r t i n some r e a c t i o n s ,  f i f t h l y , , t h a t the thermal decompositions o f d i s u l f i d e s  and might  w e l l be complicated by the e x i s t e n c e o f v a r i o u s complex equilibria. Since the thermal decompositions of d i s u l f i d e s might IT  be  16 extremely complicated, i t was f r u i t f u l way  a g a i n c l e a r t h a t the only  o f e l u c i d a t i n g these decompositions would  be through an e x t e n s i v e a n a l y t i c a l  investigation.  CHAPTER  II  EXPERIMENTAL WORK The  thermal decomposition of methyl d i s u l f i d e  s t u d i e d i n the gas  phase.  The  d i s u l f i d e s were decomposed  i n a c l o s e d e x t e r n a l l y heated r e a c t i o n v e s s e l and a c t i o n was and  followed  by the o b s e r v a t i o n  mental c o n d i t i o n s were v a r i e d by v a r y i n g  the  system, by changing the  r a t i o o f the r e a c t i o n v e s s e l , and stances t o the r e a c t i n g  the  re-  o f pressure changes  by the a n a l y s i s o f the r e a c t i o n m i x t u r e .  temperature o f the  was  The  experi-  pressure  surface  and  t o volume  by adding v a r i o u s  sub-  system.  Reagents The  d i s u l f i d e s used i n t h i s i n v e s t i g a t i o n were o b t a i n e d  from Eastman Kodak Company, Rochester, New The  York.  methyl d i s u l f i d e gave n e g a t i v e t e s t s f o r hydrogen  s u l f i d e and  f o r mercaptans.  Analysis  for i t s total sulfur  content by the lamp method i n d i c a t e d the t h e o r e t i c a l amount w i t h i n the  experimental e r r o r o f a few  t i v e index o f n  2 0  percent.  _ 1.526I + 0 . 0 0 0 2 agreed w i t h i n  Its refracthe  experi-  mental e r r o r w i t h the v a l u e o f 1.52599 measured by Vogel  and  18. Cowan37  on highly p u r i f i e d material. In view of t h i s  close agreement, methyl d i s u l f i d e was not subjected to elaborate p u r i f i c a t i o n .  Any remaining low b o i l i n g im-  p u r i t i e s would have been largely removed by the procedure of d i s t i l l i n g o f f a f r a c t i o n of the l i q u i d d i s u l f i d e held i n the storage bulb i n the vacuum apparatus before using i t i n the experimental work;cany high b o i l i n g impurities would have been largely removed by the two plate f r a c t i o n a t i o n that the d i s u l f i d e received before being i n t r o duced into the reaction vessel. The ethyl d i s u l f i d e had n  2 0 d  = .1.5066 £ 0 . 0 0 0 2 .  The  value reported by Vogel and Cowan^? i s 1 . 5 0 7 0 4 , and the value reported by Bezzi^ i s 1 . 5 0 7 0 .  Like methyl d i s u l f i d e ,  i t was fractionated i n the vacuum apparatus before use. Description of the apparatus The apparatus used to study the thermal  decomposition  of d i s u l f i d e s was an a l l - g l a s s s t a t i c system consisting e s s e n t i a l l y of a heated reaction v e s s e l connected to an evacuating system, to a sampling system, to an arrangement f o r pressure measurement, and to storage vessels f o r the reactants. The direct measurement of pressure with a mercury manometer open to the reaction vessel has not proved s a t i s factory f o r studying the thermal decomposition of d i s u l f i d e s . In the present investigation i t was found that the accuracy of a manometer was destroyed a f t e r a few minutes x e  19. posure to the reaction products, the mercury column being depressed by a black deposit which formed on the walls. In h i s work, Patrick-*- found that d i s u l f i d e condensed i n the manometer unless the - manometer was heated; heating altered the density of the mercury and made the pressure readings inaccurate.  Moreover, Patrick found that mercury  vapour from the manometer entered into the reaction f o r ming a black compound. meter was  Since the use of a mercury mano-  c l e a r l y quite unsatisfactory, i t was  decided i n  the present investigation to use a glass Bourdon gauge f o r measuring  pressure.  The reaction vessel and Bourdon gauge are shown i n detail in Fig.l. diameter of 55 mm. i t was  The pyrex reaction vessel had an  outside  and a volume of approximately 235  ml.;  connected to the evacuating system by a c a p i l l a r y  side arm and a $ 10/30  ground glass j o i n t .  The gauge was  mounted v e r t i c a l l y above the reaction vessel and the j a c ket of the gauge was  connected by a s i m i l a r j o i n t to a  system f o r adjusting and measuring pressure.  A tube pro-  jecting from the bottom of the reaction vessel permitted the introduction of s o l i d materials into the vessel. The movement of the pointer of the gauge was  followed  by means of an eighteen power microscope with a graduated scale i n the eye piece.  For most pressure measurements  the gauge was used simply as a n u l l i n d i c a t o r ; sure of a i r i n the jacket of the gauge was  the  adjusted  presexactly  20.  t o balance the unknown p r e s s u r e i n the r e a c t i o n v e s s e l ; the unknown pressure was  then read from a c l o s e U-tube  mercury manometer connected gauge was  to the j a c k e t .  The  used i n s t e a d as a d i r e c t measuring  calibrated  instrument  when f i l l i n g the r e a c t i o n v e s s e l , when o b s e r v i n g very r a p i d p r e s s u r e changes, or when making approximate p r e s s u r e measurements.  The gauge was  o f l e s s than 1/4  s e n s i t i v e t o a p r e s s u r e change  mm.  •-  A diagram o f the e n t i r e apparatus  i s shown i n F i g . 2.  w i t h the r e a c t i o n v e s s e l i n p o s i t i o n i n the f u r n a c e . l o w i n g the method o f B r y c e 3 4 f r overcoming the 0  o f o b t a i n i n g vacuum t i g h t  s e a l s w i t h the two  Fol-  difficulty  r i g i d cone  and socket j o i n t s connecting the r e a c t i o n v e s s e l t o the r e s t o f the system, the upper socket was end o f a double " s p r i n g " was  loop o f 5 mm*  mounted a t the  glass tubing.  This glass  s u f f i c i e n t l y f l e x i b l e t o permit t i g h t s e a l s  t o be o b t a i n e d i n both j o i n t s .  P l i c e n e wax  was  found  very  s a t i s f a c t o r y f o r making the s e a l s ; i t d i d not develop l e a k s d u r i n g s e v e r a l months o f almost t o above 1 0 0 °  continuous  any  heating  C.  The apparatus was  evacuated  by a mercury d i f f u s i o n  pump backed by a r o t o r y o i l pump. c o o l e d i n dry i c e - a c e t o n e prevented from r e a c h i n g the pumping system.  Two  t r a p s , F^ and  F^,  the r e a c t i o n vapours Trap F^ served a l s o f o r  c o l l e c t i n g products, and f o r p r e v e n t i n g mercury vapour from d i f f u s i n g t o the r e a c t i o n v e s s e l and the galleries.  connected  A d i s c h a r g e tube a t t a c h e d t o the system  was  A B M D E P G H I J K T  Reaction vessel Bourdon gauge .Manometer Glass spring Discharge tube Traps Reactant storage v e s s e l s Preheating vessels Gas storage .vessels Sampling p i p e t t e s Furnace Stopcocks  21 used t o i n d i c a t e when a s u f f i c i e n t vacuum had been obtained; when no D.C. power was a v a i l a b l e an e x t e r n a l T e s l a c o i l was used i n s t e a d . The  pressure i n t h e lower g a l l e r y could be measured by  a second Bourdon gauge, B^.  The r e a d i n g s o f t h i s gauge  were only approximate, f o r i t was b u i l t t o be durable r a t h e r than t o be s e n s i t i v e .  I t would w i t h s t a n d a pressure  dif-  f e r e n t i a l o f g r e a t e r than one atmosphere. A c o n s i d e r a b l e problem i n t h e study o f d i s u l f i d e s i s t o be a b l e t o f i l l  the r e a c t i o n v e s s e l w i t h d i s u l f i d e t o  an adequate gas p r e s s u r e .  T h i s problem was overcome i n the  present i n v e s t i g a t i o n through t h e use o f the l a r g e p r e h e a t i n g v e s s e l s , H, a t t a c h e d t o the lower g a l l e r y .  These  v e s s e l s c o u l d be immersed i n hot water t o p r o v i d e a l a r g e volume o f gaseous d i s u l f i d e a t a r e l a t i v e l y h i g h p r e s s u r e . With t h i s arrangement i t was p o s s i b l e t o o b t a i n p r e s s u r e s o f methyl d i s u l f i d e o f 230 mm., s u l f i d e o f 40 mm.  and pressures o f e t h y l d i -  i n the r e a c t i o n vessel.  These p r e s s u r e s  may be compared t o t h e maximum p r e s s u r e s 60 mm. and 13mm. r e s p e c t i v e l y obtained by Patrick-*- who simply preheated t h e d i s u l f i d e s i n t h e i r s m a l l storage b u l b s . The d i s u l f i d e s , which a r e l i q u i d a t room  temperature,  were s t o r e d i n s m a l l b u l b s a t t a c h e d t o t h e p r e h e a t i n g vess e l s , r a t h e r than i n t h e p r e h e a t i n g v e s s e l s  themselves.  T h i s arrangement e l i m i n a t e d t h e p o s s i b i l i t y o f decomposing the s t o r e d d i s u l f i d e through repeated p r e h e a t i n g .  I t also  provided f o r two f r a c t i o n a t i o n s o f the d i s u l f i d e b e f o r e i t s  22.  i n t r o d u c t i o n i n t o the r e a c t i o n  vessel.  The storage b u l b s were f i l l e d s m a l l bore s i d e arms.  i n the u s u a l way  through  Only a middle f r a c t i o n o f the s t o r e d  d i s u l f i d e was used i n the experiments,  the t o p and bottom  q u a r t e r s b e i n g d i s c a r d e d t o remove any i m p u r i t i e s .  Suffi-  c i e n t d i s u l f i d e f o r s e v e r a l runs was t r a n s f e r r e d from i t s storage v e s s e l t o the preheater by c o o l i n g t h e l a t t e r i n a bath o f dry i c e - a c e t o n e .  The cooled d i s u l f i d e i n the p r e -  h e a t e r was thoroughly pumped o f f t o remove any t r a c e s o f air. Gaseous compounds, such as hydrogen s u l f i d e ,  methyl  mercaptan, and n i t r i c oxide were s t o r e d i n the one l i t e r g l a s s bulbs a t t a c h e d t o the upper g a l l e r y . were f i l l e d  These b u l b s  through t h e ground g l a s s j o i n t s t h a t were  a t t a c h e d t o them. The r e a c t i o n mixture c o u l d be sampled i n the p i p e t t e s , I.  These c o u l d be a t t a c h e d t o t h e c a p i l l a r y connections o f  the r e a c t i o n v e s s e l o r t o the lower To overcome the problem  gallery.  o f condensation o f the d i s u l -  f i d e s , the lower g a l l e r y and t h e c a p i l l a r y connections o f the r e a c t i o n v e s s e l were wound w i t h Chromel r e s i s t a n c e wire and heated w i t h an e l e c t r i c c u r r e n t .  The Bourdon gauge  attached t o the r e a c t i o n v e s s e l was heated d i r e c t l y i n the furnace; t h e second gauge a t t a c h e d t o the g a l l e r y c o u l d be heated e l e c t r i c a l l y .  In the course o f an experiment t h e  lower g a l l e r y was maintained a t a temperature  of approxi-  mately 105° C., w h i l e the c a p i l l a r y connections were s l i g h t l y  23.  hotter.  The main stopcock a t the entrance t o the r e a c t i o n  v e s s e l was  not heated, but i t was  found t o become hot a f t e r  l o n g h e a t i n g o f the a d j a c e n t t u b i n g . The stopcocks i n the heated p a r t s of the system were s e a l e d w i t h Dow  Corning " S i l i c o n e High Vacuum" stopcock  grease which m a i n t a i n s i t s c o n s i s t e n c y up t o 2 0 0 ° C. may  It  be noted t h a t a f t e r prolonged h e a t i n g t h i s grease forms  a gel.  U n f o r t u n a t e l y the d i s u l f i d e s d i s s o l v e d  Silicone  grease very r a p i d l y so t h a t f r e q u e n t r e g r e a s i n g was sary.  neces-  The stopcocks i n the remainder o f the system were  s e a l e d w i t h Apiezon M stopcock grease which has a b e t t e r c o n s i s t e n c y than s i l i c o n e . The r e a c t i o n v e s s e l was heated by an e l e c t r i c furnace c o n s t r u c t e d by P a t r i c k . ^  The f u r n a c e c o n s i s t e d o f a q u a r t z  core, t h r e e i n c h e s i n diameter, wound i n t h r e e s e c t i o n s w i t h Chromel r e s i s t a n c e w i r e and i n s u l a t e d w i t h s e v e r a l i n c h e s o f powdered a s b e s t o s . the furnace was  The opening i n the top o f  i n s u l a t e d by s e a l i n g i n the r e a c t i o n ves-  s e l w i t h a mixture of powdered asbestos and Alundum cement. Each s e c t i o n of the f u r n a c e winding was  connected i n s e r i e s  t o a v a r i a b l e r e s i s t a n c e , and the t h r e e s e c t i o n s were connected i n p a r a l l e l t o the power supply.  Adjustment  of the  v a r i a b l e r e s i s t a n c e s made i t p o s s i b l e t o o b t a i n a constant temperature  over the l e n g t h o f the r e a c t i o n  The temperatures  vessel.  a t the t o p and the bottom o f the  r e a c t i o n v e s s e l were measured by two Chromel-Alumel couples p l a c e d a g a i n s t the w a l l o f the v e s s e l .  thermo-  A two  way  24. s w i t c h i n the potentiometer  c i r c u i t p e r m i t t e d r a p i d conse-  c u t i v e r e a d i n g o f the two thermocouples.  The  were c a l i b r a t e d by comparison w i t h a standard  thermocouples platinum  r e s i s t a n c e thermometer over the range o f temperatures i n the  used  investigation.  Automatic c o n t r o l of the furnace temperature was t a i n e d by means o f an e l e c t r o n i c thermoregulator a relay.  ob-  operating  The c l o s i n g o f the r e l a y c o n t a c t s s h o r t e d out a  c o n t r o l l i n g r e s i s t a n c e i n the power supply and i n c r e a s e d the c u r r e n t t o the f u r n a c e . furnace c i r c u i t s  and  i s shown i n F i g . 4.  The thermoregulator, was  A diagram o f the c o n t r o l  of the type invented by  Coates/^  c o n t r o l l e d by a r e s i s t a n c e thermometer i n the  furnace.  The operation of the instrument  i s based on the r e v e r s a l of  phase o f the out-of-balance E.M.F. o f an A.C. occurs on p a s s i n g from one The thermoregulator  8  b r i d g e which  s i d e o f balance to the o t h e r .  c i r c u i t i s shown i n F i g . 3.,  in  which the powerpack and r e l a y c i r c u i t s have been omitted for simplicity. an A.C.  The main c i r c u i t c o n s i s t s o f f o u r p a r t s :  b r i d g e c o n t a i n i n g the r e s i s t a n c e thermometer and  a standard v a r i a b l e r e s i s t a n c e , a c i r c u i t f o r a m p l i f y i n g the output from the b r i d g e , a c i r c u i t f o r c o n v e r t i n g t h i s output t o v a r i a b l e D.C.  v o l t a g e , and a r e l a y d e v i c e .  The A.C. b r i d g e c o n s i s t s o f a c e n t e r tapped T^ and two  r e s i s t a n c e s R-^ and R » 2  One  transformer,  o f these i s the t h e r -  mometer, and the other i s the v a r i a b l e r e s i s t a n c e which can be a d j u s t e d t o balance the b r i d g e a t any d e s i r e d  temperature.  F I G . 3. CIRCUIT DIAGRAM OP THE THERMOREGULATOR  25. The  output  of the b r i d g e , eg^, i s a m p l i f i e d by  a p p l i e d t o the g r i d of V voltage e anode  g 2  .  of V  2  and  as the much l a r g e r a l t e r n a t i n g  An a l t e r n a t i n g v o l t a g e e  i s applied to  & 2  which, t h e r e f o r e , w i l l pass c u r r e n t only  2  d u r i n g the h a l f c y c l e s i n which e  i s positive.  the magnitude o f the anode c u r r e n t i  The anode c u r r e n t generates  a c r o s s R_  3  R.,  1  The anode c u r r e n t o f  on  g r i d voltage  a potential difference  which i s smoothed by C.,  t o the g r i d o f the output  Hence  depends not only  the magnitude but a l s o on the phase o f the A.C. e~ •  the  and C , and a p p l i e d  4*  t r i o d e V_  3  2*  as the D.C.  . .  voltage e  g3  c o n t r o l s the furnace through the  relay. For o p e r a t i o n , the n e g a t i v e g r i d b i a s o f j u s t e d by the v a r i a b l e r e s i s t a n c e R^  i s so  ad-  t h a t the r e l a y i s a t  the t r i p p o i n t when the b r i d g e i s b a l a n c e d .  I f the  furnace  i s too c o l d , the b r i d g e i s unbalanced; a s m a l l a l t e r n a t i n g voltage e  i s impressed  on the g r i d o f V , a m p l i f i e d , and  ol  impressed  1  on the g r i d o f V^.  S i n c e the g r i d v o l t a g e e  of o p p o s i t e phase t o the anode v o l t a g e a » e  2  rent on  i s decreased. i s decreased  t  ^  i e  a n o c  *e  is  cur-  Consequently the n e g a t i v e .grid b i a s  and the c u r r e n t ±  z  through the r e l a y i s  i n c r e a s e d ; the r e l a y c o n t a c t s c l o s e , s h o r t i n g out the  con-  t r o l l i n g r e s i s t a n c e i n the power supply and i n c r e a s i n g the c u r r e n t t o the f u r n a c e .  I f the furnace becomes too hot,  the b r i d g e again i s unbalanced, but t h i s time the voltage e  i s In phase w i t h the anode v o l t a g e e  sequently i  2  & 2  grid .  i n c r e a s e s , eg^ becomes more n e g a t i v e , i  Conz  decreases,  F:»go  5  0  Powfir a u n p l y  f o r the  therrrorftgulator  26. and the relay contacts open. In the apparatus constructed in the presBnt investigation,  was a Type 167-D  transformer.  The anode supply to V" was provided by the 2  110 volt A.C. mains. triode, and V  110 to 6 volt center tapped  V  1  was a 6SJ7 pentode, V  a 6B4G power output triode.  D.C. anode supply to  and  2  a  6SF5  The 350 volt  was provided by the f u l l  wave r e c t i f i e r and smoothing circuit shown in Fig. 5; the powerpack, T and  2  was a Thordardson T-13R13 transformer,  was a type 80 f u l l wave r e c t i f i e r .  resistance R  2  in  The variable  in the A.C. bridge was a standard 0.1 to 1000  ohm decade dial resistance box.  The resistance thermo-  meter was a Type M molybdenum resistance thermometer obtained from Weiller Instruments Ltd., New York; the resistance windings were incased i n quartz, and the whole was contained in a stainless steel jacket. The relay circuit i s illustrated in Fig. 4 .  It con-  sisted of a Sunvic Type 602 vacuum relay with suitable series and shunting resistances. current ±  z  During operation the  passed continuously through the relay and varied  only slightly from one side of balance to the other. A l though the Sunvic relay i s not advertised for this type of operation i t was found to be quite satisfactory; the t r i p point, about 16.1 m.a. did not vary appreciably during several months of almost continuous use. The advantages of this type of thermoregulator have been pointed out by Coates.3^  The sensitivity 'can be i n -  creased simply by increasing the A.C. amplification between  27 the  b r i d g e and V^.  The instrument can be used over the  whole range o f temperatures a t which the r e s i s t a n c e thermometer i s u s a b l e .  The adjustment o f temperature by v a r y i n g  a r e s i s t a n c e i s simple and r a p i d .  The adjustment o f the  c i r c u i t so t h a t the b r i d g e balances a t the r e l a y p o i n t i s not c r i t i c a l . tions i s small.  trip  The e f f e c t o f mains v o l t a g e v a r i a -  F i n a l l y , s m a l l phase displacements do not  a f f e c t the o p e r a t i o n a p p r e c i a b l y :  i n t h i s connection the  instrument i s f a r e a s i e r t o handle than those which depend on the use o f gas f i l l e d  relays.  The instrument was a b l e to c o n t r o l a water bath t o w i t h i n 0.03° C. at 50°  C,  more s e n s i t i v e w i t h a two  and i t c o u l d have been .made stage a m p l i f i e r .  In the a i r  furnace, however, the degree o f temperature c o n t r o l  was  l i m i t e d by the slow response o f the r e s i s t a n c e thermometer t o e x t e r n a l temperature changes.  I f the power supply  was a d j u s t e d so t h a t the h e a t i n g and c o o l i n g r a t e s were very s m a l l , the f u r n a c e c o u l d be c o n t r o l l e d t o w i t h i n 0 . 2 5 ° C. or l e s s , b u t A i n g e n e r a l work the degree o f temperature t r o l was more u s u a l l y t o w i t h i n 0 . 5 °  C.  con-  I t i s recommended  t h a t i n f u t u r e work a r e s i s t a n c e thermometer be used which has a more r a p i d response. To operate the t h e r m o r e g u l a t o r the f o l l o w i n g ^ procedure should be f o l l o w e d :  (1)  a m p l i f i e r t o zero g a i n .  Shunt out; the r e l a y , and s e t the (2)  Turn on the independent  D.C.  and A.C. main switches and a l l o w the instrument t o warm up. The i n i t i a l h i g h c u r r e n t through the r e l a y w i l l f a l l gradu-  28.  a l l y to less than one-half i t s i n i t i a l value.  (3)  When  the current has f a l l e n to below the load l i m i t of the relay, about 29 m.a., cut i n the relay and allow i t to warm up.  (4)  When the whoQe system has reached e q u i l i -  brium, adjust the negative g r i d bias of  so that the  relay i s at the t r i p p o i n t when the bridge i s balanced. Since the output from the bridge i s negligible at zero gain, t h i s operation consists simply i n adjusting R^ u n t i l the p i l o t l i g h t i n the relay output c i r c u i t indicates the t r i p point, about 16 m.a. temperature.  (6)  (5)  Adjust R  2  to the required  Increase the gain gradually as the f u r -  nace approaches the desired temperature, being careful that the current i (7)  z  does not exceed the relay load l i m i t .  P e r i o d i c a l l y check the adjustment of R_.  Description of a t y p i c a l experiment. When the temperature of the furnace was changed widely, i t was necessary to allow several hours f o r the temperature d i s t r i b u t i o n along the length of the furnace to reach equilibrium.  Therefore, the furnace was not heated up from  room temperature each day but was maintained continuously at temperatures close to those of the experiments. At* the beginning of a day's experiments the thermoregul a t o r was turned on and used at low s e n s i t i v i t y to adjust the furnace to the desired temperature.  When t h i s tempera-  ture was reached the adjustments of the thermoregulator were checked, the gain was adjusted to give maximum sensit i v i t y , and the furnace was allowed to reach i t s equilibrium.  29. temperature d i s t r i b u t i o n .  The r e s i s t a n c e s c o n t r o l l i n g t h e  temperature d i s t r i b u t i o n r e q u i r e d separate adjustment f o r each temperature. - The c u r r e n t i n t h e windings o f the lower g a l l e r y and t h e c a p i l l a r y t u b i n g was turned on and t h e i r temperature was r a i s e d i n stages t o approximately 1 0 5 ° C. The mercury d i f f u s i o n pump was used t o evacuate t h e apparatus  t o a "black vacuum". When a s u f f i c i e n t vacuum had been a t t a i n e d , t h e t e l e -  scope was focussed on the p o i n t e r o f t h e gauge and a d j u s t e d so t h a t t h e p o i n t e r r e s t e d on an a r b i t r a r y zero on t h e s c a l e . A d i f f u s e l i g h t behind t h e gauge a i d e d i n o b t a i n i n g a sharp image o f t h e p o i n t e r . B e f o r e b e g i n n i n g a day's experiments t h e d i s u l f i d e i n 5  the  p r e h e a t i n g v e s s e l was c o o l e d i n a bath o f d r y i c e and  acetone and t h o r o u g h l y pumped o f f t o remove any t r a c e s o f air.  When ready t o i n t r o d u c e t h e r e a c t a n t d i s u l f i d e  into  the  r e a c t i o n v e s s e l , the p r e h e a t e r was immersed i n a beaker  of  h o t water, from 5 0 ° C. t o 1 0 0 ° C. depending on t h e i n i t i a l  pressure d e s i r e d , and a few minutes were allowed f o r t h e d i s u l f i d e t o r e a c h maximum temperature and p r e s s u r e . As soon as p o s s i b l e a f t e r the r e a c t i o n v e s s e l had been the  hot water was removed from the p r e h e a t e r t o a v o i d de-  composing  t h e remaining d i s u l f i d e .  The r e a d t i o n v e s s e l had t o be f i l l e d the  filled,  carefully  since  gauge would be broken i f a l a r g e p r e s s u r e d i f f e r e n c e  were allowed t o occur between the d i s u l f i d e i n t h e v e s s e l and the  a i r i n t h e j a c k e t o f t h e gauge.  During the f i l l i n g  30. operation the pointer of the gauge had to be continuously observed to see that such a pressure difference did not occur, and since the mercury manometer could not be watched simultaneously i t was not possible to regulate the amount of disulfide admitted to the reaction vessel in the usual way.  One of two special procedures had to be used. When a pressure of less than 100 mm. of reactant was  desired the amount admitted to the reaction vessel could be controlled with some accuracy by using the gauge as a direct measuring instrument.  After closing taps T^, T^, and T^,  air was admitted to the jacket of the gauge until the pointer rested on a predetermined scale division representing • mately one-half the desired disulfide pressure.  approxi-  Then by  suitable manipulation of taps T-^ and T^, disulfide was admitted to the reaction vessel u n t i l the pointer had reached a predetermined scale division on the other side of the zero point.  The operation took from two to ten seconds,  depending on the condition of the bore of tap T , on the condition of the capillary tubing, and on the speed of manipulation . When a pressure of greater than 100; mm of reactant was desired, the reaction vessel and the jacket of the gauge had to be f i l l e d simultaneously i f the gauge were not be be broken.  The operation was accomplished by appropriate mani-  pulation of taps T-p Tj, and T , and took from 15 to 30 1Q  seconds.  The amount of disulfide admitted to the reaction  vessel was controlled approximately by f i l l i n g the vessel to  31.  the maximum possible pressure, as controlled by the temperature of the preheater. usually within 10 mm.  The i n i t i a l pressure obtained was of the pressure desired.  The course of an i n d i v i d u a l experiment was followed by pressure versus time measurements.  The time measurements  were made with a large e l e c t r i c Gray-Lab Universal Timer, which was started at the completion of the f i l l i n g operation. The d i s u l f i d e pressure i n the reaction vessel decreased very rapidly to a minimum at the s t a r t of an experiment. During t h i s period the gauge was not zeroed but was used to indicate pressure change d i r e c t l y . mum,  The " i n i t i a l " , or mini-  pressure was measured by adjusting the a i r pressure  i n the jacket of the gauge u n t i l the pointer read zero.  This  procedure was d i f f i c u l t to execute quickly and accurately, but fortunately the occurrence of an induction period i n the reaction allowed s u f f i c i e n t time f o r the adjustments to be made.  Pressure readings after the i n i t i a l one were made by  the simpler and more precise prodedure of admitting a small excess of a i r into the gauge jacket and allowing the normal increase of pressure during reaction to return the pointer to  zero.  Since the pointer always passed through zero point  slowly, and since i t always reached zero from the same side, the exact point taken as zero could be duplicated i n successive readings. The a n a l y t i c a l experiments were terminated by sampling the reaction mixture at a certain stage of the reaction. procedure was to open tap T  2  and admit the mixture to the  The  32. sampling of  p i p e t t e w h i l e simultaneously evacuating the j a c k e t  t h e gauge.  sampling  The f r a c t i o n o f m a t e r i a l t r a n s f e r r e d t o the  p i p e t t e ( u s u a l l y about 80% ) was c a l c u l a t e d from t h e  p r e s s u r e s i n t h e r e a c t i o n v e s s e l before and a f t e r  sampling.  At t h e c o n c l u s i o n o f an experiment taps TQ_ and T^ were opened and t h e r e a c t i o n v e s s e l and gauge j a c k e t were evacuated s i m u l t a n e o u s l y .  L i k e a l l o p e r a t i o n s i n which t h e  r e a c t i o n v e s s e l and j a c k e t o f t h e gauge were evacuated o r f i l l e d , t h i s o p e r a t i o n had t o be done c a r e f u l l y t o a v o i d b r e a k i n g the gauge.  In s p i t e o f great care gauges were  broken o c c a s i o n a l l y ; t h e i r replacement  caused  considerable  delay i n the i n v e s t i g a t i o n . The pressure-time  curve f o r a t y p i c a l experiment.  The p l o t o f pressure change versus time f o r a t y p i c a l experimental run i s shown i n F i g . 6 .  T h i s run was made a t  a temperature o f 3 4 1 ° C. w i t h an i n i t i a l pressure o f 7 3 . 0 mm. of  methyl d i s u l f i d e i n the r e a c t i o n v e s s e l . I t w i l l be seen t h a t t h e pressure changes i n t h e thermal  decomposition r a t e curve.  o f methyl d i s u l f i d e f o l l o w e d a very complex The curve c o n s i s t e d o f (1) a p e r i o d o f r a p i d l y  decreasing pressure,  (2) an i n d u c t i o n p e r i o d , (3) a ^ p e r i o d  of  a u t o a c c e l e r a t i n g r a t e up t o a maximum, and (4) a / p e r i o d  of  d e c r e a s i n g r a t e a f t e r t h e maximum. A f t e r t h e maximum r a t e had been passed t h e r a t e de-  creased t o a low value q u i t e s h a r p l y , but not t o z e r o . pressure then  The  continued t o r i s e slowly f o r an a p p a r e n t l y i n -  definite period.  Fig. 7.,  which p r e s e n t s the r e s u l t s o f an  I  0  10  20  30  TIME IN MINUTES Fig. 6 T y p i c a l p r e s s u r e - t i m e curveo Decomposition of 73.0 mm. of methyl d i s u l f i d e at 341 °C. 0  33. extended run a t 341° C.,  i l l u s t r a t e s t h i s behavior.  t h i s experiment the r a t e had decreased  to a low  a f t e r 60 minutes, but the pressure was  still  r a t e o f 1 mm.  per hour a f t e r 10 hours.  In  value  r i s i n g at a  Attempts to f i t  the p o r t i o n o f the curve s t a r t i n g a t the maximum r a t e t o i n t e g r a t e d order equations were e n t i r e l y without It  success.  seemed probable t h a t the c o n t i n u i n g p r e s s u r e r i s e  60 minutes was The  after  due mainly t o secondary r e a c t i o n s .  f i n a l pressure i n c r e a s e i n the r e a c t i o n was  determined  s i n c e i t was  c l e a r l y of l i t t l e  not  significance.  Pressure i n c r e a s e s of 105% o f the i n i t i a l p r e s s u r e were recorded i n experiments a t 341° C. and 373° C., cases the pressure was s t i l l r i s i n g s t e a d i l y .  but i n b o t h  Clearly  r e s u l t s d i d not c o n f i r m Patrick's^- r e p o r t t h a t the p r e s s u r e i n c r e a s e i n the r e a c t i o n i s 70% of the  these  final  initial  pressure. At, temperatures was  a t which the r a t e o f the main r e a c t i o n  measurable, the i n i t i a l p r e s s u r e decrease o c c u r r e d so  r a p i d l y t h a t i t was initial  q u i t e i m p o s s i b l e t o determine the t r u e  p r e s s u r e o f d i s u l f i d e i n the r e a c t i o n v e s s e l .  experiments a t 316° C. w i t h i n i t i a l p r e s s u r e s of the of  95 mm.,  first 15 mm.  the pressure decreased  by about 1 0 mm.  almost  complete i n one minute.  illustrated in Fig. 8.  order  i n the  15 seconds, and the t o t a l measurable decrease was  In  o f about  This behavior i s  At h i g h e r temperatures  a l a r g e de-  crease probably o c c u r r e d d u r i n g the f i l l i n g o p e r a t i o n . stead o f c a l c u l a t i n g pressure changes from the  In-  indeterminate  200  166 PRESStJRE  150  mm i Hg,  100  50  0  0  100  200  300  400  500  TIME IN MINUTES Pigo 7 Pressure-time curve i l l u s t r a t i n g the l a t e stages o f the reaction,. Decompos i t i o n of 166 mm. of methyl d i s u l f i d e at 341 °C. 0  600  34. i n i t i a l pressure i n the reaction vessel i t was necessary therefore to calculate them from the minimum pressure existing during the induction period. Whether or not this minimum pressure was i n reality the "true i n i t i a l pressure" of a homogeneous gas reaction could only be shown by further experiments. Patrick^ did not observe such an i n i t i a l pressure .decrease.  It i s possible that i t did not occur i n the quartz  reaction vessel that he used. The reproducibility of pressure-time curves Since i t was d i f f i c u l t to control the amount of disulfide admitted to the reaction vessel i n i t i a l l y , and since i n addition the magnitude of the "observable" pressure decrease depended on the length of the f i l l i n g operation, i t was a l most impossible to duplicate conditions of minimum pressure in successive experiments.  Therefore, to determine whether  or.not the results cbf individual experiments were reproducible i t was necessary to compare the results of experiments of differing i n i t i a l pressures by using the rate law found for the reaction. A number of runs were made at 316° C. i n which the i n i t i a l pressures varied from 75 to 89 mm; the results were recalculated to a single i n i t i a l pressure of 80 mm. and are compared i n Fig. 8.  (These runs vary in length because  they were made as part of the analytical work.) From Fig. 8. i t w i l l be seen that the extent of the pressure increase was not completely reproducible in the later part of the reaction.. However both the maximum rate and the  Flgo  8  e  R e o r o d u c l b i l i t y of the pressure-time curves at 3 1 6 ° C . Minimum D r e s s u r e s r e f e r r e d to 80.0 nan, of d i s u l f i d e .  l e n g t h of the i n d u c t i o n p e r i o d were r e p r o d u c i b l e . In a p p r o x i mately 40 runs made a t 316° C. the i n d u c t i o n p e r i o d never d i f f e r e d from 12 minutes by more than one minute,  and i n  more than o n e - h a l f of these runs i t d i f f e r e d from the u s u a l l i s minutes by l e s s than 15 seconds.  The  reproducibility  of  the maximum r a t e w i l l become e v i d e n t when the c a l c u l a t i o n  of  rate constants i s considered. The r e s u l t s o f F i g . 8 . show r a t h e r worse r e p r o d u c i b i l i t y  than was  usual.  F o r example, the pressure-time curves  t a i n e d from s i x c o n s e c u t i v e hydrogen  sulfide  analytical  runs d i d not d i f f e r from each o t h e r by more than 1 mm. r e f e r r e d t o the same s c a l e as F i g . 8.  At h i g h e r  the curves were more r e p r o d u c i b l e than a t 316° In  ob-  when  temperatures  C.  order t o determine whether or not the r e s u l t s showed  a g e n e r a l d r i f t as the experimental work progressed, the pressure-time curves f o r the n i n t h and f i f t y - f i f t h compared.  These runs were s e l e c t e d because  pressures d i f f e r e d only s l i g h t l y . in F i g . 9. sure,.  runs were  their  initial  The comparison  i s made  w i t h b o t h curves r e f e r r e d t o the same i n i t i a l  There i s no s i g n i f i c a n t d i f f e r e n c e between the  pres  two  curves. Although the e f f e c t was  not d e f i n i t e l y e s t a b l i s h e d , i t  appeared t h a t the r e s u l t s became l e s s r e p r o d u c i b l e i f the hot r e a c t i o n v e s s e l were exposed s u r f a c e e f f e c t i s suggested.  to a i r between r u n s .  I t appeared a l s o t h a t  A  results  which d i f f e r e d w i d e l y from the normal d i f f e r e d a l s o i n the extent o f the i n i t i a l  pressure decrease, but the enormous  60  0  10  20  30  TIME IN MINUTES  P i g . 9e Comparison of the p r e s s u r e - t i m e curves f o r the n i n t h and f i f t y - f i f t h runs© Decompositions of methyl d i s u l f i d e at 341 ° C 0  36. d i f f e r e n c e i n r a t e between the p r e s s u r e decrease  and the  p r e s s u r e i n c r e a s e made i t d i f f i c u l t t o v e r i f y t h i s . o b servation. The  i n i t i a l pressure decrease Since an understanding  i n the r e a c t i o n  o f the pressure decrease  i n the  r e a c t i o n w i l l clarify the meaning o f the experiments t o be d e s c r i b e d , i t i s a d v i s a b l e t o a n t i c i p a t e some r e s u l t s o f l a t e r work and d i s c u s s the p r e s s u r e decrease I t w i l l be r e c a l l e d t h a t t h e i n i t i a l  at this point.  pressure  decrease  i n t h e r e a c t i o n made i t necessary t o s p e c i f y the c o n d i t i o n s of the experiments by the minimum pressure o f the i n d u c t i o n p e r i o d r a t h e r than by the p r e s s u r e o f d i s u l f i d e admitted t o t h e r e a c t i o n v e s s e l .  originally  That t h i s minimum p r e s s u r e  was an e f f e c t i v e standard f o r c o r r e l a t i n g the r e s u l t s soon became e v i d e n t .  L a t e r work showed moreover t h a t the minimum  pressure was a c t u a l l y the more fundamental q u a n t i t y .  The  a n a l y t i c a l r e s u l t s showed t h a t t h i s constant minimum p r e s sure was due e n t i r e l y t o unreacted main disappearance period.  d i s u l f i d e and t h a t t h e  of d i s u l f i d e occurred a f t e r the i n d u c t i o n  Thus t h e minimum p r e s s u r e was t h e " e f f e c t i v e  t i a l p r e s s u r e " o f t h e r e a c t i o n and the p r e s s u r e  ini-  decrease  was not p a r t o f the main r e a c t i o n . S e v e r a l p o s s i b l e causes o f t h e p r e s s u r e decrease had t o be c o n s i d e r e d :  (1) a d s o r p t i o n o f t h e r e a c t a n t on t h e s u r f a c e  of the pyrex r e a c t i o n v e s s e l ,  (2) r e a c t i o n w i t h the f o r m a t i o n  37. of v o l a t i l e polymeric products,  (3)  t i o n o f i n v o l a t i l e products, or (4)  r e a c t i o n w i t h the forma-, r e a c t i o n with t o t a l  ad-  s o r p t i o n o f the p r o d u c t s . Whichever was two r e s t r i c t i n g  the c o r r e c t p r o c e s s , i t was  conditions.  subject to  F i r s t , as evidenced by  the  shape o f t h e pressure-time curves, the process had t o reach a s t a t e o f e q u i l i b r i u m a t the s t a r t Second, the d i s u l f i d e  of the i n d u c t i o n p e r i o d .  had t o be the only v o l a t i l e  i n the system d u r i n g the i n d u c t i o n p e r i o d . gether, these two  c o n d i t i o n s immediately  substance  Considered t o -  e l i m i n a t e d the  p o s s i b i l i t i e s o f f o r m a t i o n of i n v o l a t i l e products or  adsorbed  products s i n c e the e q u i l i b r i u m would n e c e s s a r i l y have been of the'' form i .  (RSSR)  gas  K  B  ' , and the minimum p r e s s u r e would . q. c  have been a f u n c t i o n o f temperature amount o f d i s u l f i d e was  not the case.  the o n l y v o l a t i l e  o n l y , independent  admitted t o the r e a c t i o n v e s s e l .  substance  e l i m i n a t e d the p o s s i b i l i t y  being of  products.  the r e s u l t s obtained were i n harmony w i t h the i d e a o f  the i n i t i a l decrease  i n pressure b e i n g due t o a d s o r p t i o n o f  the r e a c t a n t d i s u l f i d e . below.  This  Moreover, the c o n d i t i o n of d i s u l f i d e  r e a c t i o n with the f o r m a t i o n of v o l a t i l e polymeric All  o f the  Some o f these r e s u l t s a r e l i s t e d  In c o n s i d e r i n g the v a l i d i t y of these o b s e r v a t i o n s i t  w i l l be r e a l i z e d  t h a t the decrease  s t a r t e d d u r i n g the  o p e r a t i o n ; however, s i n c e the r e a c t i o n v e s s e l could be w i t h i n two  seconds,  decrease was  filling filled  i t i s b e l i e v e d t h a t the g r e a t e r p a r t o f the  actually  observed.  38. (1)  The pressure decrease was larger the larger the  i n i t i a l pressure but the relation was not linear.  At 316° C.  with an i n i t i a l pressure of 50 mm. the observed decrease was about 4 mm; with an i n i t i a l pressure of about 95 mm. the observed decrease was 15 mm. (2)  The extent of the decrease appeared to be less  the higher the temperature.  At 291° C. the pressure de-  creased by more than 21 mm. to a minimum of 80 mm., while at 316° C. the decrease was about 15 mm. (3)  The rate of the pressure decrease was much less  sensitive to temperature than the rate of the main reaction: At 373° C. observed pressure decrease of 1 mm. occurred i n about 10 seconds.  At 291° C. the corresponding time for  the decrease, calculated from the activation energy of the maximum rate, should have been 22 minutes, but actually the decrease was 90% complete after only 1 minute. Assuming 10 seconds and 60 seconds to represent, approximately, times of equal percent pressure decrease at the two temperatures, i t was possible to estimate the activation energy for the rate of pressure decrease.  The calculation  gave E « 16 k.cal./mole, correct to order of magnitude only. Such a low activation energy i s i n harmony with the adsorption mechanism. It i s much less than the 45 k.cal./mole activation energy of the main reaction. (4)  At 291° C , the addition of 11 mm. of hydrogen sulfide  to the reaction vessel before addition of a minimum pressure of  39. 80 mm.  of d i s u l f i d e decreased  the magnitude o f the  decrease from g r e a t e r than 21 mm.  t o only 4 mm..  pressure  Hydrogen  s u l f i d e i s known to adsorb on g l a s s . (5) creased  The magnitude o f the pressure  decrease was i n -  s l i g h t l y when t h e . s u r f a c e t o volume r a t i o of the  r e a c t i o n v e s s e l was  i n c r e a s e d by a f a c t o r 6.5.  This  be-  h a v i o r would be expected i f the extent of a d s o r p t i o n were s l i g h t l y dependent on the number o f a c t i v e s i t e s .  Since  r a t e o f a d s o r p t i o n should be d i r e c t l y p r o p o r t i o n a l t o  the  the  s u r f a c e t o volume r a t i o , i t would be u s e f u l i n f u t u r e work t o study the r a t e s o f pressure  decrease i n packed and  un-  packed v e s s e l s . •(6) was  When most of the d i s u l f i d e i n the r e a c t i o n v e s s e l  pumped o f f r a p i d l y a t the completion  decrease,  the remaining  pressure  of the  pressure  i n the v e s s e l i n c r e a s e d .  D e s o r p t i o n was i n d i c a t e d . C o n s i d e r a t i o n w i l l show t h a t the a d s o r p t i o n of i s the only mechanism of the pressure p l a i n a l l these r e s u l t s and f  decrease which w i l l  s a t i s f y the e q u i l i b r i u m and  l y t i c a l conditions discussed previously. concluded was  disulfide  w i t h some confidence  ex-  ana-  A c c o r d i n g l y i t was  t h a t a d s o r p t i o n o f the  reactant  actually occurring. I t i s p o s s i b l e t o p r e d i c t an a c t i v a t i o n energy f o r the  pressure  decrease from a simple  c o l l i s i o n model f o r a d s o r p t i o n .  I f we assume t h a t the r a t e o f a d s o r p t i o n can be as the product  represented  o f (1) the number of molecules s t r i k i n g  s u r f a c e per second, and  the  (2) the f r a c t i o n of these which have  40. energy greater than the a c t i v a t i o n energy, e"^^^, then from k i n e t i c theory we may d e r i v e the f o l l o w i n g expression f o r the i n i t i a l r a t e o f adsorption: dP/dt  =  P S(8RT)  2  E  ~  E  /  R  T  41TTM)  where "P" i s the pressure of d i s u l f i d e , M the molecular weight, S the surface t o volume r a t i o of the r e a c t i o n v e s s e l , E the a c t i v a t i o n energy, and T the absolute temperature.  At 316° C.  and 100 mm. pressure, the i n i t i a l ' r a t e was estimated a t 1mm./sec. Using t h i s estimate and the values S » 1.0, M = 9 4 , R » 8\.31 x 10"^ erg./deg./mole the a c t i v a t i o n energy was c a l c u l a t e d as: E  calc. =  1  ^ « k  cal  »/ ole m  This value i s of the same order as observed a c t i v a t i o n energy of 16 k.cal./mole.  Thus both the frequency f a c t o r - o f the r a t e  and the a c t i v a t i o n energy are of the c o r r e c t order f o r an adsorption process. The existence of the i n d u c t i o n period showed that there could not have been appreciable heterogeneous r e a c t i o n of the adsorbed d i s u l f i d e with desorption of the products.  Therefore  i t was reasonable t o assume t h a t the two processes of ads o r p t i o n and gas r e a c t i o n were independent.  However the com-  p l i c a t i o n was evident that as the main r e a c t i o n proceded the adsorbed d i s u l f i d e would desorb and enter i n t o the r e a c t i o n : t h e r e f o r e , i t would be impossible t o determine the exact amount of d i s u l f i d e decomposed a t any stage o f the ..reaction. I t was r e a l i z e d too t h a t the adsorption and desorption would  41. a f f e c t the r e p r o d u c i b i l i t y of the pressure-time curves i f the amount of d i s u l f i d e adsorbed was dependent on the condition of the surface of the reaction v e s s e l :  thus the surface  e f f e c t mentioned previously i s explained. The dependence of the r a t e of pressure change on the i n i t i a l pressure of methyl d i s u l f i d e ' The dependence of the rate of pressure change on the i n i t i a l pressure of methyl d i s u l f i d e was investigated over a range of i n i t i a l pressures from 24 mm. to 230 mm.  As usual,  the minimum pressure of the induction period was taken to represent the e f f e c t i v e i n i t i a l pressure of the reaction. Since i t was d i f f i c u l t to duplicate any i n i t i a l  (minimum)  pressure no? attempt was made to reproduce the r e s u l t s . Instead, the self-consistency of a large number of experiments was taken as implying that the r e s u l t s were reproducible. A series of runs was made at 3;41° C  with various mini-  mum pressures of d i s u l f i d e i n the reaction vessel.  The pres-  sure-time curves f o r these runs are reproduced i n F i g . 1 0 . The dependence of the extent of reaction at any t ime on the minimum pressure was investigated using data from the experiments i l l u s t r a t e d i n F i g . 1 0 . Table I I .  The data are l i s t e d i n '  With the exception of the run at 24 mm., the percent pressure increase W p / P i . ) m  n  pressure at a l l four times.  w  a  s  independent of the minimum  I t was concluded therefore that  the extent of the pressure increase i n the thermal  decomposition  Figo 1 0 s .  P r e s s u r e - t i m e c u r v e s f o r the decompositions of v a r i o u s i n i t i a l p r e s s u r e s o f m e t h y l d i s u l f i d e a t 341 ° C . (See also F i g . 10b.)  42 of methyl disulfide i s f i r s t order with respect to the " i n i t i a l " pressure of disulfide. Table I I Extents of reaction for various i n i t i a l pressures of methyl disulfide  Run  Minimum pressure mm.  1  Percent pressure increase 8 min.  10 min.  15 min.  20 min.  24.0  45  62*  76  85  2  57.5  36|  53  68*  74*  3  73.0  54  68*  74  4  92.5  38*  53  66*  73  5  108.0  43  54*  68*  73  6  141.8  35  48  63  68  7  166.5  42  53  67*  74  8  229.5  42*  53*  68  73  The dependence of the maximum rate of pressure rise on the minimum pressure was also investigated.  The maximum  rates were determined by plotting the curves of Fig.10. on a greatly enlarged scale and determining their maximum slopes.  The order was determined by graphical solution of  the equation: Order  • d log.(dP/dt)max. d log.P . min. Fig.11 shows the plot of log.(dP/dt)max. versus log.P . • mm.  200  Y  &  5  10  15 TIME  20 IN  25  30  MINUTES  Fig« 10b«, P r e s s u r e - t i m e ' c u r v e s f o r t h e decomp o s i t i o n s of v a r i o u s i n i t i a l oressures of m e t h y l d i s u l f i d e a t 341 °C.  43.  The line of slope 1.0 i s drawn through the points, and lines of slope 1.5 and 0.5 are drawn for comparison.  Since  the correct slope of the graph was clearly unity, i t was concluded that the maximum rate of pressure increase i n the thermal ..decomposition of methyl disulfide i s f i r s t order with respect to the " i n i t i a l " pressure of disulfide. The rate constants calculated from the f i r s t order equation (dP/dt)max.  =  k F m  ± . n  are presented in Table III. They were indeed constant over a wide range of pressures.  The mean value of 0.00208 +_  0.00007 sec.-l was used in later calculations.  It w i l l be  noted that these rate constants were calculated on the basis of the minimum pressure of the reaction rather than the true i n i t i a l pressure; their constancy supports the idea that the minimum pressure i s actually the effective i n i t i a l pressure of the reaction. The dependence of the length of the induction period on the i n i t i a l pressure i s interesting.  At 341° C. the i n -  duction period of 2 to 2\ minutes was almost independent of the i n i t i a l pressure for pressures below 140 mm.. run at 166.5 mm.,  In,the  however, i t was considerably shortened,  and i n the run at 229.5 mm.  i t lasted only one minute.  The  independent behavior at low pressures i s shown more clearly by some results at 316° G. of 159-5 mm.,  SO.O mm.,  In runs with i n i t i a l pressures  and 55.25 ram., observed the i n -  duction periods were 12 rain., 11* min., and 13 min.,  44.  respectively. Table III Reaction rates and rate constants at 3 4 1 ° G. for various i n i t i a l pressures of methyl disulfide  Run  Minimum pressure mm.  Maximum rate mmx/min  Rate constant min-1  1  24.0  3.00  0.125  2  57.5  6.80  0.118  3  73.0  9.17  0.125  4  92.5  11.3  0.122  5  108.0  13.4  0.124  6  141.8  18.5  0.130  7  166.5  20.3  0.122  a  229.5  30.0  0.131  The effect of packing the reaction vessel It was of interest to determine the Effect that packing of the reaction vessel would have on the rate of the reaction. The reaction vessel was packed as uniformly as possible with short lengths of. pyrex tubing, the ends of which had been f i r e polished to eliminate possible catalysis by sharp edges.  In this way, the surface to volume ratio was i n -  creased from approximately 1 . 0 to 6 . 5 .  The results of three  experiments under these conditions are presented i n Fig. 1 2 . ,  70  60  50 CHAN^F  w .  o f  HP,  40  30  UNPACKED  20  PACKED  10  0  A. 3n  40 TIMF  F i j o 12, methyl  50  60  70  80  (MINUTES)  E f f e c t o f t a c k i n g t h e r e a c t i o n veas»i . Decomposition? d i s u l f i d e (30.0 nun, minimum p r e s s u r e ) at 316 ° C  90  45  referred t o a single minimum-pressure of 80 mm. of disulfide. The effect of packing the reaction vessel was clearly very small•  The magnitude of the - pressure decrease was i n -  creased slightly.  The induction period was almost unaffected;  the slight lengthening from 11| - 12 minutes to 13 minutes was very probably due to. adsorption of the reaction products. The only appreciable effect seemed to be a lowering of the f i n a l pressure attained, an effect which also suggested adsorption of the products. occur,  Such adsorption was known to  (see - "The determination of hydrogen sulfide").  The comparison of the results from the packed and unpacked vessels enabled two conclusions to be drawn:  (1) The  induction period i s not affected by the surface area of the vessel.  Therefore i t i s not due to chain termination on  the walls of the vessel, until the surface becomes poisoned by the. reaction products.  (2) The reaction i s essentially  homogeneous. The dependence of. the rate, of pressure change on temperature The dependence of the rate of pressure change on temperature was not investigated with thoroughness; i t was f e l t that at the present time an accurate knowledge of the activation energy would be of l i t t l e help i n understanding the mechanism of the reaction. However, a short investigation was made because an approximate knowledge of the activation energy was desirable, and because i t was desirable to know whether the different stages of the reaction depended  46.  on temperature i n the same way. The variations with temperature of the i n i t i a l pressure decrease, the length of the induction period, the time of the maximum rate, and,the magnitude of the maximum rate were studied at four temperatures between 3 1 6 ° C. and 373° C. At 3 1 6 ° G. and at 341° C. the results were averaged from a large number of experiments. taken from a single run.  At 36O5  0  C. the results were  At 372 C. the reaction was too 0  rapid to permit the determination of a pressure-time curve from a single run; instead, the results of two successive runs, i n which the times of pressure reading were staggered, were used to construct a single smooth curve. In these fast reactions the reaction vessel was f i l l e d i n two seconds by using low i n i t i a l pressures of 60 mm. and by using the gauge as a direct measuring,instrument; the f i r s t few readings after the induction period were obtained directly from the gauge without reference to the mercury manometer. The lengths of the induction period, and the time required for the reaction to reach i t s maximum rate were obtained from runs at low pressures.  The results are listed i n  Table IV. A graph of ln.k versus l/T i s presented i n Fig. 13. >The linearity of the graph shows that the Arrhenius Equation holds over the range of temperatures studied. tion energy calculated E  The activa-  from the slope of the graph i s :  = 45 k.cal./mole.  I  45 k. cel. ,/molp  47. The error i s probably not greater than  4*k.cal./mole.  This value may be-compared to the 51 k.cal./mole vation energy calculated by P a t r i c k .  acti-  1  Table IV Dependence of the reaction on temperature  Temperature °C.  Induction period sec.  Time of max. rate sec.  Rate constant sec."*-1  316  690  I860  0.00042  341  120  420  0.00208  157  O.OO67  65  0.0116  360£  35  373  19  .  From the- rate constant -at 341° C. the frequency f a c tor was calculated to be approximately 10^3 s e c . "  1  By adapting the Arrhenius equation i t was possible to calculate a c t i v a t i o n energies from the length of the i n duction period at various temperatures. k  «=  Ze- / E  If:  R T  then the time taken t o complete any p a r t i c u l a r stage of the reaction i s : t But " l  r t  =  (function of p r e s s u r e J Z ^ e ^ / ^ .  the length of the induction period i s independent  of pressure at the low pressures used.  Therefore:  48.  I and  m  Ce  E/RT  ln(I) = C + E/RT  The activation energy can be determined graphically. In Fig. 14. the graph of ln(I) versus l/T and the similar graph of ln(time of the maximum rate) versus l/T have been plotted together. The linearity of both graphs indicates independence of pressure. The activation energies calculated from the slopes of the graphs are: 48.7 k.cal./mole 45.0 k.cal./mole The activation energies calculated from the maximum rate of pressure increase, the duration of the induction period, and the time taken to reach the maximum rate, do not differ from each other by more than the experimental error.  Therefore i t i s possible that the length of the  induction period i s governed by the processes which determine the maximum rate. It i s interesting to note that in the later stages of the reaction the time taken to achieve a given percent pressure increase i s considerably more sensitive to temperature than the length of the induction period or the time taken to achieve the maximum rate.  At 373° C. the time  taken to reach 100% pressure increase was 20 minutes.  From  the temperature coefficient of the induction period we would expect the time taken to reach the same pressure increase at 341° C. to be 127 minutes, but experimentally i t was found to be 365 minutes. to be real.  The difference i s believed  This result would be explained i f the adsorp-  1.50  1.70  1.60  10 /T ?  F i g c 1 4 o Dependence on t e m p e r a t u r e o f : L l ) the l e n g t h o f t h e i n d u c t i o n p e r i o d , (2) t h e t i m e a t w h i c h tho maximum r a t e is attained, (1)  Ej  «  48.8  k.cal./mole  (2)  E  =  45.0  k.cnl./mole  M  t  49. tion of products were reduced at high temperatures. However, the temperature coefficient of the rate of reaction may actually be larger i n the later stages of the reaction than i n the i n i t i a l stages.  The conclusion would follow  that differing processes are involved. The analysis of the reaction mixture It has been emphasized already that the nature of the thermal decomposition of methyl disulfide would only be elucidated by extensive analytical investigation.  Accor-  dingly, the greater part of. the experimental work of the present investigation was the preparation of accurate methods for the analysis of the reaction mixture and the application of these methods to the study of the reaction. Samples of the reaction mixture were obtained for analysis in the gas sampling pipette.  When the number of moles  of any constituent in the sampling pipette had been determined i t s partial pressure in the reaction vessel could be calculated.  This calculation, based on the ideal gas law,  is illustrated later.  Therefore, the problem of analysis  was reduced, to the determination of the number of moles of each constituent in the sampling pipette. The most important constituent of the reaction mixture was the disulfide i t s e l f .  Hydrogen sulfide and mercaptans  had been shown to be present by Patrick^ and they were identified early i n the present investigation. In addition, monosulfides, polysulfides, free sulfur, carbon disulfide,  I  50. thiophene, and saturated and unsaturated hydrocarbons were suspected of being present.  As most of these compounds  were later identified the mixture was clearly complex. The analysis of any mixture of organic sulfur compounds i s made d i f f i c u l t by a paucity of specific analytical reagents.  If unsaturated hydrocarbons are present the analysis  is s t i l l more d i f f i c u l t because "double bonds" react with the useful oxidation reagents permanganate, iodine, bromine, and hypochlorite. Conversely, the presence of sulfur compounds makes the determination of double bonds d i f f i c u l t . Unsaturated.hydrocargons-were  believed to be present i n the  reaction mixture. The methods available for the analysis of mixtures of sulfur compounds are complex schemes of analysis.  These  schemes rely on methods of selectively separating the various constituents. estimation.  They depend heavily on indirect techniques of In principle they are not desirable for accu-  rate quantitative work but no alternative methods are available, As yet no general scheme has been devised for the anal y s i s of sulfur compound mixtures.  Rather, i t has been neces-  sary to differentiate analytically between systems according to the classes, of sulfur compounds that are present, according to the particular homologues present, the presence or absence of double bonds, the physical state of the system _ (solution or gas), the range of concentration, and the accuracy desired. Of necessity, the schemes that have been devised  have been designed for the analysis of particular, types of mixtures under particular conditions. Although.usually adequate for the purposes of their design, they can not be applied at random to other systems or other conditions. Most of the existing analytical schemes have been designed for the analysis of petroleum or "benzol" mixr tures.39|40,41  They deal with compounds of relatively high  molecular weight at relatively high concentrations (O.Ol to 1% S) i n organic solvents; designed for routine work, they are not required to give exceptional accuracy.  One scheme  has been designed especially for the analysis of low molecular weight compounds,^ but i t i s not applicable i n the 2  presence of double bonds.  The examination, evaluation, and  generalization i n these schemes of the very large amount of work that has been done on the analysis of sulfur compounds was very useful i n the present problem.  In particular the  monumental paper of Ball*"® was of inestimable help. How1  ever, i t must be emphasized that none of these existing schemes were directly suitable to the analysis of the methyl disulfide decomposition system. The analysis of the methyl disulfide decomposition system was approached as a new analytical problem.  An exami-  nation of the properties of the system showed that although certain existing analytical techniques were not applicable certain new techniques could be used. now discussed:  (1)  These properties are  The substances present included most of  52. the classes of organic sulfur compounds with unsaturated hydrocarbons i n addition.  Therefore the choice of possible  methods was severely restricted.  However, i n contrast to  the double bonds met i n petroleum systems, the double bonds in the present system were present probably entirely as ethylene.  Therefore iodine was a possible reagent.  "(2) The  particular homologues present were the lowest members of their respective series, and therefore, the most volatile and most soluble i n water.  The v o l a t i l i t y and solubility  of methyl mercaptan and methyl sulfide destroyed the aceuracy of the partition and precipitation methods normally used to. separate them from various constituents of the mixture (see - "the analytical methods for mercaptan"). However, the high v o l a t i l i t y of methyl mercaptan was a distinct advantage i n the determination of methyl disulfide.  (3) In  general only a single homologue from each series was present in appreciable quantity. Therefore the analytical methods could.be designed particularly to suit single specific homologues.  (4) The compounds existed i n the gas phase.  This  state was advantageous as i t permitted the analysis for hydrocarbons by gas fractionation. tricted choice of solvents.  It also allowed unres-  (5) The compounds were present  only in very small quantities, 1 mm. of gas i n the reaction vessel corresponding to 0.000005 moles.  The resulting low  concentration of material i n the solutions, 0.0001 to 0,05% sulfur, lowered the accuracy of the volumetric methods,  53. prevented the use of certain precipitation separations, and. eliminated the use of the lamp method for the determination of total sulfur.  The loss of the lamp method  was particularly unfortunate because, i n consequence, the determination of monosulfides became impossible by any modification of present methods. The accuracy that was desired i n the present analyt i c a l workowas higher than the accuracy achieved In previous analytical schemes. In general i t was found that the accuracy of existing methods could be improved i f f i r s t l y , they were adapted to suit the particular homologue present, and i f secondly, each analysis at each concentration were treated as a special problem rather than a routine one. The system of analysis that was f i n a l l y developed for the disulfide decomposition.system  was extremely useful,  but much work remains to be done before the complex of methods i s completely satisfactory.  The existing erratic  method for the determination of disulfides was modified to give accurate results for methyl disulfide.  Unfortunately  the presence of polysulfides i n the mixture interfered with some of the analyses by this method.  A direct method was  designed for the determination of hydrogen sulfide which was completely satisfactory.  The existing direct methods  for the determination of mercaptans were shown to be greatly inaccurate when applied to methyl mercaptan.  Of the several  indirect methods tried instead, one gave adequate results;  54.  however, further development would be desirable.  Standard  methods were used for the determination of carbon disulfide, thiophene, and free sulfur.  An approximate method of gas  fractionation was used to analyse for hydrocarbons. The standard copper acetylide method of identifying acetylenes was found to be invalidated by the presence of carbon d i sulfide. Wo method was devised for the analysis of methyl monosulfide i n the low concentrations of the present system. The otherwise suitable bromine method^ was of no use i n the presence of double bonds.  The only other method was that  of removing the monosulfide. (plus disulfide) from the mixture as a mercurous or mercuric nitrate complex and estimation of the total sulfur removed;^ unfortunately the concentrations of the present system were below the solubility of the complexes, and below the range of the lamp sulfur method. Attempts to devise new methods of analysis were hindered by the d i f f i c u l t y of separating methyl sulfide from hydrogen sulfide, methyl mercaptan, methyl disulfide, and free sulfun The separation from the sulfhydryl compounds by precipitation or partition methods was prevented by the apparent solubility of methyl sulfide i n water, by i t s v o l a t i l i t y , and by the ease with which 1th forms complexes with metal saltB^ e.g. with AgNO^.^  The most promising method of sepa-  ration, gas fractionation, would not be sensitive to small quantities.  The analysis of methyl sulfide was f i n a l l y  55.  abandoned i n the present investigation.  However, i t i s to  be hoped that i n future work the possibility of i t s determination as the sulfoxide or sulfone w i l l be investigated. The development of the methods of analysis i s described very briefly i n the following sections.  In the  development of the methods for hydrogen sulfide, mercaptan, and disulfide, the author was assisted by Mr. G. D. Maingot. It i s recommended that the reader consult the independent report of Maingot^ for a fuller discussion of some aspects of the problem and for a more detailed account of some of the development work. The direct method for the determination of mercaptan The independent determination of hydrogen sulfide and mercaptans i s complicated by the similarity of their reactions*  In most schemes of analysis only one of these  constituents i s determined directly, usually the mercaptan; the other i s estimated indirectly from the total sulfhydryl in the mixture. The direct methods for the determination of mercaptan were investigated f i r s t .  For this work the mixture was  dissolved i n AnalR petroleum ether, boiling range above 1 2 0 ° C.  This solvent gave negative tests for hydrogen sul-  fide, mercaptan, disulfide, free sulfur, carbon disulfide and thiophene.  Its low solubility i n water made i t unneces-  sary to correct for the loss of solvent i n partition separations.  56.  Since hydrogen sulfide i s t h e more reactive of the two components a l l methods of determining mercaptan directly require the preliminary removal of hydrogen sulfide from the mixture.  The best method that has been devised for the  separation i s the treatment of the organic solution with aqueous acidified cadmium c h l o r i d e . ^  This method was used  in the present work. The determination of mercaptans i n " o i l " that i s free of hydrogen sulfide i s not d i f f i c u l t .  The now standard  Borgstrom-Reid^ method was used in the present work, essent i a l l y i n the modification of Ball.^°  In this method the  mercaptans are precipitated.. by standard silver nitrate as the silver mercaptides; the excess silver nitrate i n the three phase system i s back titrated with ammonium thiocyanate to a f e r r i c alum end point. The application of the Borgstrom-Reid method to the present system was.complicated  by the low concentrations i n -  volved, by the v o l a t i l i t y of methyl mercaptan, and most of a l l by the property, unique amongst the s i l v e r mercaptides, that silver methyl mercaptide i s coloured.  The necessary  use of 0.01 N. standard solutions instead of the usual 0.1 N. solutions made the detection of the titration end point difficult.  The end point i s usually described as a "salmon  pink" colour, but in the present work i t was evidenced only by a slight colour change from pale yellow to orange-yellow. This colour change was partially obscured by the suspension  57 of greenish-yellow silver mercaptide; i t was more seriously obscured i f traces of hydrogen sulfide were present because the suspension was then a reddish-brown colour.  The addi-  tion of alcohol, which was sometimes necessary to break emulsions i n the three phase system of o i l , water, precipitate, tended to give the precipitate an orange t i n t .  It  was possible after much practice to obtain reproducible end points, but the method i s not recommended for future work with methyl mercaptan.  With other mercaptans the method  is excellent. As far as possible the f i l t r a t i o n of the solution had to be avoided because i t a ppeared that the precipitated mercaptide adsorbed silver nitrate and only desorbed i t after an excess of thiocyanate had been added.  It was observed  f i r s t l y , that, the i n i t i a l - silver nitrate t i t r e was always too high, and secondly, that the degree of dispersion of the precipitate altered a t the end point.  When f i l t r a t i o n  was unavoidable i t was done only after adding a small but detectable excess of thiocyanate.  Filtration through glass  wool was the most satisfactory method. The greenish-yellow colour of silver methyl mercaptide was used as a qualitative test for methyl mercaptan.  The  other silver mercaptides are white. The complete procedure that was used for the direct determination of mercaptan i s now described briefly. further details the reader should consult Maingot.^  For  58. The procedure for direct determination of mercaptan The gaseous samples existed in the 400 ml. gas. pipettev  50 ml. of AnalR petroleum ether was ad-  mitted to the pipette with a minimum of a i r , and the whole was shaken for about 15 minutes. The solution was run into a 100 ml. volumetric flask which was made up to volume with the washings of the pipette.  The glass stopper was inserted tight-  l y to prevent loss of,the volatile (b.p. 5° C.) mercaptan to the a i r . Aliquot portions were transferred to stoppered separating funnels and treated with one sixth their volume of acid 10% cadmium chloride solution.  After  separation, this treatment was repeated with a smaller amount of cadmium chloride solution to test for complete removal of hydrogen sulfide. A measured portion of the separated o i l layer was pipetted into a glass:stoppered  erlenmeyer flask  containing a measured excess of 0.01 N. silver n i t rate.  With the greased stopper insetted, the flask  was shaken periodically for about 10 minutes to allow the mercaptan to react.  Ferric alum indicator  was.added and the excess silver nitrate back t i t rated with 0.01 N. ammonium thiocyanate:  Initially  an excess of thiocyanate was added and the flask was. shaken to obtain fast reaction of the adsorbed silver nitrate. Then a small excess of silver nitrate  59.  was added and the excess titrated to the end point.  (The end point should be observed by i n -  tense transmitted l i g h t ) .  This procedure was  repeated until the end point had been reproduced within 0.05 ml. five or six times. The moles of mercaptan were calculated from the equation: RSH + AgNO^  =  AgSR + HNO^  Note 1. Provided their containers are kept sealed, solutions of methyl mercaptan i n petroleum ether are quite stable,  c.f. Ball^®-  mercaptans i n naphtha are not stable. Note 2. The petroleum ether solvent does not absorb hydrochloric acid from the acid dadmium chloride solution. Note 3. To obtain reproducible results, the conditions of the cadmium chloride treatment should be as standardized as possible, (see following discussion). Note 4. Since methyl mercaptan i s appreciably soluble i n water, i t i s unnecessary either to shake the o i l with silver^ nitrate for the usual one hour or, alternatively, to add large quantities of alcohol. Note 5. I f free sulfur i s present i n the solution the greenish-yellow precipitate gradually  60. turns to a reddish-brown, probably by the reaction  2 AgSR + S  * Ag S + RSSR.  The only effect i s to obscure the end point. Tests of the method were made with methyl mercaptan obtained from Eastman Kodak, 98% to 100% pure, which was in a globe in the vacuum apparatus.  stored  To test the sampling  and titration procedures, methyl mercaptan was admitted to the reaction vessel, sampled, and analysed i n the usual way except for the cadmium chloride treatment. of 146 mm. as 137 mm., air.  A f i r s t sample  of mercaptan in the reaction vessel was analysed but the sample was believed to contain traces of  A second sample of 31«5 mm.  was analysed as 32.5  which contained no a i r  mm.  Tests of the effect of the cadmium chloride treatment were not as successful.  When a 0.01  N. petroleum ether solu-  tion of methyl mercaptan was treated with an equal volume of acid 5% cadmium chloride solution about 20% of the mercaptan was removed.  Since the loss was probably due to  solution of the mercaptan in the aqueous layer i t was supposed, that i t could be minimized by use of a smaller volume of cadmium chloride solution.  Therefore a 0.003 N. solution  (the only mercaptan s t i l l available) was treated with onesixth i t s volume of 10% cadmium chloride.  The loss was  only reduced to 10%, and i t was concluded that the mercaptan concentration affects the loss.  61. "When a mercaptan solution was treated with cadmium chloride solution which contained precipitated cadmium sulfide the loss was much more serious.  It i s believed  that methyl mercaptan which had dissolved i n the aqueous layer was adsorbing on the cadmium sulfide precipitate. Three pieces of evidence supported this adsorption mechanism.  (1) Mercaptans are known to adsorb on metal sul-  fides. ^  (2) I f a solution of CdS and Gd(SR) were acidi2  fied and extracted with petroleum ether the recovery of mercaptan was very low.  (3) In the actual analyses i l -  lustrated i n Fig. 16. the error i n mercaptan depended on the amount of hydrogen sulfide present. It was not possible to estimate the loss of mercaptan in the cadmium chloride treatment, but errors of 30% i n the direct mercaptan analysis of the reaction mixture were not considered unreasonable.  Since the error occurred i n the  preliminary removal of hydrogen sulfide, i t i s clear that other direct methods would have given equally poor results. In spite of the error i n the method i t was considered valuable to use i t i n the actual analytical work.  Since  i t was a direct method i t served as an important check on the indirect method that was developed later.  By standar-  dizing the conditions of the cadmium chloride treatment i t was possible, at least, to obtain consistency i n the results. •r  The uniqueness of the d i f f i c u l t i e s encountered i n applying the Borgstrom-Reid method to methyl mercaptan i l l u s trates the non^generality of analytical methods for sulfur  Fie;. 16. Pressure— t ime curves, f o r mercaptan as determined* by the dJree*- --.nd i n d i r e c t methods. DeconDOsitior. of 80.0 mm of methyl d i s u l f i d e at 7,\6, °C.  62. compounds. The direct method for the determination of hydrogen sulfide. Since the direct methods for mercaptan analysis were of low accuracy, i t became essential to develop a method for the direct determination of hydrogen sulfide.  The met-  hod had to be sensitive to 0.0000025 moles of hydrogen sulfide (* mm. i n the reaction vessel) and had to be applicable in the presence of mercaptans.  Eventually an excellent  method was developed i n which the hydrogen sulfide i s dissolved i n dilute a l k a l i , precipitated and separated from the mercaptans as cadmium sulfide, and estimated by oxidation of the cadmium sulfide with iodine. Solution of the hydrogen sulfide i n organic solvents was attempted f i r s t because the use of organic solutions would have made a simple simultaneous determination of hydrogen sulfide plus mercaptan possible.  However, the treat-  ment of 0.00025 moles of hydrogen sulfide (about 50  mm.)  with 100 ml. of petroleum ether dissolved only 50% of the gas,  and i t was concluded that organic solvents would be un-  satisfactory. The solvent-that was used f i n a l l y was 0.5 N. (2%) aqueous sodium hydroxide.  This solvent was quite satisfactory pro-  vided that the hydrogen sulfide was dissolved i n the absence of a i r and that the solution was used immediately: i f these precautions were not observed there was loss of hydrogen sulfide by oxidation.  It i s well known that both hydrogen  63 s u l f i d e and mercaptans a r e o x i d i z e d  rapidly i n basic  solutions. The  cadmium s u l f i d e was p r e c i p i t a t e d by adding  5$ cadmium c h l o r i d e t o the b a s i c s o l u t i o n .  neutral  The cadmium  hydroxide and the cadmium mercaptides which were a l s o p r e c i p i t a t e d had t o be removed by a d j u s t i n g the  solution.  the a c i d i t y o f  T h e o r e t i c a l c a l c u l a t i o n s , which a ssumed no  l o s s o f hydrogen s u l f i d e , from s o l u t i o n , showed t h a t a s o l u t i o n o f pH 1 should d i s s o l v e the mercaptides without d i s s o l v i n g measurable q u a n t i t i e s  o f cadmium s u l f i d e .  However  the use o f pH 1 d i d cause s i g n i f i c a n t l o s s o f cadmium s u l f i d e , and i t was c l e a r t h a t the a c i d i t y would have t o be a d j u s t e d w i t h i n  narrow l i m i t s .  e t h y l mercaptide d i s s o l v e d  T e s t s showed t h a t  e x a c t l y a t t h e methyl orange end  p o i n t , whereas methyl mercaptide r e q u i r e d a c i d by about one pH u n i t .  cadmium  a s o l u t i o n more  An a c i d c o n c e n t r a t i o n o f 0.005 M.  was found t o d i s s o l v e both mercaptides completely w h i l e causing n e g l i g i b l e l o s s o f cadmium s u l f i d e . I t was d i s c o v e r e d by M a i n g o t ^ t h a t the extremely f i n e l y divided  cadmium s u l f i d e p r e c i p i t a t e i s r e t a i n e d  t i v e l y by an.asbestos mat.  quantita-  A c c o r d i n g l y the cadmium s u l f i d e  was separated from t h e s o l u t i o n by f i l t r a t i o n through a gooch c r u c i b l e equipped w i t h an.asbestos mat.  I t was neces-  s a r y t o wash t h e p r e c i p i t a t e v e r y thoroughly t o remove adsorbed mercaptan.  With methyl mercaptan water washing was  adequate, but w i t h h i g h e r mercaptans a l c o h o l washing would probably have been n e c e s s a r y .  64 The direct oxidation of suspended cadmium sulfide precipitates by iodine was investigated by Maingot^ with weighed samples of especially purified material.  The oxidations were  quantitative to the limit of the accuracy of the weighed samples, 0.0001 gm.  Accordingly the precipitated cadmium  sulfide was determined by a volumetric iodometric method. In spite of the low concentration reagents that had to be used this method was extremely precise. The complete procedure for the analysis of hydrogen sulfide i s now described.  This procedure i s for the determina-  tion of amounts of hydrogen sulfide from 0.0000025 to 0.00025 moles, i.e. \ to 50 mm. pressure i n the reaction vessel. To handle larger amounts i t would be necessary to use smaller aliquots. The procedure for the direct determination of hydrogen sulfide The sample was originally contained in the gas pipette.  50 ml. of 2$ NaOH was admitted to  the pipette without admitting any a i r , and the whole was shaken for 15 minutes.  The solution  was run into a 100 ml. volumetric flask which was made up to volume with the washings of the pipette. This solution was used immediately. To 25 ml. aliquots in 125 ml. glass stoppered erlenmeyer flasks were added an equal volume  65.  of 5% neutral cadmium chloride solution and a drop of methyl orange.  1 N. HCl was added to i  the end point, followed by a few drops i n excess When the stoppered flask was shaken the end point would return as the marcaptides dissolved. The procedure of adding excess was continued until the acid colour was just permanent.  The solution  was neutralized exactly and 6 drops (0.25 ml.) of acid was added i n excess.  The flask was stop-  pered and shaken until any remaining mercaptide dissolved.  (Cadmium mercaptides sediment rapidly  to the bottom of the flask whereas cadmium sulfide remains suspended in the solution.) The contents of the flask were filtered -  through a gooch crucible with a thick asbestos mat.  The high dispersion of the precipitate  made this operation particularly easy.  The pre-  cipitate was washed thoroughly with d i s t i l l e d water.  '  -  '  The precipitate and the asbestos mat were transferred to a 125 ml. glass stoppered flask. An excess of 0.01 N. iodine solution...was added followed by hydrochloric acid.  In a typical  case 20 ml. of iodine and 5 ml. of 6 N. acid were added to cadmium sulfide equivalent to 7 ml. of iodine in 20 ml. of wash water.  The flask was  66.  stoppered, shaken vigorously to disperse the precipitate, and allowed to stand for 20 minutes. The excess iodine was back titrated with standard 0.01 N. sodium thiosulfate solution using starchiodide as the indicator.  The moles  of hydrogen sulfide were calculated from the equation: H_S + I_ 2 2  »  S + 2 HI  The time that elapsed between the admittance of the sodium hydroxide to the pipette and the acidification of the solution was never more than one-half hour; no appreciable oxidation occurred i n this time. With low concentration samples two concurrent analyses could be completed in two hours, but with amounts of 0.0002 moles (40 mm.)  each  f i l t r a t i o n required about one and one-half hours under suction sufficient to boil water, and the total time for two analyses was often four hours. Duplicate analyses usually checked within 0.05 ml. of iodine, (g mm.). The accuracy of the method was checked with pure hydrogen sulfide and with mixtures of methyl mercaptan and hydrogen sulfide.  The preparation of the hydrogen sulfide i s  described later; i t s purity was checked by direct oxidation of the gag with standard iodine solution; for a sample of 49.25 mm. i n the reaction vessel the calculated pressure was 50.0 mm.  Samples of pure hydrogen sulfide were prepared  by admitting a measured quantity of the gas to the reaction  67 vessel, and sampling i n the usual way.  These samples  were analysed by the f u l l procedure except that the acidity was adjusted to the methyl orange end point rather than to 0.005 M.  The results of analyses on four standard  samples are shown in Table V.  The agreement between ob-  served and calculated pressures i s quite good. Table V. Analysis of pure hydrogen sulfide samples  Observed pressure  Note:  Calculated pressure  73.75  mm.  71.0  mm.  71.5  mm.  71.0  mm.  29.5  mm.  28.5  mm.  5.5  mm.  5.0  mm.  A l l pressures refer t o the reaction vessel at 316° G  The mixtures were prepared by combining standard solutions of hydrogen sulfide and mercaptans in the proportions and concentrations of the actual experimental work. They were analysed for hydrogen sulfide by the f u l l procedure. The results of these analyses are shown i n Table VI.  The  agreement between the calculated and measured pressures was excellent. It would have been desirable to test the method with mixtures containing sulfide, disulfide, etc., but i t i s not thought that any of these compounds could have interfered  68.  with the method. Even.if these compounds had dissolved in the alkali solvent, even i f they were present during the iodine oxidation stage due to adsorption on the cadmium sulfide precipitate, they would not have interfered appreciably; tests showed that neither methyl sulfide nor methyl disulfide reacted with iodine. Table VI Analysis for H S i n the presence of CH SH 9  CH^SH  Error  H S measured .  i.  .  .  observed -.  40.5 mm.  24*5  48.5 mm.  14.75 mm.  Note: a.  mm.  - 0.25 mm.  24.25 mm. 14.25"mm.  a  - 0.5  mm. .  A l l pressures measured i n reaction vessel at 316° C. Duplicated with acidities 0.0005 M., 0.001M., 0.05' M.  The precision of this analytical method w i l l become evident when i t s application to the reaction mixture i s described.  See Fig. 15.  The method for the determination of hydrogen sulfide plus mercaptan With the accuracy of the method developed for hydrogen sulfide, i t became feasible to determine mercaptans indirectly from the total hydrogen sulfide plus mercaptan i n the mixture. Accordingly i t became imperative to develop a method for the determination of total SH i n the mixture.  TIME IN MINUTES Figo 15. relation 80.0 mm.  The f o r m a t i o n o f h y d r o g e n s u l f i d e i n to p r e s s u r e I n c r e a s e . Decomods.ition o f o f m e t h y l d i s u l f i d e at 316 °C.  69. The use of hydrocarbon solvents was excluded by the insolubility of hydrogen sulfide. A method for the determination in basic solution would have been the most desirable since the solution used for the hydrogen sulfide analysis could have been used also for the total analysis.  Therefore  i t was attempted to adapt the silver nitrate method (c.f. direct determination of mercaptan) to analysis of a basic solution of the reaction mixture.  Essentially, the method  was to precipitate the sulfide and mercaptide in the basic solution^ add n i t r i c acid until the solution was just acid, and back titrate the excess silver nitrate with thiocyanate. The method worked well on pure hydrogen sulfide solutions giving results only a few percent lower than the cadmium sulfide method. With pure mercaptan solutions the results were usually only a l i t t l e high, possibly due to adsorption of silver nitrate on the mercaptide precipitate. However, the analysis of hydrogen sulfide - mercaptan mixtures by this method met with no success whatever.  The  conditions  of pH, the time of shaking with silver nitrate, and the dilution were varied, and lengthy shaking of the precipitates in slightly acid solution was tried, but the results were consistently low.  Moreover, the end points were ob-  scured by a copious precipitate, f i l t r a t i o n was  unsatis-  factory, and the end points were not permanent. Some of the results obtained by this method have been reported by Maingot.44  The analysis of the more complex reaction mixture  70. gave s t i l l worse results.  It i s recommended that the  method be completely abandoned in future work. Certain other methods were t r i e d for the determination of hydrogen sulfide plus mercaptan in basic solution. One of these consisted in adding standard iodine directly to the basic solution followed immediately by acid.  Un-  fortunately the hydrogen sulfide reacted almost immediately with k moles of iodine.  Since any error in the hydrogen  sulfide analysis would thus be magnified fourfold with respect to the mercaptan, the method was abandoned. The use of a l k a l i solutions was f i n a l l y abandoned. Instead the total hydrogen sulfide plus mercaptan was mined by direct oxidation of the gases with iodine. complete method i s now  deterThe  described.  Procedure for the determination of hydrogen sulfide plus mercaptan by direct oxidation with iodine. The gaseous samples existed in the sampling pipette.  A measured excess.of 0.01  M. iodine solu-  tion in potassium iodide was admitted to the sampling pipette with as l i t t l e a i r as possible.  The  whole was shaken for from 20 minutes to 30 minutes, depending on the excess. The solution was run into a suitable flask viand titrated to the starch-iodide end point with 0.01 N. sodium thiosulfate. The moles of mercaptan  71. were calculated from the equations: HS + I  2  = S + 2 HI  RSH + I  2  - RSSR + 2 HI  2  Any error in. hydrogen sulfide i s doubled i n mercaptan. The analysis of a 49.25 mm. sample of hydrogen sulfide by this method gave a value 50.0 mm.  The slight error may  be due to loss.of iodine to a i r while transferring the solutions.  Unfortunately i t was not possible to test the method  with mercaptans; the samples of ethyl and methyl mercaptan which remained at this stage of the investigation were impure and time did not permit their purification.  However  i t was known that iodine oxidation i s a common method of determining mercaptans i n the absence of unsaturated hydrocarbons, and i t was confirmed that a solution of iodine plus potassium iodide neither oxidized nor reduced methyl disulfide.  It was concluded therefore that the oxidation i s  stoichiometric. The effect of monosulfides on the oxidation was tested by allowing 10 mg. of methyl sulfide to stand with 20 ml. of 0.01 N. iodine i n a stoppered flask for ten minutes. The iodine, was analysed as 19.9 ml. and the loss was considered negligible.  It was necessary to assume that any double bonds  that were present i n the reaction mixture would be unreactive to iodine.  This assumption was reasonable since ethylene was  the most probable olefin.  72 The simplicity, of this direct method, the rapidity with which i t can be performed, and the precision of the iodine titration, are advantages of the method.  It i s  recommended for future work with the suggestion that i t be tested more rigorously. The determination of hydrogen sulfide i n the reaction. The amounts of hydrogen sulfide present at various stages of the reaction were determined by the cadmium sulfide method.  A series of runs were made i n which the  reaction mixture was sampled at various times.  Since each  analysis required a separate run i t was necessary to choose standard conditions for a l l the runs.  The temperature of  316° C. which was chosen was low enough to permit the sampling to be done i n a short time compared to times of appreciable reaction.  Therefore the main part of the  reaction could be studied i n detail.  A l l the analytical  runs were made with a minimum pressure as close as possible to SO mm., t h i highest pressure which could be reproduced approximately, and the results were calculated to represent a hypothetical run with a minimum pressure of exactly 80 mm. At the temperature and pressure of the standard reaction, 80 mm. and. 316° C , the i n i t i a l pressure decrease was about 15 mm., the induction period was about 11* minutes and the maximum rate.occurred at approximately 31 minutes. Unfortunately the pressure-time scurves at 316° G. were less reproducible than at 341° G.; probably this was due to  i  73.  increased adsorption of both reactant and product at the lower temperature. When sampling at this t emperature i t was observed that immediately after sampling the pressure i n the reaction vessel increased rapidly.  It was necessary to read the  pressure after sampling very quickly i f the readings were to be correct.  Pressure increases of 10 mm. i n excess of  those expected from the normal reaction were recorded i n 10 minutes. of products.  The increase could only be due to desorptlon Therefore the analytical results could not  be expected-; to a ecount quantitatively for a l l the products of the reaction. The treatment of the analytical results i s now i l l u s trated.  From the data from run number 6 (below) we cal-  culate the partial pressure of hydrogen sulfide i n the reaction vessel.  The data for this calculation are as  follows: Minimum pressure of the run, P min. Pressure at time of sampling, P^.  81.75  mm.  124*5  mm.  32.5  mm.  Pressure drop during sampling P^  90.0  mm.  Time of sampling  43 3 / 4  min.  Volume of the reaction vessel, Vj_  246  ml.  Volume of the sampling pipette, v~2.  490  ml.  5  ml.  Pressure after sampling,  P  g  Volume of the connecting tubing V 3 t  Temperature of the reaction vessel  589°  K.  74.  The sample was dissolved i n 100 ml. of base, and two 25 ml. aliquots were analysed: Iodine added  -  20.00 ml. of 0.0102° N.  Thiosulfate required: —  Aliquot 1. - 13.80 ml. Aliquot 2. » 13.85 ml.  Thiosulfate normally  =  0.0100^ N.  Correcting for the normality of the thiosulfate, the iodine used was 6.35  ml.  Therefore the moles of hydrogen sulfide  in the pipette were: Y moles  -  100 ml. x 6.35 ml. x 0.0102 2  x  1000  The amount of hydrogen sulfide which l e f t the reaction vessel during the sampling was the fraction P^/P^, of the amount originally i n the vessel.  A portion of this fraction was  l e f t in the connecting tubing.  Therefore the number of  moles of hydrogen sulfide i n the N moles . • Y moles x P /P » Y x 124.5 x 92.0 f  d  reaction vessel were: x (v^V^J/Vg  £21  490  The partial pressure of hydrogen sulfide i n the reaction vessel i s calculated from the gas law. P(ILS) 2  -  N RT/V  *  -  26.25 mm.  N x 0.0820 x 589 x 1000 x 760 mm. 246  Correcting this value by using the f i r s t order property of the extent of reaction, the pressure which would have been obtained i n the hypothetical run with minimum pressure 80.0 mm.  75.  is: PMHpS)  2 6 . 2 5 mm.  x  A  P (H S) f  2  «  25.75  30.0 81.75  mm.  mm.  mm.  It is to be noted that the assumption of ideal behavior i s reasonable at the elevated temperature of the reaction vessel.  No assumption of ideality at the temperature  of the pipette has to be made. The calculation requires that, the particular constituent under investigation should not condense in the connecting tubing, and that the relative proportions of the constituents, i n the reaction vessel be undisturbed during the sampling. The results of seven analytical runs for hydrogen sulfide are listed i n Table VII.  When the pressure-time curve  for each of these runs was calculated to an i n i t i a l pressure of 80 mm.  of disulfide, the resulting curves from a l l runs  were closely similar.  Therefore i t was assumed that the  respective results were reproducible.  Only the curve from  the 90 minute run differed appreciably from the others. The results of Table VII were used to construct a graph of the amount of hydrogen sulfide present In the mixture versus the time.  This graph i s shown i n Fig. 1 5 .  The pre-  cision of the analytical method i s at once evident from this graph, but i t i s possible that the accuracy i s only within 5%. The shape of the hydrogen sulfide curve has been compared to the shape of the pressure-time curve i n Fig. 1 5 .  76. The curve f o r t o t a l pressure has been reduced by a factor of one-half.  I t w i l l be seen that the increase i n hydrogen  s u l f i d e p a r a l l e l s the t o t a l pressure increase f a i r l y closel y i n the early stages of the reaction, with the hydrogen s u l f i d e accounting f o r 5Q% of the t o t a l pressure r i s e . exact correspondence  An  would not be expected i n view of the  various adsorption processes occurring. After 40 minutes the two curves lose t h e i r p a r a l l e l i s m , and at two hours the two rates are approximately equal (see F i g . 21.). Table VII Hydrogen s u l f i d e present at various stages of the reaction P . - 80 mm., t = 316° C.  Run number  Minimum pressure mm.  Sampling time min.  P(H S)  P (H S)  7  88.0  10  1.1  1.0  2  77.2  2l£  3.6  3.8  4  85.8  3©i  12.3  11.5  6  81.8  43 3A  26.2  25.8  5  80.2  61  31.9  31.8  3  81.5  91  38.0  37.2  1  79.5  120  42.2  42.5  2  f  2  In the course of the hydrogen s u l f i d e analyses i t was noticed that the sodium hydroxide solutions of the reaction  77 mixture were pale yellow i n colour. This colour was a t t r i buted to polysulfides in solution or to free sulfur which added on to the sodium sulfide present.  The colour was  very faint i n the analyses after 10 minutes and 20 minutes, was much s tronger i n the a nalysis after 3:0 minutes, and was s t i l l more intense i n the later analyses.  Therefore i t was  supposed that the formation of free sulfur or polysulfides roughly paralleled the formation of hydrogen sulfide. The- determination of mercaptan i n the reaction The standard conditions that were used for the hydrogen sulfide runs were used also for the mercaptan runs, v i z . 80.0 mm. and 316° G.  The analyses were made by the i n -  direct, iodine method, considered reliable, and by the direct silver nitrate method. In the determination of mercaptan by the silver nitrate method the silver mercaptide precipitate was the greenishyellow colour of the methyl mercaptide.  It was concluded  that no large amounts of the higher mercaptans were present. If the silver mercaptide system was allowed to stand for times of the order of one-half hour before t i t r a t i o n , the colour of the precipitate turned to a reddish shade. This was considered to be evidence of the presence of free sulfur or of labile polysulfide sulfur reacting i n a manner similar to the "doctor reaction": S  +  2 Ag(SR)  -» Ag S 2  + RSSR  78. In the determination of mercaptan by the indirect iodometrie method, the values for hydrogen sulfide were taken from the hydrogen sulfide formation curve of Fig. 15. Since the pressure-time curves for corresponding runs i n the hydrogen sulfide and. total sulfhydryl analyses were not.always equivalent, there was some question as to the exact values of hydrogen sulfide that should be used. results were calculated.  Finally, two sets of  First, on the assumption that the  amounts of hydrogen sulfide produced were independent of small variations i n the total pressure, a set of results was calculated by subtracting the experimental hydrogen sulfide P*(HgS) values from the corresponding total values. Second, on the assumption that, variations i n pressure were paralleled exactly by variations i n the hydrogen sulfide produced, the P'(H S) values were corrected accordingly before subtraction ifrom the total values.  The two sets are  called respectively, Set A, and Set B. The complete results of the mercaptan determinations are listed i n Table VIII. These results have been plotted as pressure versus time curves i n Fig.. 16.,  The values of  Set A gave a smoother eurve than those of Set B, and the former values were generally slightly lower than the latter. The smoother curve was given the greater weight.  The  curve obtained by the silver nitrate method follows the other two curves quite closely for about 25 minutes, after which i t diverges markedly.  Comparison of Figs. 1 6 . and 15.  79.  shows strikingly that the divergence of the two curves i s a function, of the hydrogen sulfide present.  The magnitude  of the divergence i s of the order of that which, was expected from the tests that had been made on the silver nitrate method.  Therefore i t was concluded with some confidence  that the results from the indirect method were close to the truth. Table VIII Mercaptan present at various stages of the reaction P  min  =  8 0  «°  '>  ma  Indirect iodine method  Time min.  t  Direct silver nitrate method  Mercaptan (mm.) Set A.  = 3 1 6 ° G.  Set B.  Time min.  Mercaptan mm.  9  1.0  22£  26.0  23.0  21  18.5  30  43.0  43.0  313A  37.5  46  53.2  54.0  46  46.2  6li  58.8  61.0  60i  46.0  121  57.5  57.0  120  42.5  122h  41.0  Set A of the mercaptan values determined by the iodine method together with the value for 9 minutes of reaction determined by the silver nitrate method have been replotted i n  80.  Fig. 1 7 . as the best values.  The resultant pressure-time  curve for the formation of mercaptan i s compared to the pressure-time curve for the total pressure .increase. The two curves are quite dissimilar.  The mercaptan curve shows  an induction period of about the same length as the total pressure curve.  After the induction period the rate of  formation of mercaptan rises sharply to a maximum before 2 0 minutes, whereas the rate of pressure increase rises slowly to a maximum after 3 0 minutes.  The formation of  mercaptan clearly takes place before the main pressure i n crease and before the corresponding formation of hydrogen sulfide.  The total pressure increase cannot correspond to  the decomposition of methyl disulfide. The formation of mercaptan takes place without appreciably increasing the pressure of the system. . At 1 5 minutes the mercaptan pressure i s 1 0 mm., whereas the total pressure increase i s only lit mm.  At 2 0 minutes the mer-  captan pressure i s 2 0 mm. whereas the total pressure increase is only 5 mm.s  The same conclusion may be reached directly  from the fact that the pressure change parallels the later formation of hydrogen sulfide. Apparently the mercaptan reached a maximum pressure between 5 5 and 8 5 minutes and thereafter decreased slightly in pressure.  It was thought quite possible that this de-  crease was not real.  In order to obtain further evidence  on this point a; mercaptan analysis was made after 2 4 * minutes  t  0  20  40  •  r  60  80  100  TIME IN MINUTES  Pig. 17o F o r m a t i o n o f mercaptan i n r e l a t i o n t o the p r e s s u r e i n c r e a s e . ) D e c o m p o s i t i o n o f 80.0 mm. o f m e t h y l d i s u l f i d e at 3 1 6 ° C .  .  120  81.  y , : ,  of a run at 373° C.  The result obtained by the silver  nitrate method was RSH - 27.5 mm.  for P . « 80.0 min.  mm.  If we assume that we can compare the results at the two temperatures using the activation energy of the maximum rate of pressure increase, we may calculate approximately that the same pressure of mercaptan would be present after 2400 minutes at 316° G.  The value of 27.5 mm.  at 2400  minutes may then be compared to the average value of 41.7 mm.  at 120 minutes determined by the same method.  In making  the comparison i t must be remembered f i r s t l y that the error in the silver nitrate method would be much larger after 2400 minutes than after 120 minutes, and secondly that the greater adsorption of disulfide at the lower  temperature  could eventually cause a correspondingly greater formation of mereaptan.  Therefore the difference of 14 mm.  between the  two values i s not necessarily outside the error of the comparison.  It seems i n any case that i f the mereaptan does  disappear after a maximum the rate of the disappearance i s extremely slow.  '  -  Since the rate of disappearance of mercaptan i s certainly very slow, i t follows that the formation of mercaptan must come to an end after approximately one hour of reaction at 316° G. The determination of free sulfur i n the reaction For the determination of free sulfur the method of B a l l ^  \  82. was used, with, less concentrated reagents but without modification.  This method, based on the Doctor reaction, depends  on the addition of a known quantity of butyl mercaptan to the " o i l " , oxidation of the butyl mercaptan by the free sulfur present, and determination of the mercaptan used up.  The  reactions are: (1)  Mercaptide formation: 2 BuSH +  (2)  »  Pb(SBu)  »  PbS  *  ? h S 0  +  2  2 NaOH  +  2  S  +  BuSSBu  Sulfide removal: PbS  +  H SG^  HS  +  GdGl  2  (4)  2  Doctor reaction: Pb(SBu)  (3)  NaPbG>  2  —*  2  GdS  ^  +  H  S 2  +  2 HG1  Mercaptan regeneration: Pb(SBu).  +  H S0, o  »  BuSH +  PbSO,  The mercaptan i s determined before and after the reaction by the three phase silver nitrate method, excellent for* butyl mercaptan.  A blank i s run on a portion of the same " o i l "  which has been treated with mercury to remove free sulfur. The blank corrects for the loss of butyl mercaptan to a i r , and for side reactions. AnalR petroleum ether was used as the solvent in the present work. A silver nitrate treatment was used for the preliminary removal of hydrogen sulfide and mercaptan. reagents were 0.01 N.  The  The method was found to give positive  results i f free sulfur were present, but i t s accuracy was not tested.  83. Reactions were run for 120 minutes under standard conditions and the products i n the gas pipette were analysed for free sulfur.  The results were: Free sulfur = 0.0 +. 0.5  mm.  This result was particularly interesting i n view of certain observations made during the hydrogen sulfide and the d i sulfide analyses which suggested the presence of free sulfur. (See "The determination of free sulfur i n the reaction"). The result does not mean that free sulfur i s not a product of the reaction, but rather that free sulfur i s not sampled by the method used.  When -£he products which had  accumulated from several runs i n the dry ice trap were analysed by the same method, they were found to contain free sulfur i n quantity. It was d i f f i c u l t to estimate the amount formed i n a single run. The method for the determination of disulfide In the presence of unsaturated hydrocarbons, disulfides must be determined by reduction to the corresponding mercaptans and estimation of the mercaptans by some standard method.  The extremely attractive bromine oxidation method^  cannot be used in the presence of double bonds. The reduction of disulfides to mercaptans has been done in v arious ways, but B a l l ^ i s of the opinion that refluxing with zinc  and glacial acetic acid gives the most repro-  ducible results.  Since the results of Ball were most erratic  never-the-less, i t was imperative that his procedure be  8J+. investigated and i f possible modified to give more consistent results. 'Samples of methyl disulfide were prepared for reduction by weighing the pure disulfide i n sealed capillary tubes and breaking the tubes under AnalR petroleum ether. The samples corresponded to 50 to 80 mm. pressure i n the reaction vessel at 316° C.  The samples were refluxed with  zinc and glacial acetic acid, and the mercaptan formed was determined by silver nitrate precipitation methods; these methods were known to be accurate i n the absence of hydrogen sulfide.  In a series, of t r i a l reductions the reflux  times were varied from 2 to 6 hours, and the amounts of zinc and acetic acid were also varied.  The results varied  from 30% to 90% reduetionaand they were not reproducible. When, unknowingly, acetic acid containing a l i t t l e water was used, reductions of 120% and 136% were obtained and there were clearly side reactions producing hydrogen sulfide.  The method was not reliable. Eventually i t was found that reproducible results  could be obtained by using a stronger acid and doing the reductions at:.lower temperature.  In the new method the  reduction was carried out on a water bath at 100° C. rather than at the reflux temperature of greater than 120° G., and the acetic acid used contained approximately 6% by weight of water.  A series of eight reductions run for times of 3  hours gave results consistently i n the range 111* +. 1*%.  85.  The reduction was complete i n less than three hours, and the results were unaffected by further heating. of methyl monosulfide had no effect.  The addition  This consistency was  considered adequate for the present work. The extent of the reduction was markedly dependent on the strength of the acid used.  At the lower temperature  glacial acetic acid gave a reduction of less than 1% i n three hours.  Therefore, for the application of the method,  a large fresh quantity of 9k% acid was prepared and this acid was used for a l l the analyses.  A t r i a l reduction using  the new acid indicated a correction factor of  100/115.  Using  this correction factor reductions were run at various concentrations of disulfide.  The results are shown i n Table IX  calculated as pressures i n the reaction vessel at 316°  C.  The reduction was unaffected by the concentration range. Table IX The reduction of methyl disulfide  Disulfide present  66.5 17.2  mm. mm.  Disulfide found (corrected)  66.2 17.9  mm. mm.  The complete procedure for analysis of the reaction mixture required the preliminary removal of hydrogen sulfide and  86. mercaptans by shaking the tpetr oleum ether solution with silver nitrate, and the removal of free sulfur by shaking with mercury.  The complete procedure as used in a typical  analysis was as follows. Procedure for the determination of disulfide The samples existed in the gas pipette. 50 ml. of AnalR petroleum ether, boiling range above 120° C., was admitted to the pipette with as l i t t l e a i r as possible.  (RSH + 0  * RSSR).  The pipette was shaken for 15 minutes to dissolve the disulfide.  The solution was run into a 100  ml. volumetric flaelk, which was made uptto volume with the washings of the pipette. About 60 ml. of the o i l solution was treated with 10 ml. of 0.1 N. silver nitrate i n a separating funnel to remove hydrogen sulfide and mercaptans, decanted off, retreated with a second 10 ml. portion, and washed with d i s t i l l e d water. The o i l was transferred to a 125 ml. glass stoppered flask and 1 to 2 ml. of mercury was added. The flask was stoppered and shaken vigorously for several hours.  The length of shaking  depended on the particular run but i t was generally about 8 to 12 hours.  The mercury and the mercuric  sulfide precipitate were filtered off. If necessary a second treatment was made.  87. 20 ml. aliquots of the solution were placed i n 250 ml. flasks fitted with ground glass joints, followed by 15 gm. of, 30 mesh zinc and 50 ml. of 94% by weight acetic acid.  The flask was immedi-  ately connected to the reflux system which consisted of an efficient condenser connected at the top through a stopcock to a trap.  The trap contained  25.00 ml. of 0.01 M. standard silver nitrate which collected the mercaptan formed.  When samples of  greater than 60 mm. were analysed i t was advisable to have a second trap attached.  The flask was  heated by a boiling water bath for 3 hours. At the end of 3 hours the contents of the trap were back titrated with standard 0.01 N. ammonium thiocyanate to the ferric alum end point.  Any mer-  captan remaining i n the flask was determined by diluting i t s contents with water, separating off the o i l layer, f i l t e r i n g i t , and determining the mercaptan by the Borgstrom-Reid method,  (see:—"the d i r -  ect method for mercaptan"). The moles of disulfide were calculated as onehalf the moles of mercaptan, RSH/2., and corrected appropriately. The time of heating was not c r i t i c a l .  The reaction was  probably complete i n about 2\\ hours and the later heating served only to complete the transfer of the volatile methyl mer-  as.  captan to the silver nitrate trap.  Since the estimation  of the mercaptan i n the trap was both accurate and simple, whereas.the estimation of the mercaptan i n the flask required a questionable partition separation, the time of heating was chosen to make the transfer complete.  With  methyl disulfide the amount of mercaptan l e f t i n the flask was usually too l i t t l e to determine, i.e. less than * but  with higher disulfides the transfer would not have  been complete. the  mm.,  The silver nitrate method i s excellent i n  absence of an o i l phase. The main disadvantage of the procedure was i t s length.  The complete procedure usually took about 20 hours. To test the method, methyl disulfide was admitted to the  reaction vessel at the low temperature of 193° C , sam-  pled, and analysed by the f u l l procedure. The silver nitrate used contained added silver sulfide. of 100/115 was used.  The correction factor  The result i s shown i n Table X.  The  excellent agreement was probably better than could be expected generally. Table X Analysis of pure methyl disulfide  Measured pressure  Calculated pressure  70.0 mm  70.2 mm  39. The determination of disulfide in the reaction The procedure just outlined was used to analyse the reaction mixture for disulfide at various stages of the reaction.  The standard conditions were as usual:  P •„ • min The resuls are listed i n Table XI.  80.0 mm.,  t - 316° C.  Table XI Disulfide present at various stages of the reaction P . = 80.0 mm., t • 316° C m i n  Time min.  Disulfide mm.  9  79.5  21  60.2  31 3/4  45.0  46  .  32.5  59  60  27.5 28.0  122 122i  29.0 22.0  Duplicate reductions on the same sample gave results which checked within +. .75 mm.  or less.  However, complete  duplicate runs did not give such reproducibility at 122 minutes. Disulfides other than methyl disulfide were not present in appreciable amounts. Under the conditions used, any ethyl or higher mercaptan formed (e.g CH^SCh^SH) would have  90 remained i n the f l a s k i n a p p r e c i a b l e  amounts, but i n a l l  cases the mercaptan was found e n t i r e l y i n the t r a p .  More-  over, the mercaptide p r e c i p i t a t e s appeared pure i n t h e . early  analyses. The  p r e l i m i n a r y treatment o f the s o l u t i o n s t o remove  f r e e s u l f u r produced c o n s i d e r a b l e  quantities of a black  p r e c i p i t a t e , presumed t o be mercuric s u l f i d e .  Since f r e e  s u l f u r was not present i n the gas p i p e t t e , t h i s apparent f r e e s u l f u r could o n l y have been due t o l a b i l e Unfortunately,  polysulfides.52  the q u a n t i t i e s o f m e r c u r i c s u l f i d e were not  estimated, but i t i s the authors o p i n i o n t h a t the q u a n t i t i e s corresponded t o perhaps 10 t o 20 mm.  o f monatomic s u l f u r i n  the l a t e r stages o f the r e a c t i o n , i . e . t o macro q u a n t i t i e s of p o l y s u l f i d e s .  The y e l l o w c o l o u r i n g o f b a s i c  solutions  o f the r e a c t i o n mixture, and t h e f o r m a t i o n o f s i l v e r  sul-  f i d e i n the mercaptan t i t r a t i o n s are f u r t h e r evidence o f polysulfides. In t h e f i r s t two d e t e r m i n a t i o n s t h e s i l v e r methyl mercaptide p r e c i p i t a t e s appeared as pure as those from the r e d u c t i o n o f pure d i s u l f i d e samples: p r e l i m i n a r y  treatment  w i t h mercury f o r only one hour was q u i t e s u f f i c i e n t t o remove the t r a c e s o f l a b i l e s u l f u r .  However, i n t h e l a t e r  deter-  minations the p r e c i p i t a t e s tended p r o g r e s s i v e l y t o a d i r t y brown c o l o u r .  The presence o f s i l v e r s u l f i d e i n d i c a t e d by  t h i s c o l o u r could only be a t t r i b u t e d t o the r e d u c t i o n o f p o l y s u l f i d e s from the o r i g i n a l m i x t u r e .  Since treatment o f  91. the mixture with mercury for 12 hours or 20 hours did not improve the precipitates i t was concluded that these polysulfides did not contain "labile" sulfur. It i s possible that two kinds of polysulfide w ere formed in the reaction, (1) branched chain polysulfides RS (:S) R, and (2) straight chain polysulfides R-S-S -R. X  X  x  The branched chain compounds would be the more l a b i l e ,  (see  Appendix I ) . Both types would Cause errors in the disulfide determination. The experimental results for disulfide have been plotted i n Fig. 18.  At 9 minutes of reaction the disulfide  accounts approximately for the entire total minimum pressure of 80 mm.  ( i t w i l l be remembered that traces of hydrogen  sulfide and mercaptan are present).  Further, the analytical  points indicate an i n i t i a l autoacceleration.  Since a l l the  evidence pointed to the reality of the induction period, the earlier part of the disulfide curve was drawn to follow the total pressure curve. The f i r s t point on the disulfide curve i s considered to be accurate to within 1 mm., a few percent. error.  and the second point to within  However the later points are definitely in  (1) The error caused by non labile polysulfides may  be illustrated by an example.  The reduction of methyl tetra-  sulfide would produce both the mercaptide and silver sulfide in the trap. C^S^C^  2 CH3SH  -  2  HS 2  1B A n a l y t i c a l p r e s s u r e - t i m e curve f o r disulfide. Decomposition of 80.0 mm. of methyl d i s u l f i d e at 3 1 6 ° C . PIgo  0  92. Therefore one molecule of tetrasulfide would appear as three molecules of apparent disulfide; 21 mm. of disulfide could be i n reality 7 mm. of tetrasulfide.  (2)  If the  polysulfide sulfur were labile and were removed by the mer,curie sulfide treatment, one molecule of polysulfide would appear as one molecule of disulfide.  Therefore the experi-  mental values for disulfide were really: (RSSR) plus (RS_R —  labile) plus (x-1)(RS R —  non labile)  The magnitude of the errors caused by the polysulfides could not be determined from the results available.  However,  the fact that, the polysulfides were present i n macro quant i t i e s made i t probable that in the later stages of the reaction the entire apparent pressure of disulfide was really due to lesser pressures of polysulfide.  This conclusion  becomes even more probable when the otherwise remarkable fact i s considered that the decomposition of disulfide comes almost to an end while the apparent pressure of disulfide i s s t i l l 25 mm.  (Fig. 18.).  Further evidence on this point i s  considered later. It was of interest to measure the apparent disulfide pressure after much longer times of reaction.  The products  of a reaction at 373° C. were analysed after 24* minutes and the results were calculated to 316° C. on the assumption that the change of temperature does not change the character of the reaction significantly.  The apparent disulfide pres-  sure 0 f 10 mm. at 2400 minutes may be compared to the average  93. value of 25 mm. at 120 minutes.  It i s possible either that  the polysulfides are decomposed slowly or that higher nonvolatile polysulfides are formed.  The author prefers the  former possibility.  1  Analysis for thiophenes The method described by A l l e n ^ was used for the e s t i mation of thiophenes.  '  The gaseous sample was dissolved i n 100 ml. of AnalR petroleum ether. 10 ml. of a fresh solution of isatin in concentrated sulfuric acid (0.01 gm./lOO ml.) was added to a 19 ml. aliquot and the mixture was allowed to stand for 1 hour.  When thiophenes were present,a blue Colour deve-  loped i n the a cid layer. A sample was analysed at the 120 minute stage of the standard reaction and no blue colour was observed.  Since  the test was not gery sensitive at the low concentrations used, i t was only possible to say that no more than 1 mm. of thiophene could have been present, i.e. less than 1/140 of the tcfcaL mixture. Analysis of the products i n the dry ice trap collected from several runs at 373° C. indicated the presence of thiophenes in trace quantities.  It was possible to estimate  the amount as less than 1/200 of the reaction mixture after 24-2 minutes. It was concluded that the thermal decomposition of methyl disulfide does not produce thiophenes i n significant quantity.  94.  Analysis for carbon disulfide. The colorimetric method of Callan, Russel-Henderson and S t r a f f o r d 4 9 was used for the estimation of carbon disulfide.  Piperidine was substituted for diethylamine. The  method was only approximate and one of the xanthate methods is recommended for future work. The gaseous sample of the reaction mixture was dissolved i n 100 ml. of AnalR petroleum ether, and 1 ml a l i quots were taken for analysis.  1 ml. of piperidine solution  was added (1 ml./lOO Ml. of petroleum ether), followed after 1 minute by 1 ml. of cupric acetate solution (0.03 gm: CuAc / 2  100 ml. of 100% ethyl alcohol), and the mixture was made up to 10 ml. with 100% ethyl alcohol.  If carbon disulfide was  present a yellow to brown colour developed i n the solution. By comparing the solution with standards i n even bore test tubes i t was possible to estimate the concentration of carbon disulfide approximately. Qualitative analyses showed the presence of carbon d i sulfide.  Quantitative analyses were.made on samples from  the 120 minute stage of the standard reaction.  The results  are shown in Table XII.- It i s evident that carbon disulfide is an important product of the decomposition. Analysis for hydrocarbons The analysis for hydrocarbons was complicated by the presence of large quantities of mixed sulfur compounds. Since these compounds adsorb markedly i t was not possible to use the mass spectrometer.  Since bromine reacts with  95. hydrogen sulfide (4 moles), free sulfur (3 moles), mereaptan (3 moles), disulfide (5 moles), monosulfide (2 moles), this reagent could.not be used to determine double bonds (1 mole). unusable.  Permanganate and hypochlorite were also  Infra red analysis might have been of some use  in spite of the small quantities of material available, but time did not permit us to investigate this possibility. Table XII Carbon disulfide present in the reaction min. '» 316° C. p  =  8 0  0  mm  t  =  Total pressure increase  Time  60  120 min.  Carbon disulfide  mm.  15 + 5 mm.  Essentially the analysis for hydrocarbons requires the preliminary removal of the sulfur compounds.  This separa-  tion could possibily be done chemically with reagents sueh as silver nitrate, mercury, mercuric nitrate or concentrated sulfuric acid, but there i s danger of losing the small quant i t i e s of hydrocarbons present. Probably the best method would have been preliminary gas fractionation followed by chemical treatment for hydrogen sulfide.  Unfortunately time  did not allow the necessary investigation. A crude estimation of the low boiling hydrocarbons present was made by freezing out the sulfur compounds. Standard reactions ( P  m i n  = 80.0 mm.,  t «= 3l6° C.) were run for 120  96. minutes.  They were sampled  by t h e s p e c i a l p r o c e d u r e o f  a l l o w i n g the pressure i n the r e a c t i o n v e s s e l t o decrease u n t i l i t equalled the pressure i n the p i p e t t e .  The p i p e t t e  was connected t o t h e a p p a r a t u s shown i n F i g . 19.  In t h i s  a p p a r a t u s t h e t r a p was o f n e g l i g i b l e volume compared t o t h e pipette. The t r a p was c o o l e d f i r s t i n l i q u i d n i t r o g e n , -193° At t h i s t e m p e r a t u r e , hydrogen and methane have  C.  significant  vapour p r e s s u r e s whereas a l l t h e o t h e r p o s s i b l e p r o d u c t s a r e s o l i d s w i t h n e g l i g i b l e vapour p r e s s u r e s .  The p r e s s u r e  o f t h e system dropped r a p i d l y from about 30 mm. mm.,  and t h e n t o 0.0  hours.  + 0.25  mm.  t o about 11  after a period of several  Assuming t h a t hydrogen and methane do n o t adsorb  on t h e o t h e r s o l i d p r o d u c t s , i t was p o s s i b l e t o c a l c u l a t e t h a t t h e s e Hydrocarbons compose l e s s t h a n 1% o f t h e r e a c t i o n p r o d u c t s a f t e r 120 minutes o f t h e s t a n d a r d r e a c t i o n .  The  method was n o t s e n s i t i v e t o t r a c e q u a n t i t i e s . W i t h t h e t r a p s e a l e d o f f from t h e p i p e t t e , t h e b a t h was changed t o s o l i d - l i q u i d normal pentane, -130°  G.  At  t h i s temperature t h e vapour p r e s s u r e s o f a c e t y l e n e , 'ethylene, and e t h a n e , a r e r e s p e c t i v e l y 8 mm.,  100 mm.,  and 40 mm.,  t h a t o f hydrogen s u l f i d e i s a p p r o x i m a t e l y 2 mm.47  and  A l l the  o t h e r s u l f u r compounds a r e s o l i d s w i t h n e g l i g i b l e vapour pressures.  The p r e s s u r e o f h y d r o c a r b o n s was c a l c u l a t e d from  the e q u i l i b r i u m gas p r e s s u r e c o r r e c t e d f o r hydrogen s u l f i d e . The r e s u l t i s p r e s e n t e d i n T a b l e X I I I .  The r e s u l t i s v e r y  FIG.  19,  FRACTIONATING APPARATUS FOR THE ANALYSIS OF HYDROCARBON  97.  approximate and can only be regarded as indicating that hydrocarbons are present in quantity. Table XIII Hydrocarbons present in the reaction P. = 80.0 mm., t = 316° C. m  Time mins. 120  Total pressure * mm. 140  H  2  —  + GH ™  mm.  4  "  'Other hydrocarbons mm.  0.0  25  Stoichiometric considerations make i t extremely unlikely that the saturated compound, ethane, i s present i n large amounts. view.  The absence of hydrogen and methane supports this  The formation of acetylene would presumably have re-  quired the dehydrogenation of ethylene. Such dehydrogenation is considered unlikely in view of Bryce's observation ^4 that the reactions of hydrocarbons with sulfur vapour at 320° C. produce no more than traces of acetylene.  Therefore the 25  mm. of hydrocarbons are considered to be mainly ethylene. An attempt was made to analyse for acetylene by the copper acetylide method described by Treadwell and Hall.50 i t was found that carbon disulfide interferes with the analysis by giving a dark red precipitate.  This precipitate was pre-  sumed to be a xanthate compound. Like the acetylide, the xanthate dissolves i n 6 N. HCl, but only slowly.  The copper  acetylide method cannot be used in the presence of carbon d i sulfide.  98.  The involatile products of the reaction One of the products of the reaction was a tar which collected in the connecting capillary tubing when the system was evacuated.  It appeared black to reflected light and a  reddish-brown to transmitted light.  The tar did not collect  in the reaction vessel i t s e l f which was no more than tinted a faint brown after 70 runs.  It collected in the capillary  in quantity after the f i r s t run feut did not f i l l the capillary in 70 runs.  It i s possible that the tar was degraded  in the hot reaction vessel and repolymerized  in the cooler  capillary; or i t i s possible that the tar had a small but significant vapour pressure at the temperature of the reaction vessel.  The nature of this tar was not determined,  but i t could probably be placed i n the vague category of polysulfide tars which accompany the thermal reactions of many sulfur compounds.  Further elucidation would be of con-  siderable interest. The contents of the dry ice product trap were an intense orange-tinted yellow.  This colour may have been due i n part  to the free sulfur which was identified there, to degradation products of the tar, to the polysulfides formed in the reaction or to other  products.  After a large number of runs the trap was found to contain traces of a black solid.  This material, which was i n -  soluble both i n organic and i n aqueous solvents, may have been free carbon.  It was not identified.  99. Clearly the reaction produces a complex of high boiling material.  It i s significant that the formation of the re-  duced products, hydrogen sulfide and mercaptan, requires the simultaneous formation of dehydrogenated or polymeric products of carbon and sulfur. Correlation of the analytical results for disulfide  f  hydro-  gen sulfide and mercaptan The experimental pressure-time curves for disulfide and mercaptan have been plotted together in Fig. 2 0 .  A curve  for the apparent amount of disulfide decomposed has been con structed by subtracting the amount of apparent disulfide present at time " t " from the i n i t i a l minimum pressure of 8 0 . 0 mm.  The significance of the dashed curves awaits dis-  cussion. It w i l l be observed that in the early stages of the reaction, where the two curves are accurate, the curve for the mercaptan formed coincides with the curve for the d i sulfide decomposed. (1)  Two important conclusions may be drawn.  The mercaptan i s a primary product of the decomposition  of methyl disulfide.  (2)  For each mole of disulfide one  mole of mercaptan i s formed. It has been pointed out already that the experimental curve for disulfide i s actually a graph of (RSSR) plus (RS R — X  labile) plus (x-lXRSyR —  non labile); after the  f i r s t few points the curve i s widely in error.  The magni-  tude of this error may now be considered. After about 6 0  100  TIME IN MINUTES P i g . 20 The b e h a v i o r o f d i s u l f i d e i n the r e a c t i o n D e c o m p o s i t i o n o f 80,0 mm o f m e t h y l d i s u l f i d e a t 316 °Go 0  100. minutes of reaction the experimental disulfide curve levels off; at the same time, the formation of mercaptan, a primary product, comes to an end. mental curve shows 25 mm.  Yet at the same time the experiof disulfide remainingl  It w i l l  be shown later that this phenomenon i s not due to inhibition by the reaction products. absence of disulfide!  Therefore i t must be due to the  The true disulfide pressure must be  close to zero at 25 minutes, and the 25 mm.  of apparent d i -  sulfide must in reality be a lower pressure of polysulfide. Stoichiometric considerations w i l l be shown to support this view.  Accordingly the dashed curve i n Fig. 20. has been  drawn as a probably truer representation of the disulfide present.  This curve i s drawn primarily for i l l u s t r a t i o n .  The curve for the amount of disulfide decomposed, calculated as (P_,.„ minus P^), also suffers from the error min. • t ' in the disulfide analyses. allow for the 15 mm. sorbed.  Moreover, the P i m  n  does not  of disulfide which i s i n i t i a l l y ad-  Desorption of this disulfide as the reactant i s  used up causes additional error in the curve.  Decomposi-  tion of adsorbed material would have exactly the same effect^ but unless such decomposition i s catalysed by the reaction products, i t cannot occur (c.f. presence of the induction period).  Since the rate of desorption must be of the same  order as therapid rate of adsorption, i t i s possible to suggest a correction to the curve,  ( l ) - Since the amount  of desorption up to 20 minutes cannot be large, the curve must be reasonably accurate up to this point.  (2)  Since  101.  the d i s u l f i d e i s almost decomposed a t 6 0 minutes the c o r r e c t i o n a t t h i s p o i n t must be c l o s e t o the f u l l 15 mm.  At 1 2 0  minutes the t r u e q u a n t i t y o f d i s u l f i d e decomposed i s probably  (15 p l u s 80 = 9 5 ram.) r a t h e r than (80 minus 2 5 = 5 5  Therefore the a r b i t r a r y dashed  mm.)  curve i n F i g . 2 0 . i s c o n s i d e r e d  to r e p r e s e n t the d i s u l f i d e decomposed more c l o s e l y than the d i r e c t l y calculated  curve.  The a n a l y t i c a l r e s u l t s f o r mercaptan, hydrogen and d i s u l f i d e are c o r r e l a t e d i n F i g . 2 1 .  sulfide,  Pressure-time  curves are p l o t t e d f o r t h e t o t a l p r e s s u r e i n c r e a s e , and f o r the hydrogen  s u l f i d e and mercaptan p r e s e n t .  The estimated  curves f o r d i s u l f i d e p r e s e n t and d i s u l f i d e decomposed a r e also plotted.  S e v e r a l important c o n c l u s i o n s may be drawn.  The i n d u c t i o n p e r i o d i s r e a l .  Only t r a c e s o f products  were observed b e f o r e 9 minutes, and the d i s u l f i d e was not observed t o decompose.  Probably because the products adsorb,  the t r u e i n d u c t i o n appears t o come t o an., end s l i g h t l y b e f o r e the t o t a l pressure b e g i n s t o i n c r e a s e . Methyl mercaptan i s formed as a primary product o f t h e decomposition.  One mole o f mercaptan i s formed  f o r each  mole o f d i s u l f i d e decomposed, and t h e r e i s no p r e s s u r e i n crease.  S i n c e the decomposition must produce a second  "frag-  ment" o f s t o i c h i o m e t r i c composition CSH^ the q u e s t i o n a r i s e s o f why t h e r e i s no p r e s s u r e i n c r e a s e .  The primary r e a c t i o n  comes t o an end a f t e r about 6 0 minutes. Since methyl mercaptan and d i s u l f i d e account f o r the  20  0  40  60  TIME IN  80  100  MINUTES  21. C o r r e l a t i o n of the a n a l y t i c a l results,, o f " 8 0 . 0 mm. of methyl d i s u l f i d e a t 316 °C.  Figo  120 «  Decomposition  102. entire total constant pressure in the i n i t i a l stages, of the reaction, methyl monosulfide, which was not estimated, can not be a primary product of the reaction. The total pressure increase parallels the formation of hydrogen sulfide up to about 6 0 minutes.  Therefore, i n  the period of 10 to 6 0 minutes the other volatile products, hydrocarbons, carbon disulfide, and polysulfides, must be formed at the same time as hydrogen sulfide. products are formed later than the mercaptan.  A l l these The magni-  tude of the pressure increase i s about 1.5 times the pressure of hydrogen sulfide. After 6 0 minutes the rate of the pressure increase f a l l s relative to the rate of hydrogen sulfide production until at 120 minutes the two rates are the same. It may be significant that i t i s at about 6 0 minutes that the pressure of disulfide has been estimated to f a l l to zero.  After  120 minutes the pressure continues to rise almost indefinitely; i t i s clear that secondary reactions are extensive. If the estimated curve for decomposed disulfide i s accepted, i t becomes clear that some disulfide must be decomposed as hydrogen sulfide and allied products are used up. The stoichiometry of the reaction A l l the numerical results quoted in the present section refer to the standard reaction at 316° C. with P i m  mm.  n  = 80.0  103. The constituent of the reaction mixture may be summarized.  The methyl disulfide i s the major constituent  of the system in the early stages of the reaction, but i t has probably dropped to a few millimeters at 6 0 minutes. The - products formed i n largest quantity are hydrogen sul-. fide and mercaptan (chiefly or entirely methyl mercaptan); both are reduced relative to the disulfide.  The formation  of mercaptan stops after about 6 0 minutes, but the formation of hydrogen sulfide continues throughout the entire reaction; eventually the hydrogen sulfide pressure may exceed that of the mercaptan.  The major volatile dehydro-  genated products, are carbon disulfide and hydrocarbons which are presumed to be chiefly ethylene.  The other major vola-  t i l e products are polysulfides and free sulfur.  A tar i s  an important non volatile product. Hydrogen and methane.are not present i n measurable amounts. Thiophene occurs only in traces.  Higher disulfides  are probably absent. The pressures of the various constituents at 120 minutes of reaction have been listed i n Table XIV".  The pressures of  hydrogen sulfide, mercaptan, carbon disulfide and ethylene have been listed as directly analysed.  Disulfide has been  estimated as zero, and polysulfides as 8 mm.  Since free  sulfur exists as Sg at 316° C. i t s pressure i s very small. The sum of the pressures of the individual constituents is 148 mm. of 140 mm.  It may be compared to the observed total pressure Considering the inaccuracies involved, the agree-  104. ment i s quite good; i t may be concluded that the analytical results have some validity.  The agreement supports the  view that the true disulfide pressure i s zero; i f the 25 mm. of apparent disulfide were not identified with polysulfides, the calculated pressure of 165 mm. would be far too high. Table XIV Constituent pressures at 120 minutes P .  = 80.0 mm., t = 316° G. .  Comments  . i Estimated pressure mm.  •  V  Compound  Measured pressure mm.  GH SSCH  25  HS 2  42  42  RSH  58  58  3  3  •> •  S  cs  2  RS R X  CH CH 2  2  Involatile products  Due to polysulfides  Low pressure as Sg  0  0  • 15  Approximate  15  •  Assume RS.R 4 Approximate  8  25  25 0  0 Total Measured total pressure Estimated disulfide decomposed  148 140 95  The agreement i n the early part of the reaction between the sum of the mercaptan and disulfide pressure, and the  105. t o t a l p r e s s u r e , and a t 120  minutes between the c a l c u l a t e d  and measured t o t a l p r e s s u r e s l e a d s t o the c o n c l u s i o n t h a t the r e a c t i o n produces  no other products i n q u a n t i t y .  The t o t a l carbon found i n the v o l a t i l e products c o r responds t o 70  mm.  o f methyl d i s u l f i d e ; the t o t a l s u l f u r c o r -  t o 80  mm.  and the t o t a l hydrogen to 78  responds  mm.  The  amount of d i s u l f i d e decomposed i s c l e a r l y g r e a t e r than the d i r e c t l y c a l c u l a t e d 55  mm.  Indeed, when i t i s remembered  t h a t i n a d d i t i o n t o the v o l a t i l e products t h e r e are  adsorbed  products and i n v o l a t i l e products, i t i s e v i d e n t t h a t the amount of decomposed d i s u l f i d e must be c l o s e t o the i n d e pendently estimated 95  mm.  The v o l a t i l e products do not account f o r amounts o f c a r bon, hydrogen, and s u l f u r , which correspond r e s p e c t i v e l y t o 25  mm.,  17  mm.,  and 15  respond t o 10 mm.  mm.,  of d i s u l f i d e .  These amounts c o r -  o f a h y p o t h e t i c a l compound 5^o^3* C  Such  a compound, chosen s i m p l y t o i l l u s t r a t e the r a t i o s of the atoms, has the r e q u i r e d low hydrogen but h i g h s u l f u r content of polysulfide t a r .  A l t e r n a t i v e l y , i t i m p l i e s polymeric p r o -  ducts because the r a t i o o f carbon t o hydrogen i s 1:2.  Thus  the presence o f the t ar i s i n a c c o r d w i t h the s t o i c h i o m e t r y . The e f f e c t o f mercaptan on the r a t e o f r e a c t i o n . Since methyl mercaptan i s one o f the major products o f the r e a c t i o n , i t was  of i n t e r e s t t o d i s c o v e r whether or not  i t s presence a f f e c t s the normal r e a c t i o n . The methyl mercaptan, b.p.  5°  C., was  an Eastman Kodak  106.  product of at least 98% purity; i t was frozen in liquid air, pumped off to remove any traces of a i r and stored i n a globe. 18.5 mm.  of the mercaptan was admitted to the reaction ves-  sel at 3 4 1 ° C. and allowed to remain there for 55 minutes. At the end of this time no pressure increase had been recorded. After evacuation, a fresh 18.5 mm. of methyl mercaptan was admitted to the reaction vessel followed by 4 7 . 5 mm. of methyl disulfide, and the reaction was followed i n the usual way.  The pressure-time curve for this experiment i s plotted  in Fig. 22.  A pressure-time curve for a normal reaction at  5 7 . 5 mm. has been recalculated to 4 7 . 5 mm.  and plotted for  comparison. It i s apparent that the addition of methyl mercaptan had very l i t t l e effect on the decomposition.  Since i t did  not affect the rate of pressure increase (hydrogen sulfide) i t could not have affected the earlier stage of mercaptan formation.  It cannot take part in any intermediate stages  of the reaction. The effect of hydrogen sulfide on the rate of reaction Pure hydrogen sulfide was prepared by an adaptation.iof the method of Biekford and Wilkinson*?  The reaction i s :  1  CaS + MgN0 + 2 H 0 3  2  — »  Ca(N0 ) + Mg(0H) + HgS 3  2  2  The hydrogen sulfide was passed through aqueous barium hydroxide, dried with calcium chloride and phosphorus pentoxide, and frozen i n a liquid nitrogen trap attached to a storage  TIME IN MINUTES  P i g . 2 2 . The e f f e c t of added rr-ercaptan on the reaction© Decompositions of methyl, d i s u l f i d e at 341 °C.  O  RSSR = 47,25 nun.  •  RSSR = 47.25 mm.,  RSH ~ 18-.5 mm«  107. globe on the apparatus.  The trap was sealed o f f and eva-  cuated, to remove traces of a i r .  After warming to -72° C.  the hydrogen s u l f i d e was admitted to the globe. A preliminary run at 341° C. showed that hydrogen s u l fide has a very marked effect on the rate of the reaction. To study the e f f e c t more closely a series of runs were made.at the lower temperature of 316° C. with constant amounts of d i s u l f i d e and varying amounts of hydrogen s u l fide.  Four experiments were made with a d i s u l f i d e pressure  of 52.25 mm. and hydrogen s u l f i d e pressures of 0.0, 10.25, 34.0, and 52.0 mm.  The pressure-time curves f o r these ex-  periments are reproduced i n F i g . 23. It i s evident that hydrogen s u l f i d e has a large effect on the rate of the reaction.  The induction period i s A o r -  tened, the time required to reach the maximum rate i s shortened, and the maximum rate i s increased.  The values of  the maximum r a t e , the induction period, and the time of the maximum rate are l i s t e d i n Table XV. It i s remarkable that i n a l l four reactions the maximum rate of pressure r i s e occurs a f t e r the same t o t a l pressure increase of 13.7 mm.  Such a pressure increase corresponds  approximately to 6.5 mm. of hydrogen s u l f i d e formed. It was found that the time required to reach the maximum, rate, i . e . the time required to reach a given extent of reaction, depends i n a simple way on the hydrogen s u l f i d e present at the time of the maximum r a t e .  A r e l a t i o n was  0  10  . 2 0  . 3 0  40  • 50 "  Fie-* 2 3 E f f e c t o f h v d r o g e n s u l f i d e on t h e t h e r m a l d e c o m p o s i t i o n o f m e t h y l RSSP = 52.25 ran, H 2 s"= r e s p e c t i v e l y : (1} 0.0 (2) 10.25 mm. (3) 34.0 mm. 0  disulfide.. (4) 52.0 xmru  60  108. assumed of the form: (l/time)  = ^ 2 S a x . rate* k  m a x <  r a t e  H  S)  The hydrogen sulfide present at the time of the maximum, rate was calculated as the amount added plus 6.5 mm. The plot of log.(l/time) versus log.fH^S) i s presented in Fig. 24.  The graph i s a very good straight line with a slope  of 0.56.  The linearity of the graph supports the view that  the added hydrogen sulfide i s not used up to any large extent.  However, i t i s d i f f i c u l t to explain the simplicity  of the relation: l/  t  = k(H S)i 2  The agreement of this relation with the experimental results may very well be fortuitous. Table XV Effect of hydrogen sulfide.on the reaction of 52.25 mm. of disulfide  Added H S 2  mm. 0.0  Induction period min. 13  Time of max. Maximum rate rate mm./min. min. 34.0  0.123  10.2  44  20.5  0.147  34.0  24  12.0  0.198  52.0  1*  9.7  0.213  The dependence of the maximum rate of pressure rise on the hydrogen sulfide concentration was investigated by assu-  log.  (K2S.added  +  6,5  mm.)  F i g * 24* The e f f e c t o f added h y d r o g e n s u l f i d e on t h e time a t w h i c h t h e maximum r a t e i s attained» Decompositions of methyl d i s u l f i d e at 3 1 6 ° C . Units  are m i l l i m e t e r s  and  minutes  t  0  109. ming a relation of the form: dP/dt  = k(H S)J . 2  The plot of log.(dP/dt) versus log.(H S) 2  25.  t  is shown i n Fig.  The scatter of the points about the straight line i s  within the experimental error of the measurement of the tangent rates.  The slope of the line i s 0.25 and therefore,  the order "n i s approximately one-quarter ( i ) . M  It w i l l be realized that these results refer only to those parts of the decomposition which produce a pressure increase, that i s , to the reactions producing hydrogen sulfide.  To interpret these results one must know how the  addition of hydrogen sulfide affects the reaction producing mercaptan.  For example, the simple dependence of the rate  of pressure change on the hydrogen sulfide present suggests that the reactions producing this pressure change are catalyzed directly by hydrogen sulfide, but this simple dependence may not be real.  The increase i n the maximum rate  may be due, at least i n part, to the accelerating effect of hydrogen sulfide on the rate of formation of the primary products.  Similarly, the shortening i n the induction period  of the pressure increase may be due to a shortening i n the induction period of the primary reaction. The clue to the interpretation l i e s i n the result that the pressure-time curves, with added hydrogen sulfide or without i t , a l l level off sharply after the same t o t a l pressure increase.  It is shown i n the discussion that this result  o.e  1.0  1.2 LOG.  (E S 2  1.4 ADDED  +  6,5  1.6 mm)  F i g , 25 The e f f e c t of added hydrogen s u l f i d e on the maximum r a t e of pressure increase* Decompositions of me'thvl d i s u l f i d e at 316 °C. 0  U n i t s are m i l l i m e t e r s and minutes  1.8  110.  suggests t h a t the f o r m a t i o n o f hydrogen s u l f i d e i s a t r u e secondary  r e a c t i o n depending on t h e p r i o r occurrence o f t h e  primary r e a c t i o n producing mercaptan.  I t follows that  h y d r o g e n . s u l f i d e shortens t h e i n d u c t i o n p e r i o d o f t h e p r i mary r e a c t i o n and a c c e l e r a t e s i t . A d i r e c t a n a l y t i c a l study o f t h e e f f e c t o f hydrogen s u l f i d e on t h e r e a c t i o n would be h e l p f u l . The e f f e c t o f hydrogen s u l f i d e on t h e f i n a l  pressure  S i n c e hydrogen s u l f i d e has such a marked e f f e c t on the r a t e o f t h e decomposition,  i t was o f i n t e r e s t  whether o r not i t a l s o a f f e c t s  to discover  the f i n a l pressure a t t a i n e d .  Two runs were made a t 3 7 3 ° C. by t h e t e c h n i q u e  des-  cribed previously f o r observing f a s t r e a c t i o n s , with approximately 6 0 mm. o f d i s u l f i d e p r e s e n t . of a d s o r p t i o n were minimized  The p o s s i b l e e f f e c t s  under these c o n d i t i o n s .  40.25  mm. o f hydrogen s u l f i d e was present i n one o f these r u n s . The pressure-time curves f o r t h e two runs are compared i n Fig. 2 6 .  I t i s e v i d e n t t h a t the a d d i t i o n o f hydrogen s u l f i d e  does not change t h e f i n a l , p r e s s u r e reached i n the r e a c t i o n . T h i s r e s u l t l e a d s t o two suggestions..  (1)  The added  hydrogen s u l f i d e i s not a p p r e c i a b l y used up i n the r e a c t i o n . (2)  Hydrogen s u l f i d e i s not i n v o l v e d i n any e q u i l i b r i a  which i n v o l v e p r e s s u r e changes. The e f f e c t o f t h e r e a c t i o n mixture on t h e r a t e o f the reaction I t was important t o know t h e e f f e c t o f the complete  0  5  15 .  10  20  TIME IN MINUTES Flg 26 The e f f e c t o f added h y d r o g e n s u l f i d e on t h e l a t e 3 t a g e s o f t h e r e a c t i o n . D e c o m p o s i t i o n s o f m e t h y l d i s u l f i d e at. 373 ° C . 0  0  •  RSSR = 59.5  °  RSSR = 59.5 mm,,  mm. H S = 40.25 ram. 2  111. reaction mixture on the rate of decomposition of methyl disulfide. A normal decomposition of 56.75 mm.  of methyl disul-  fide (minimum pressure) was allowed to proceed for 52 minutes until the period, of rapid reaction was over.  At  this time a fresh quantity of disulfide of 50.0 mm. minimum pressure was injected into the reaction vessel.  The  resulting reaction was followed by pressure measurement. The total pressure-time curve for the complete reaction has been plotted in Fig. 27. as curve 1.  The curve  that would have been obtained i f the f i r s t decomposition had been allowed to proceed normally has been plotted as curve IA.  The net pressure-time curve for the second disul-  fide sample was obtained by subtracting curve IA. from curve 1; i t i s plotted as curve 2. minimum pressure of 56.75 mm.,  Curve 2, recalculated to a has been superimposed as curve  2A on the pressure-time curve of the f i r s t sample in such a way that the two curves coincide at the maximum rate. The presence of the reaction mixture reduced the apparent induction period, as measured by the pressure change, to only 2 minutes.  Considering that the true induction  period comes to an end slightly before the f i r s t detectable pressure increase, i t i s quite possible that the true i n duction period i s removed entirely.  This effect i s i n con-  trast to the effect of hydrogen sulfide alone. calculated that the 21 mm.  It may be  of hydrogen sulfide formed at  52 minutes from the f i r s t sample would have reduced the i n -  0  10  20  -  30 Figo  40 27  c  50 60 70 . 80 90 100 E f f e c t of the r e a c t i o n p r o d u c t s , (316 °C.)  112.  d u c t i o n p e r i o d o f the second o n l y t o 34 The  presence o f the r e a c t i o n mixture reduced t h e p e r i o d  of autoacceleration mum  minutes.  s l i g h t l y but i t d i d not a f f e c t the maxi-  r a t e o f pressure  increase.  The s p e c i f i c r a t e  constants  c a l c u l a t e d from the pressure-time curves f o r the two d i s u l f i d e samples were i d e n t i c a l w i t h i n the experimental e r r o r , 0.00040  s e c " and 0 . 0 0 0 4 1 s e c " . 1  1  This f a c t i s i l l u s t r a t e d  c l e a r l y by the superimposed curves 1 . and 2A.  The b e h a v i o r  i s i n c o n t r a s t t o t h e b e h a v i o r caused by hydrogen s u l f i d e alone. I t i s not wise t o draw many c o n c l u s i o n s o f an i n j e c t i o n r u n . one  from t h e r e s u l t s  In a system as complex as t h e present  t h e r e may be v a r i o u s  s i d e r e a c t i o n s which would not o c -  cur i n a normal decomposition.  However the r e s u l t s do sug-  gest t h a t the e f f e c t o f t h e complete r e a c t i o n mixture i s simply  t o remove the t r u e i n d u c t i o n p e r i o d and i n i t i a t e t h e  formation  o f mercaptan a t an a c c e l e r a t e d  rate.  I t w i l l be noted t h a t the decomposition r a t e o f the second sample decreased from t h e maximum more r a p i d l y than the r a t e o f t h e normal r e a c t i o n .  T h i s e f f e c t c o u l d be due  to an, a l t e r a t i o n o f the mercaptan hydrogen s u l f i d e  ratio,  but i t might a l s o be due t o some s i d e r e a c t i o n o f p a r t o f the added d i s u l f i d e w i t h one o f the products;  free sulfur  would be an example. I t w i l l be r e c a l l e d t h a t the formation  o f mercaptan i n  the normal r e a c t i o n comes t o an end a t about 6 0 minutes. Since  the r e a c t i o n products do not i n h i b i t the decomposition  113. of disulfide i t may be concluded that formation of mercaptan comes to an end. because, the disulfide has fallen to a low pressure. The effect of n i t r i c oxide on- the rate of reaction .The effect of n i t r i c oxide on the rate of the reaction was investigated to test for the presence of free radical chains. Unfortunately time did not permit the preparation of pure n i t r i c oxide.  The material used had been prepared for  an undergraduate laboratory; i t was not of high purity. The experiments were done at 316° G. with 80. mm. of disulfide in the packed, reaction vessel.  Consecutive runs  were made with 1.75 mm.,  and 5.5 mm. of  added n i t r i c oxide.  0.5 mm.,  0.0 mm.,  The resulting pressure-time curves are  plotted i n Fig. 28. Nitric oxide has very l i t t l e effect oh the rate of the reaction.  The most noticeable effect" i s a shortening of the  induction "period, i n two of the runs; however after the i n i t i a l increase i n pressure the rate f a l l s below normal and the curves approach the normal curve.  This behavior suggests  that n i t r i c oxide i s able to start and stop reaction chains, but i t does not indicate whether such chains occur i n the normal decomposition. The pressure-time curves with nitric oxide were not easily reproducible.  114. The thermal decomposition of ethyl d i s u l f i d e It was of interest to compare the thermal decompos i t i o n of methyl d i s u l f i d e with the thermal decomposition of eShyl d i s u l f i d e .  Accordingly a few experiments were made  with ethyl d i s u l f i d e . The thermal decomposition of ethyl d i s u l f i d e was studied at 318^° G.  The decompositions were followed only by pres-  sure measurement.  The highest pressure of ethyl d i s u l f i d e  that was obtained i n the reaction v e s s e l , 40 mm., may be compared to the maximum 13 mm. obtained by Patrick."^" The pressure-time curve f o r a t y p i c a l reaction of ethyl d i s u l f i d e i s shown i n F i g . 29.  The r e p r o d u c i b i l i t y  of t h i s  curve i s i l l u s t r a t e d i n F i g . 30. by the results of three experiments.  The curve i s similar i n shape to the pressure-  time curves of the thermal decomposition of methyl d i s u l f i d e . There i s an induction period of 2 minutes, followed by a short period of autoacceleration.  The maximum rate occurs  at 5 to 6 minutes. The t o t a l pressure increase i s much larger i n the case of ethyl d i s u l f i d e than i n the case of methyl d i s u l f i d e . The reaction reaches 100% pressure increase between 20 and 25 minutes, whereas under comparable conditions of maximum rate, the decomposition of methyl d i s u l f i d e reaches 100% pressure increase only after 300 minutes. It w i l l be noticed that after the reaction rate has f a l len to a low value, i t r i s e s again f o r a short time.  This  10  20  30  TIME IN MINUTES 29o Decomposition of E t h y l D i s u l f i d e T y p i c a l pressure-time c u r v e . Initial pressure = 2 8 , 0 mm., temperature = 3 1 8 ^ ° C  Flgo  115. effect, which was also noticed by Patrick^-, appears to be reproducible. Fig. 31. presents three pressure-time curves with different i n i t i a l pressures of ethyl disulfide.  The per-  centage pressure increases for these three curves are listed in Table XVI for various times.  The percentage pressure i n -  crease at any time i s independent of the i n i t i a l pressure. Therefore the reaction i s f i r s t order with respect to the i n i t i a l pressure. Table XVI Extents of reaction for various i n i t i a l pressures of ethyl disulfide Extent of reaction  Time minutes  P  0  - 18.25 mm.  P  • 28.0  0  (%)  mm. P  Q  -  40.0  5.0  16  17  15  7.5  38  39  38  10.0  55  56  60  12.5  72  74  73  15.0  85  85  86  20.0  102  96  96  25.0  115  104  108  mm.  Rate constants were calculated from the f i r s t order equation (dP/dt)max.  =  kP  Q  116.vThe r a t e s were determined by drawing tangents t o t h e p r e s sure-time c u r v e s . XVII.  The r a t e constants a r e l i s t e d i n Table  They a r e f a i r l y constant f o r i n i t i a l p r e s s u r e s o f  28. mm. o r h i g h e r  t  but they f a l l o f f a t 18 mm.  The mean  o f t h e h i g h p r e s s u r e v a l u e s i s o.00197 sec.""l and the s t a n dard d e v i a t i o n i s 0.00011 sec.~^-. The decomposition o f e t h y l d i s u l f i d e produces  copious  q u a n t i t i e s o f , f r e e s u l f u r which c o l l e c t s on t h e w a l l s o f t h e apparatus when t h e r e a c t i o n v e s s e l i s evacuated.  In t h i s  r e s p e c t t h e decomposition d i f f e r s from t h a t o f methyl  disul-  fide. The decomposition o f e t h y l d i s u l f i d e produces an i n volatile tarsimilar methyl d i s u l f i d e . of a i r .  i n appearance  t o t h e t a r produced by  T h i s . t a r chars when heated i n t h e absence  T h e r e f o r e i t c o n t a i n s carbon.  To summarize, the thermal decomposition o f e t h y l d i s u l f i d e d i f f e r s from the thermal decomposition o f methyl f i d e i n t h r e e important r e s p e c t s . at a lower temperature  (1)  than t h e l a t t e r .  disul-  The former o c c u r s (2)  The f i n a l  percent p r e s s u r e i n c r e a s e i s l a r g e r i n t h e former than t h e latter.  (3)  The:' former produces  copious q u a n t i t i e s o f  f r e e s u l f u r w h i l e t h e l a t t e r does n o t .  The l a s t two d i f -  f e r e n c e s suggest t h a t t h e decomposition o f e t h y l  disulfide  and t h e decomposition o f methyl d i s u l f i d e a r e e s s e n t i a l l y different  i n nature.  5  10  15  r  20  25  30  35  TIME IN MINUTES P i g , 31, Decomposition of E t h y l D i s u l f i d e ? Dependence o f the maximum r a t e on the i n i t i a l p r e s s u r e of d i s u l f i d e . Temperature = 318J °C,  117.  Table XVII Rate constants for the thermal decomposition of ethyl disulfide at 3 l 8 i ° C.  I n i t i a l Pressure  Rate Constant  mm.  sec.-l  18.2  0.00137  18.8  0.00131  28.0  0.00192 0.00186 0.00216  33.0 40.0  ,  0.00190 0.00210  Summary of the experimental results for methyl disulfide The main results obtained in this investigation may be summarized as follows: (1)  Methyl disulfide decomposes with a measurable  velocity at temperatures above 300° C.  The complex pres-  sure-time curve for the decomposition shows i n i t i a l l y a very rapid, decrease in pressure to a minimum, followed by a period of constant pressure.  This induction period i s  followed by a period of auto&atalytic build_up of the rate to a maximum and a subsequent f a l l i n g off.  The pressure  then continues to rise at a slow rate almost indefinitely. (2)  The f i n a l pressure increase i s greater than 105$  118. at both 341° C (3)  and 373° C.  The i n i t i a l pressure decrease i s larger the l a r g e r  the i n i t i a l pressure and the lower the temperature.  Prior  addition of hydrogen sulfide markedly lessens the amount of the decrease. a factor of 6.5 slightly.  Increasing the surface to volume r a t i o by increases the amount of the pressure decrease  This pressure decrease i s due to adsorption of  the reactant d i s u l f i d e on the surface of the reaction v e s s e l . The a c t i v a t i o n energy of the rate of pressure decrease i s of the order of 16 k.cal./mole., a value which might be expected i n ah adsorption process.  The frequency factor i s of the  order of the number of molecules s t r i k i n g the wall per second. Desorption of the d i s u l f i d e can occur r e a d i l y . (4)  The induction period i s r e a l :  Analyses show that  no appreciable amounts of d i s u l f i d e are decomposed during i t and na traces of products are formed.  However the true  induction period appears to end s l i g h t l y before the f i r s t pressure increase reproducible.  can be detected.  The induction period i s  I t i s independent of pressure i f the minimum  pressure i s l e s s than 140 mm.  I t s a c t i v a t i o n energy i s ap-  proximately the same as the a c t i v a t i o n energy of the maximum rate of pressure increase.  Changing the surface to volume  r a t i o of the reaction v e s s e l has no e f f e c t on the induction period. (5) . The reaction i s e s s e n t i a l l y homogeneous:  Packing  the reaction vessel has.no s i g n i f i c a n t e f f e c t on the rate of the reaction.  119. (6)  The percent pressure increase a f t e r various times  of reaction i s independent of the minimum pressure of d i s u l f i d e over the pressure range, 57 mm. to 230 mm.  At 24  mm. the percent pressure increase i s unusually high.  The  maximum rate of pressure increase i s f i r s t order with respect to the minimum pressure of d i s u l f i d e over the entire pressure range investigated, 24 mm. to 230 mm. the rate constant i s 0.00208 + 0.00007 s e c . ' (7)  At 341° C.  1  The maximum rate of pressure increase has an a c t i -  vation energy of approximately 45 k.cal./mole. and a frequency factor of approximately l O ^ s e c . " . 1  1  Activation energies  calculated from the length of the induction period and the time taken to reach the maximum rate are respectively 48.7 k.cal./mole. and 45.0 k.cal./mole. (£)  The major products of the reaction are mercaptan,  hydrogen s u l f i d e , free s u l f u r , carbon d i s u l f i d e , low b o i l i n g hydrocarbon, l a b i l e and non l a b i l e polysulfides, and an i n v o l a t i l e t a r . No other v o l a t i l e products are formed i n quantity, but there may be other i n v o l a t i l e products. mercaptan i s c h i e f l y or e n t i r e l y methyl mercaptan.  The  The  hydrocarbon, i s presumed to be c h i e f l y ethylene.. (9) amounts.  Hydrogen and methane are not formed i n measurable Thiophene i s formed only i n traces.  Monosulfides  cannot be formed i n large amounts i n the main part of the reaction. used.  Higher d i s u l f i d e s are not detectable by the.test  120. (10)  After the reaction of 80 mm. minimum pressure of  disulfide (approx. 95 mm. i n i t i a l pressure) at 316° C. for 120 minutes the total pressure i s 140 mm.  The total pres-  sure of 148 mm., calculated as the sum of the analytical constituent pressures, agrees well with the measured value. The volatile products at 120 minutes account for (a) Carbon equal to 70 mm. of disulfide (b) Hydrogen equal to 78 mm. of disulfide, and (c) Sulfur equal to SO mm. of disulfide, i.e. 75% to 85% disulfide decomposed.  The remaining 25% to  15% accounts for the large amounts of adsorbed products and for the involatile products.  The ratio of the remaining  Carbon, Hydrogen, and Sulfur, C:H:S  = '5:10:3, corresponds  to dehydrogenated or polymeric products. (11)  Methyl merdaptan i s a primary product of the de-  composition.  Its rate of formation i s practically zero  during the induction period but i t rises rapidly to a maximum at the end of the induction period.  In the standard  reaction i t reaches a maximum pressure of 60 mm. between 55 and 85 minutes; subsequently i t appears to. disappear slowly. One mole, of mercaptan i s formed for each mole of disulfide decomposed.  The reaction does not increase the pressure of  the system. (12)  The curve for the decomposition of disulfide coin-  cides with the curve for the formation of mercaptan i n the i n i t i a l stages of the reaction.  In the later stages of the  reaction the disulfide present i s not known with certainty because polysulfides, formed at the same time as hydrogen sulfide,  121. interfere with the analyses.  However, the following evidence  indicates that the pressure of disulfide i s close to zero at 60 minutes: (a) The polysulfides are formed i n macro quantities,  (b) The mereaptan and.disulfide curves level off at  about 60 minutes although the products of decomposition do not hinder the normal reaction,  (c) The sum of the pressures  of the various constituents at 120 minutes agrees with the measured total pressure only i f disulfide i s absent,  (d) Stoi-  ehiometrically, the products formed at 120 minutes account approximately for the complete decomposition of disulfide. (13)  The formation of hydrogen sulfide takes place much  later than the formation of mercaptan, and i t i s accompanied by the pressure increase of the reaction.  In the main part  of the reaction the rate of formation of hydrogen sulfide equals one-half the rate of pressure increase. After AO minutes, the time at which a l l the curves level off, the rate of pressure rise f a l l s relative to the rate of hydrogen sulfide formation until at 120 minutes the two rates are equal. (14)  Free sulfur, or labile polysulfides, mercaptan,  and hydrogen sulfide are present in traces at the end of the induction period. (15) . The prior addition of methyl mercaptan to the system has no effect on the rate of the reaction. (16)  The prior addition of hydrogen sulfide to the sys-  tem markedly shortens the induction period, shortens the period of autoaccelerating pressure rise, and increases the  122.  maximum rate of pressure r i s e .  The extent of reaction at  the. time of the maximum rate i s not affected.by the addition of hydrogen s u l f i d e .  The time at which the maximum rate i s  attained, or a l t e r n a t i v e l y the time at which the unique corresponding extent of reaction i s attained, depends to the one-half power on the hydrogen s u l f i d e present at that time. The maximum rate of pressure increase depends to the onequarter power on the hydrogen s u l f i d e present at the time of the maximum rate. (17)  The addition of hydrogen s u l f i d e to reactions at  high temperatures has no e f f e c t on the f i n a l pressure attained. (Id)  The e f f e c t of the complete reaction mixture on the  decomposition of fresh d i s u l f i d e i s to decrease the induction period to an extent that cannot be attributed to hydrogen s u l f i d e alone.  I t i s possible that the true induction period  i s removed e n t i r e l y .  In contrast to hydrogen s u l f i d e alone,  the complete reaction mixture has no e f f e c t on the maximum, rate of pressure increase. (19)  N i t r i c oxide Has no d e f i n i t e e f f e c t on the reaction.  Since the e f f e c t of surface i s also small i t has not been shown d e f i n i t e l y whether or not free r a d i c a l chains are i n volved i n the reaction. (20)  The appearance of the reaction i s complicated by  the adsorption and desorption  of both reactant and products  on the surface of the reaction vessel.  123. Summary of the experimental results for ethyl disulfide (1)  The decomposition of ethyl disulfide occurs at  lower temperatures than the decomposition of methyl disulfide. are  The pressure-time curves for the two decompositions  similar i n form but not identical.  The curve for ethyl  disulfide shows an induction period followed by a short period of autoacceleratiQn.  The maximum rate i s attained  rapidly and appears to remain nearly constant over a large part of the reaction.  The magnitude of the pressure increase  is larger i n the decomposition of ethyl disulfide than i n the decomposition of methyl disulfide. (2)  The percent pressure increase at any time i s i n -  dependent of the i n i t i a l pressure of ethyl disulfide.  The  f i r s t order rate constant at 318|° C. i s 0.00197 + 0.00011 sec.~l.  The rate constant f a l l s off at low pressures.  (3)  Copious quantities of free sulfur are formed i n the  decomposition of ethyl disulfide. (4)  An involatile carbonaceous tar i s formed which i s  similar in appearance to the tar produced i n the methyl d i sulfide reaction. (5)  The thermal decomposition of ethyl disulfide differs  in many respects from the thermal decomposition of methyl disulfide.  CHAPTER I I I DISCUSSION The two q u e s t i o n s about the thermal decomposition o f methyl d i s u l f i d e which demand answers a r e :  "What i s the  o v e r a l l nature o f t h e decomposition?"  "What are the  r e a c t i o n mechanisms?"  and  The most important q u e s t i o n a t ^ t h e  present time i s the f i r s t .  Before any reasonable  may be made t o suggest what fundamental  attempt  processes take p l a c e  i n the r e a c t i o n one must know what products a r e formed, a t what stage o f the r e a c t i o n they are formed, from what they are formed, and what r a t e laws are f o l l o w e d .  The d e s c r i p -  t i v e nature o f the o v e r a l l r e a c t i o n must be understood. Although a p a r t i a l d e s c r i p t i v e account o f t h e r e a c t i o n can be g i v e n by the present i n v e s t i g a t i o n t h e r e remain many obscurities.  Consequently i t i s not p o s s i b l e t o suggest a  complete r e a c t i o n mechanism. D e s c r i p t i o n o f the r e a c t i o n On the b a s i s o f the r e s u l t s obtained i n t h e p r e s e n t i n vestigation i t i s possible to give a p a r t i a l account o f what i s happening  i n the r e a c t i o n *  descriptive L e t us f o l l o w  125. the course of the reaction. I n i t i a l l y a quantity of the reactant d i s u l f i d e i s adsorbed on the surface of the pyrex reaction vessel. question  The  "Does t h i s adsorbed d i s u l f i d e play an important part  i n the main reaction?" the negative:  may be answered f a i r l y d e f i n i t e l y i n  Although the extent of adsorption i s increased  by packing the reaction v e s s e l , the induction period and the maximum rate of pressure increase are unaffected.  Therefore  the adsorbed d i s u l f i d e does not enter these phases of the reaction.  Although we do not have d i r e c t evidence of the  homogeneity of the reaction producing mercaptan i t would seem probable that i t i s likewise unaffected by the adsorbed d i sulfide.  What happens to t h i s adsorbed d i s u l f i d e ?  The exis-  tence of the period of constant pressure means that the adsorbed d i s u l f i d e cannot decompose r a p i d l y with the desorption of products.  I t i s probable that the adsorbed d i s u l f i d e  merely desorbs as the gaseous d i s u l f i d e i s used up. Following the adsorption there i s a r e a l induction period i n the reaction.  Traces of hydrogen s u l f i d e , mercaptan, and  free sulfur or l a b i l e polysulfides are formed, but no large quantities.  I t may be reasoned, from the probable dependence  of the "secondary" pressure increase on the primary reaction, that the end of the induction period would coincide with the start of the pressure r i s e i f i t were not f o r the adsorption of products. At the end of the induction period the d i s u l f i d e suddenly begins to decompose with the formation of methyl mercaptan,  126. mole f o r mole, and no measurable pressure increase.  The  rate of decomposition r i s e s r a p i d l y to a maximum, apparently within 10% of decomposition.  So f a r the description of the  reaction i s clear. Since methyl mercaptan accounts f o r only a part of the d i s u l f i d e decomposed the question arises of what happens to the remaining stoichiometric fragment "CSH  n  .  This material  must be removed from the system i n such a way that there i s no pressure increase i n t h i s part of the reaction. are three p o s s i b i l i t i e s :  (1)  There  I t i s added to fresh molecules  of d i s u l f i d e and removed from the system. bed on the surface of the reaction v e s s e l .  (2) (3)  I t i s adsorI t forms  non v o l a t i l e products or s l i g h t l y v o l a t i l e polymeric products. The f i r s t p o s s i b i l i t y would have meant the formation of d i sulfides of the type CH^SCHgSSCH^ i n very large quantities. However, no CH^SGH^SH was.observed  i n the d i s u l f i d e analyses.  The second p o s s i b i l i t y would require the adsorption to be both quantitative and immediate.  Although t h i s p o s s i b i l i t y  cannot be eliminated e n t i r e l y i t appears highly improbable. The t h i r d p o s s i b i l i t y appears quite probable.  I t requires  the formation of compounds of the molecular formular  c n  H2 S . n  n  Let us assume f o r the moment that the fragment CE^-S a c t u a l l y exists i n the reaction, although t r a n s i e n t l y , as the t h i o aldehyde.  We quote Sidgwick:52  The thioaldehydes are remarkable f o r t h e i r strong tendency to polymerization, much stronger than that of t h e i r oxygen analogues. They are scarcely  127. known i n the monomeric s t a t e ; they polymerize at once, mainly t o c y c l i c t r i p o l y m e r s , such as t r i t h i o f o r m a l d e h y d e , m.p. 216° C. ... i t s r i n g s t r u c t u r e has r e c e n t l y been e s t a b l i s h e d by X-ray analysis. I f three molecules of CHgS polymerized-to one molecule, pressure i n c r e a s e would be s m a l l .  the  Whether or not i t would  be d e t e c t a b l e would depend on whether or not the t r i p o l y m e r has a s i g n i f i c a n t vapour pressure a t 316°  G.  However, the  p o l y m e r i z a t i o n could w e l l be t o h i g h e r polymers.  The  pre-  sence o f a t a r i n the r e a c t i o n products supports t h i s view. T h e r e f o r e i t seems reasonable to suggest t h a t the CR^S  i s formed i n the primary r e a c t i o n and  i s removed  from the system by p o l y m e r i z a t i o n t o ( - C ^ - S - ^ p o s s i b l y of low  molecule  polymers,  volatility.  I t w i l l be r e a l i z e d  that the otherwise remarkable  for-  mation of carbon d i s u l f i d e as a major product o f the r e a c t i o n i s reasonable i f polythioformaldehydes are formed. sence of the S-C-S  bond system  mechanism f o r carbon d i s u l f i d e  The  pre-  i n the polymer suggests a formation.  The next stage of the r e a c t i o n i s the f o r m a t i o n of hydrogen s u l f i d e and a H i e d  products.  Traces of hydrogen  s u l f i d e are formed d u r i n g the i n d u c t i o n p e r i o d but i t seems improbable  t h a t they are formed by the main r e a c t i o n .  r a t e of f o r m a t i o n of hydrogen s u l f i d e  increases slowly a f t e r  the i n d u c t i o n p e r i o d u n t i l i t reaches a maximum. parallels  The  The  rate  the r a t e o f p r e s s u r e i n c r e a s e approximately.  parallelism  c o u l d be much c l o s e r i f i t were not f o r the  s o r p t i o n of d i s u l f i d e and f o r the occurrence o f a second  This de-  128. process producing hydrogen sulfide which w i l l be discussed shortly.  The other major volatile products of the reaction,  carbon disulfide, hydrocarbon, and polysulfides must be formed about the same time as hydrogen sulfide to account for the behavior of the total pressure relative to hydrogen sulfide. Between 40 and 60 minutes in the "standard reaction" the rate of formation, of hydrogen sulfide decreases sharply to a low value.  Between 60 and 120 minutes the rate i s approxi-  mately constant.  In the same period the rate of pressure i n -  crease decreases relative to the rate of hydrogen sulfide formation until the two rates are the same. It i s also between 40 and 60 minutes that the rate of formation of mercaptan f a l l s to zero, and the reason has been shown to be that the reactant disulfide has been used up.  Therefore, i t i s a  rather obvious conclusion that the main reaction producing hydrogen sulfide involves the decomposition of the disulfide. The hydrogen sulfide formed after 60 minutes must be formed by a process other thahathe main reaction.  This "second  hydrogen sulfide reaction" continues well after 120 minutes and may account i n part for the slow rise i n pressure which continues almost indefinitely i n the late stages of the reaction.  Let us consider this second reaction in more detail:  We ask the question duced?"  "From what i s this hydrogen sulfide pro-  It cannot be formed from the disulfide, and i t seems  unlikely that i t could be formed from the carbon disulfide, ethylene, or relatively small quantities of polysulfides present  129. as gaseous p r o d u c t s .  Since t h i s r e a c t i o n continues long  a f t e r 120 minutes, and s i n c e hydrogen s u l f i d e , carbon d i s u l f i d e , e t h y l e n e and p o l y s u l f i d e s account f o r a l l the gaseous products a t 120 minutes, i t appears t h a t i t must be formed from an i n v o l a t i l e product, presumably from the thioaldehyde  polymer.  A c c o r d i n g t o the a n a l y s e s about 10 mm. f i d e i s formed between 60 and 120 minutes. formed a t a n e a r l y constant r a t e i t may at l e a s t minutes.  5 mm.  o f hydrogen  Since t h i s i s  be concluded t h a t  i s formed by t h i s "second" process b e f o r e 60  The t o t a l a t 120 minutes must be about 15  Therefore, i t i s i n t e r e s t i n g  interesting  mm.  t o note t h a t the carbon d i s u l -  f i d e present a t 120 minutes i s a l s o 15 mm. is especially  sul-  The  correlation  i n view o f our s u g g e s t i o n t h a t  carbon d i s u l f i d e c o u l d most e a s i l y be produced from the aldehyde polymer.  However, i f carbon d i s u l f i d e and  hydrogen  s u l f i d e were both formed i n t h i s r e a c t i o n t h e r e would to be a c o r r e s p o n d i n g disappearance o f 10 mm.  thio-  have  of v o l a t i l e  m a t e r i a l between 60 and 120 minutes t o account f o r the equal r a t e s o f hydrogen s u l f i d e f o r m a t i o n and p r e s s u r e i n c r e a s e . Moreover,  i f t h e y a r e both formed from the t h i o a l d e h y d e p o l y -  mer one would expect an e q u i v a l e n t amount o f e t h y l e n e t o be formed.  Such a p o s s i b i l i t y i s c l e a r l y c o n t r a r y t o the ex-  perimental r e s u l t s  s i n c e i t would r e q u i r e s t i l l more v o l a t i l e  products t o be decomposed, b e s i d e s l e a v i n g almost no products i n t o which the d i s u l f i d e it  could decompose.  On the o t h e r hand,  i s d i f f i c u l t to imagine hydrogen s u l f i d e b e i n g formed  as  130. the sole product of thioaldehyde decomposition. Clearly this, process is. s t i l l obscure.  It may be that i t i s simply  a reaction of free sulfur from the polysulfides (or from Sg) with the hydrogen of the polymer.  More analytical data are  needed. Let us now consider the main reaction producing hydrogen sulfide. That this reaction uses up disulfide has been suggested by the marked f a l l i n g off i n the rate when the d i sulfide i s used up. It may also be deduced from the very fact that disulfide i s used up, or from the fact that the involatile products which have been decomposed at 120 minutes cannot account for a l l the volatile products. One of the products formed a t the same time as hydrogen sulfide i s polysulfide. The most reasonable possibility is that this should be formed by an addition reaction of the type: RSSR  + S x  * RS J . x+2  Therefore i n the following discussion i t i s advisable to distinguish between this side reaction and the main reaction producing hydrogen sulfide and a l l i e d products. The distinction i s made for semantic; reasons and does not affect the definiteness of the following arguments. At 60 minutes i t i s estimated that 90 to 95 mm. of the original .95 mm. of disulfide has been decomposed.  60 mm. of  this disulfide went to form mercaptan, and perhaps 5 mm. to form polysulfide. The remainder, which i s used up i n the production of hydrogen sulfide, i s 25 to 30 mm.  Of the 32 mm.  131. formed a t 60 minutes,  o f hydrogen s u l f i d e  be a t t r i b u t e d t o the "second" r e a c t i o n . mm.  i s of the order o f the 25  composed.  t o 30  mm.  about 5 mm.  must  The remaining  2?  of d i s u l f i d e  de-  T h e r e f o r e one mole o f d i s u l f i d e produces a p p r o x i -  mately one mole of hydrogen s u l f i d e . We now  estimate the amount o f "other v o l a t i l e p r o d u c t s "  which are produced p l u s ?.  by the main r e a c t i o n CH^SSCH^  *  E^S  I t i s p o i n t e d out f i r s t t h a t t h i s r e a c t i o n does not  i n v o l v e the decomposition  o f any other v o l a t i l e m a t e r i a l ;  the o n l y other v o l a t i l e m a t e r i a l , the mercaptan, has been shown t o be e s s e n t i a l l y  an end product of the  decomposition.  Since one mole o f d i s u l f i d e produces one^mole o f hydrogen s u l f i d e , the " t o t a l other p r o d u c t s " produced  by t h i s r e a c t i o n  must be equal t o the total pressure i n c r e a s e produced reaction.  about 5 mm. 31  The t o t a l p r e s s u r e i n c r e a s e a t 60 minutes i s  Of . :this, 15  mm.  mm.  by  mm.  i s due t o d e s o r p t i o n o f d i s u l f i d e ,  i s due t o the "second" r e a c t i o n .  The  this 51 and  remaining  I s a rough measure o f the "other v o l a t i l e products"  formed by t h i s  reaction.  The preceeding c a l c u l a t i o n  may  be checked  the products:  At 60 minutes there are 31  d u c t s " , 32  of hydrogen s u l f i d e , 60 mm.  mm.  an assumed 5 mm.  of p o l y s u l f i d e .  mm.  by t o t a l l i n g of "other p r o -  o f mercaptan, and  I f i t i s assumed t h a t  carbon d i s u l f i d e i s one o f the products o f t h i s main r e a c t i o n the t o t a l pressure i s 129  mm.  However, i f i t i s assumed  t h a t carbon d i s u l f i d e i s produced  e n t i r e l y by the  r e a c t i o n , i t i s necessary t o add about 5 mm. to g i v e 134  mm.  "second"  to t h i s  The measured t o t a l pressure o f 131  total mm.  is  132. i n reasonable agreement w i t h both t o t a l s . 31 mm.  The v a l u e o f  f o r "other p r o d u c t s " i s thus confirmed, but i t i s  not p o s s i b l e t o a s s i g n carbon d i s u l f i d e d e f i n i t e l y t o  one  or the other r e a c t i o n . We  now  ask the q u e s t i o n "What are these other  which t o t a l about 31 mm?"  I f the a p p r o p r i a t e  products  analytical  data were a v a i l a b l e f o r 6 0 minutes t h i s q u e s t i o n could be answered,fullyi the two it  The  answer would l e a d t o a s e p a r a t i o n of  processes producing, hydrogen s u l f i d e .  i s only p o s s i b l e to speculate:  At  present  The o n l y two products  choose from are carbon d i s u l f i d e and e t h y l e n e .  to  We assume  f o r the moment t h a t the carbon d i s u l f i d e i s produced  entire-  l y by the "second" r e a c t i o n .  of  I f t h i s i s so the 25 mm.  ethylene present a t 120 minutes must be produced e n t i r e l y by the main r e a c t i o n . order as the 25 t o 30 mm. roughly 27 mm.  i s o f the same  of d i s u l f i d e decomposed, the  o f hydrogen s u l f i d e formed, and the v e r y  approximate 31 mm. 31 mm.  T h i s 25 mm.  almost  o f "other p r o d u c t s " .  Moreover i f the  were too h i g h a v a l u e , the c o n d i t i o n t h a t the t o t a l  products measure 131 mm.  would r e q u i r e t h a t the carbon d i -  s u l f i d e be produced by the "second r e a c t i o n " .  Further, i t  i s p o s s i b l e t o w r i t e the f o l l o w i n g s t o i c h i o m e t r i c r e a c t i o n s which do not i n v o l v e any other m a t e r i a l s :  GH3SSCH3 S They a l s o account  *  HS  *  Sg  2  +  GH =CH  and/or  2  2  +  S  CH^CH^  f o r the f o r m a t i o n of p o l y s u l f i d e s .  On  the  133. other hand, i t must be p o i n t e d not  out that t h i s argument does  c o n s i d e r the p o s s i b i l i t y o f i n v o l a t i l e products t a k i n g  p a r t i n the r e a c t i o n and p r o d u c i n g , i n a d d i t i o n , more i n v o l a t i l e products.  Moreover i t i s d i f f i c u l t to conceive of  a mechanism f o r such a r e a c t i o n , and  d i f f i c u l t to  the c a t a l y t i c e f f e c t of hydrogen s u l f i d e . would be a s t r a n g e c o i n c i d e n c e  explain  Further, i t  f o r t h i s independent r e a c t i o n  t o have an i n d u c t i o n p e r i o d o f almost e x a c t l y the same l e n g t h as the i n d u c t i o n p e r i o d f o r the mercaptan r e a c t i o n . i t i s c l e a r t h a t more a n a l y t i c a l d a t a are The  Again  required.  q u e s t i o n , r a i s e d i n the l a s t d i s c u s s i o n , o f whether  or not the thioaldehyde  polymer i s i n v o l v e d i n the main  hydrogen s u l f i d e p r o d u c t i o n  i s very important.  the v a r i o u s s t o i c h i o m e t r i c d a t a obtained  i n the present  v e s t i g a t i o n are not able to answer i t . The suggest t h a t hydrogen s u l f i d e formation r e a c t i o n , but not  conclusively.  Unfortunately in-  k i n e t i c data  do  i s a t r u e secondary  In f u t u r e work the  question  could be approached e i t h e r by an a n a l y t i c a l i n v e s t i g a t i o n o f the e f f e c t o f hydrogen s u l f i d e on the r e a c t i o n , or by a chiometric  investigation.  The  stoi-  latter need not be d i r e c t .  In-  d i r e c t argument based on the amount o f v o l a t i l e products formed, the amount o f v o l a t i l e m a t e r i a l decomposed, the pressure  i n c r e a s e , or the magnitude of the t o t a l  pressure,  can l e a d t o an understanding o f the "second" r e a c t i o n , then q u i t e simply t i l e and tion.  total  and  t o a q u a n t i t a t i v e account o f both the v o l a -  i n v o l a t i l e compounds t a k i n g p a r t i n the main r e a c -  In t h i s connection  the general  case may  be mentioned  134. of the argument which was used to estimate the quantity of "other volatile products" of the main reaction:  The calcu-  lation requires knowledge of the ratio, "R", of the total pressure change i n the reaction concerned to the pressure change of one constituent.  We use hydrogen sulfide for  Let AX equal the mm. of volatile products decom-  example.  posed and ^Y the mm. of products other than hydrogen sulfide.  Then: AY  + AH S 2  = AP  + AX  or i n terms of moles (AY/AH S) 2  +  1  =  R  +  (AX^H S). 2  Now let us consider what substances affect the rate of the main reaction producing hydrogen sulfide.  (1) It has  been established definitely that the rate does not depend on the mercaptan concentration.  (2) Since the rate f a l l s  as the disulfide i s used up, the rate does depend on the d i sulfide concentration.  (3) The rate appears to depend on  the concentration of hydrogen.sulfide to the one-quarter power.  However the real nature of this dependence i s not  obvious:  If the hydrogen sulfide reaction depends for i t s  occurrence on. the prior formation of (CH S) by the primary 2  n  reaction, the apparent dependence of the rate on the hydrogen sulfide present may actually be an indirect consequence of an increased rate of the primary, reaction.  In any ease,  hydrogen sulfide can not be the only product which ^affects the rate:  It w i l l be recalled that 52 mm. of added hydrogen  135. s u l f i d e i s not s u f f i c i e n t , to remove e n t i r e l y the i n d u c t i o n p e r i o d o f a r e a c t i o n which r e a c h e s . i t s maximum r a t e with o n l y 7 mm.  of hydrogen s u l f i d e p r e s e n t .  depend on the presence r a t e may  (4)  The r a t e  o f the p o l y t h i o a l d e h y d e s .  depend on other products o f the hydrogen  (5)  may The  sulfide  r e a c t i o n ; f r e e s u l f u r i s an example. The  e f f e c t o f added hydrogen s u l f i d e on the magnitude  of the pressure i n c r e a s e i n the r e a c t i o n suggests  t h a t the  f o r m a t i o n o f hydrogen s u l f i d e does r e q u i r e the presence polythioaldehydes. pressure-time  I t w i l l be n o t i c e d i n F i g . 2 3 .  curves f o r the decompositions  hydrogen s u l f i d e and the pressure-time  of  t h a t the  with added  curve f o r the normal  r e a c t i o n a l l l e v e l o f f a f t e r thesame t o t a l pressure i n crease.  T h i s l e v e l l i n g o f f corresponds  to the stage o f  r e a c t i o n a t which the d i s u l f i d e c o n c e n t r a t i o n f a l l s to zeroSince the hydrogen s u l f i d e f o r m a t i o n accounts  f o r the p r e s -  sure change w h i l s t the mercaptan f o r m a t i o n i n v o l v e s no sure change, i t must be concluded  pres-  t h a t the a d d i t i o n o f hydro-  gen s u l f i d e to the system does not change the r a t i o o f hydrogen s u l f i d e t o mercaptan a t the completion o f these main r e a c t i o n s .  two  Therefore hydrogen s u l f i d e must a c c e l e r a t e  both r e a c t i o n s e q u i v a l e n t l y and must decrease periods i d e n t i c a l l y .  Otherwise,  both i n d u c t i o n  f o r example, the d i s u l f i d e  might decompose completely i n t o mercaptan before the product i o n o f hydrogen s u l f i d e s t a r t e d . independent i t i s almost  I f the two r e a c t i o n s are  i n c o n c e i v a b l e t h a t hydrogen s u l f i d e  should shorten both i n d u c t i o n p e r i o d s i d e n t i c a l l y , but i f the hydrogen s u l f i d e f o r m a t i o n depends on the presence  of  136. polythioformaldehyde such b e h a v i o r would be expected.  Thus  the evidence suggests s t r o n g l y t h a t t h e hydrogen s u l f i d e formation  i s indeed a secondary r e a c t i o n .  I f t h e formation mer,  o f hydrogen s u l f i d e uses up the p o l y -  the r e a c t i o n cannot s t a r t u n t i l some o f t h i s  has been produced by t h e primary r e a c t i o n .  material  I n t h e normal  r e a c t i o n t h i s dependence e x p l a i n s why t h e i n d u c t i o n  period  o f the mercaptan r e a c t i o n i s approximately equal t o the i n d u c t i o n p e r i o d o f the hydrogen s u l f i d e r e a c t i o n . reactions  In the  w i t h added hydrogen s u l f i d e i t e x p l a i n s why  hydrogen s u l f i d e has a g r e a t e r e f f e c t on t h e i n d u c t i o n period, o f the pressure a shortening  i n c r e a s e than on i t s maximum r a t e ;  i n the i n d u c t i o n , p e r i o d o f t h e mercaptan r e a c -  t i o n i s suggested.  The q u e s t i o n  then a r i s e s o f whether the  polymer has any e f f e c t on the r e a c t i o n a f t e r the i n d u c t i o n p e r i o d , i . e . , i s t h e dependence zero order?  I f the polymer  r e a c t s as a s o l i d o r a l i q u i d t h e r a t e might depend on i t s s u r f a c e area, but t h e homogeneity o f t h e r e a c t i o n makes t h i s Improbable. as a gas,  I f the polymer i s p a r t i a l l y v o l a t i l e and r e a c t s  o r i f i t r e a c t s , through p r e l i m i n a r y  degradation,  the r e a c t i n g e n t i t y would be i n e i t h e r an e q u i l i b r i u m o r a steady s t a t e c o n c e n t r a t i o n . being  An e q u i l i b r i u m  concentration,  independent o f t h e q u a n t i t y o f s o l i d o r l i q u i d , would  r e s u l t i n zero order dependence. but r e a c t s by p r e l i m i n a r y (CH S) 2  n  I f the polymer i s gaseous  depolymerization »  n CH S 2  137. the rate would depend on CH^S to the 1/n power; i f n i s large the dependence would-again be zero order.  Therefore  i t i s quite possible that the rate of formation of hydrogen sulfide i s independent of the concentration of the polymer, even though the polymer may be used up i n the reaction. If we assume that the formation of hydrogen sulfide requires the presence of the polymer, but i s independent of i t s concentration, i t i s possible to explain the effect of the total reaction products on the reaction. observations were  The relevant  (1) The induction period of the pres-  sure increase (hydrogen sulfide) i s markedly shortened by (2)  the reaction products.  The period of autoaccelerating  rate is only slightly affected and the maximum rate i s not affected at a l l .  (3) The total pressure increase i s lowered.  We suggest that the reaction products accelerate the formation of mereaptan but do not affect the rate of production L  of hydrogen sulfide.  The shortening (or complete removal)  of the induction period of the primary reaction would shorten the induction period of the secondary reaction without affecting i t s maximum rate.  The increased percentage of mercaptan  formed relative to hydrogen sulfide would lower the total pressure attained.  Of coursej i t i s not yet explained why  the complete reaction mixture affects only the primary reaction when hydrogen sulfide also accelerates the secondary reaction. Thus the kinetic evidence does suggest that the formation  138. . of hydrogen s u l f i d e i s a t r u e secondary q u i r e s the presence  r e a c t i o n which r e -  o f the primary products  i t appears t o be independent  (CH S)n. 2  Since  o f t h e c o n c e n t r a t i o n o f (CH S)n, 2  the dependences o f t h e r a t e on hydrogen s u l f i d e must be r e a l . We know t h a t : (AP/const.)  t  =  (H S) 2  «  t  f(t)(RSSR)  m i n >  Differentiating, G(dP./dt) where k  t  =  •-  t  (d(H S)/dt) 2  »  t  k  t  ( R S S R )  min..  f ( t ) . I t w i l l be noted t h a t t h i s e x p r e s s i o n  has been v e r i f i e d f o r " t " equal t o t h e time o f t h e maximum rate.  As r e q u i r e d , t h e time o f t h e maximum r a t e i s indepen-  dent o f t h e i n i t i a l p r e s s u r e .  Now t h e extent o f t h e p r e s s u r e  i n c r e a s e depends on the r a t i o of. hydrogen s u l f i d e formed t o mercaptan formed.  For the f i r s t  e q u a t i o n t o be v a l i d t h e  r a t i o o f hydrogen s u l f i d e t o mercaptan must be independent of t h e i n i t i a l  pressure a t t h e completion o f the two r e a c t i o n s ,  Therefore the r a t e o f f o r m a t i o n o f mercaptan must a l s o be first  order over t h e whole range o f times, and we can w r i t e  the e x p r e s s i o n s : (d(ESH)/dt)  =  (RSH). t  k '(RSSR) t  m i n >  f»(t) (RSSR) min.  The r a t e a t which d i s u l f i d e disappears depends a l s o on t h e r a t e o f a d s o r p t i o n and d e s o r p t i o n o f d i s u l f i d e , and on the rate o f p o l y s u l f i d e formation. first  The r a t e o f d e s o r p t i o n i s  o r d e r with r e s p e c t t o the amount o f adsorbed  and the r a t e o f a d s o r p t i o n i s f i r s t  disulfide  order w i t h r e s p e c t t o  the amount o f gaseous d i s u l f i d e ; a t l e a s t t o an approximation, the two processes should be f i r s t  order with r e s p e c t  t o the  139. first  order w i t h r e s p e c t t o the i n i t i a l d i s u l f i d e  pressure.  Presumably the f o r m a t i o n o f p o l y s u l f i d e s i s a b i m o l e c u l a r r e a c t i o n between d i s u l f i d e and f r e e s u l f u r .  Since the f r e e  s u l f u r must probably d i s s o c i a t e from Sg the r a t e o f t h i s r e a c t i o n should be o n l y s l i g h t l y dependent on the concentration of free sulfur. the  Therefore i t i s c o n s i s t e n t t o w r i t e  equation: -(d(RSSR)/dt)  = X"k (RSSR) i  t  t  m l a >  where t h e i * s r e f e r t o t h e d i f f e r e n t processes i n v o l v e d . I n t e g r a t i n g and assuming the k's t o be f u n c t i o n s o f time o n l y f o r a l l t h e processes we o b t a i n : ( R S S R )  min. " <  )  R S S R  ^  = t  (RSSR),. t  =  f  i  (  t  )  (  R  S  S  R  )  m i n .  F(t)(RSSR) min.  Thus i t i s probable t h a t the amount o f d i s u l f i d e i n t h e s y s tem a t any time i s f i r s t  order w i t h r e s p e c t t o t h e i n i t i a l  pressure o f d i s u l f i d e . The formation o f t h e polymer p a r a l l e l s the f o r m a t i o n of mercaptan.  The f o r m a t i o n o f e t h y l e n e probably  the f o r m a t i o n o f hydrogen s u l f i d e .  parallels  T h e r e f o r e we suggest  t h a t a l l the major c o n s t i t u e n t s o f t h e system should obey the i n t e g r a t e d f i r s t  order  (X) /(RSSR) t  m i n >  equation: =f(t)  An attempt was made t o d i s c o v e r the r a t e dependence o f the main hydrogen s u l f i d e r e a c t i o n by a t r i a l and e r r o r method. R e l a t i o n s were assumed o f the form:  140. d(H S)/dt  -  2  and graphs o f k ( X ) of  k(X) (Y) (Z) ... X  y  Z  ... versus time were compared t o a graph  x  d(H2S)/dt versus time c o n s t r u c t e d from t h e a n a l y t i c a l  results.  The l a t t e r was h o t c o r r e c t e d f o r t h e f o r m a t i o n o f  hydrogen s u l f i d e by the "second  hydrogen s u l f i d e r e a c t i o n " ,  but the c o r r e c t i o n would not be l a r g e .  As a g e n e r a l guide  i n choosing the r e l a t i o n s , the p r o b a b i l i t y was remembered t h a t a t any time, X - C ( R S S R )  m i n  . f o r any. X; t h e r e f o r e  x / y / z ... should equal u n i t y .  The v a l u e s f o r d i s u l f i d e  were taken from t h e estimated curve i n F i g . 20.  I t may be  mentioned now t h a t t h i s curve was drawn b e a r i n g i n mind t h a t approximately one mole o f d i s u l f i d e i s decomposed f o r each mole o f hydrogen s u l f i d e formed. largely i n error.  I t cannot be v e r y  The v a l u e s f o r (CH S)n were taken a s t h e 2  d i f f e r e n c e between t h e mercaptan and t h e hydrogen s u l f i d e concentrations.  The f o l l o w i n g r e l a t i o n s were t r i e d  (1)  Rate/k  =  (CH S)n  (3)  Rate/k  =  (RSSR) 4 (H S.)i(CH Sj£.  2  (2)  Rate/k  2  first:  -.. ( R S S R ) 3 A ( H S ) I  2  2  The r e s u l t s  indicated  f i r s t l y t h a t t h e r a t e must depend on d i s u l f i d e i n order t o decrease r a p i d l y a f t e r the maximum, secondly, t h a t the r a t e must depend on hydrogen s u l f i d e to  (or a l l i e d products) i n order  i n c r e a s e r a p i d l y t o t h e maximum, and t h i r d l y , t h a t (CH" S) 2  can have but l i t t l e e f f e c t on the r a t e . with our p r e v i o u s c o n c l u s i o n s .  These r e s u l t s  agree  I t appeared t h a t t h e depen-  ,dence on d i s u l f i d e must be t o a r e l a t i v e l y h i g h power and t h a t t h e dependence on "hydrogen s u l f i d e " must be t o a power much h i g h e r than one-quarter.  Therefore, the f o l l o w i n g  n  141. r e l a t i o n was  tried:.  (4)  Rate/k  =  (RSSR)(H S). 2  This  r e l a t i o n agrees s u r p r i s i n g l y w e l l w i t h the observed but no adjustment  rate,  of the estimated d i s u l f i d e curve can make  i t agree completely. tuitous i t implies  I f t h i s agreement i s more than f o r -  (1)  That a product a l l i e d to hydrogen  s u l f i d e a l s o a f f e c t s the r a t e , and p r e s s i o n must c o n t a i n a term  (2)  (RSSR)  0  i . e . , the e x p r e s s i o n should be o f the d(H S)/dt 0  4  =  That the r a t e  ex-  i n the denominator, form:  K(RSSR)(HoS)i(X) A (RSSR)*-, 3  min. Let us now  f o l l o w the r e a c t i o n a stage f u r t h e r .  After  the two main r e a c t i o n s are over the pressure continues t o r i s e almost i n d e f i n i t e l y .  We have"already d i s c u s s e d  the  second r e a c t i o n producing hydrogen s u l f i d e from the polymer or t a r .  I t would seem probable t h a t e v e n t u a l l y  e n t i r e l y degraded  the t a r i s  t o v o l a t i l e products, hydrogen s u l f i d e ,  carbon d i s u l f i d e , and ethylene f o r example.  Perhaps the pro-  d u c t i o n o f hydrogen s u l f i d e i s the i n i t i a l stage o f t h i s degradation.  During these l a t e r times o f r e a c t i o n the amount  of v o l a t i l e p o l y s u l f i d e s appears t o decrease; p o s s i b l y they are degraded  t o hydrogen s u l f i d e and other products, and  perhaps through the i n t e r m e d i a t e stage o f d i s u l f i d e f o r m a t i o n . I t i s c e r t a i n l y reasonable t o assume t h a t the r e a c t i o n RSSR + S  •»  RS^R.  is a reversible reaction.  In the l a t e r stages the mercaptan  a l s o appears  I t i s quite p o s s i b l e t h a t t h i s i s  to decrease.  142. a slow sensitized decomposition, but i t i s also possible that mercaptan adds to the double bonds present.  A possible de-  composition reaction i s : 2 CH SH  *  3  (CH" ) S 3  + HS  2  2  which has an equilibrium constant estimated at 10 at 300° G. The description of the reaction that we have made maybe summarized as follows: (1)  Adsorption of disulfide:  The adsorbed disulfide  does not, enter into the reactions significantly.  Probably  i t merely desorbs as the reactant i s ised up. (2)  Induction period:  A l l the reactions have approxi-r  mately the same induction period.  I t i s shortened by hydro-  gen sulfide and possibly removed entirely by the complete reaction mixture. (3)  It i s homogeneous and reproducible.  Decomposition of the disulfide to form methyl mer-  captan : The following reactions probably occur: •> CH"SH + CH S  GH3SSCH3  n GH S 2  3  .  2  (GH S) 2  n  The rate should be f i r s t order with respect to the minimum pressure of disulfide.  The reaction i s catalyzed by hydro-  gen sulfide and by the complete reaction mixture. (4) sulfide:  Decomposition of disulfide to produce hydrogen The following reactions probably occur: RSSR  +  (?) S  —»  HS 2  + C\ 2  + S  + (?)  » Sg + RS R X  One mole of disulfide produces one mole of hydrogen sulfide and slightly more than one mole of other volatile products.  143. Rate c o n s i d e r a t i o n s  indicate that t h i s reaction also  the second product o f the mercaptan r e a c t i o n , and must a l s o produce carbon d i s u l f i d e .  The  involves  i f so i t  rate of t h i s  t i o n appears to be independent o f the amount o f  (GH2S)  sent; the r a t e depends on the d i s u l f i d e  concentration,  the hydrogen s u l f i d e  apparently  concentration,  and  reacpre-  n  on  on  the  concentration  o f a product formed a t the same time as hydro-  gen  The  sulfide.  initial  pressure  I t i s not (5)  rate i s f i r s t of d i s u l f i d e .  The  to  the  r e a c t i o n i s homogeneous.  c a t a l y z e d by the complete r e a c t i o n mixture. Second r e a c t i o n p r o d u c i n g hydrogen s u l f i d e :  T h i s r e a c t i o n continues are over.  l o n g a f t e r the two  main r e a c t i o n s  I t probably i n v o l v e s the decomposition of  thioaldehyde and  order w i t h r e s p e c t  polymer.  I t may  the  a l s o produce carbon d i s u l f i d e ,  i f so, i t must a l s o i n v o l v e decomposition of a v o l a t i l e  product. (6)  F u r t h e r d e g r a d a t i o n r e a c t i o n s or other  side  reactions  Probably the polymeric products are completely degraded i f the r e a c t i o n i s allowed to proceed f o r a very l o n g time. mercaptan and  p o l y s u l f i d e s seem t o decompose s l o w l y .  s i d e r e a c t i o n s may I t may  Other  occur a l s o .  be noted t h a t these c o n c l u s i o n s  and  suggestions  about the r e a c t i o n s o c c u r r i n g have been drawn e n t i r e l y the experimental r e s u l t s .  They are not based on  as t o p o s s i b l e r e a c t i o n mechanisms. l i s h e d t h a t the o v e r a l l main p r o c e s s e s .  The  from  speculation  I t i s considered  estab^  reaction a c t u a l l y consists of four  Some o f the other suggestions are more specu-  144. l a t i v e i n n a t u r e , and as such, they r e q u i r e experimental verification. I t i s e v i d e n t t h a t even d e s c r i p t i v e l y the r e a c t i o n i s extremely  complex.  The present i n v e s t i g a t i o n has thrown  c o n s i d e r a b l e l i g h t on the v a r i o u s processes o c c u r r i n g , but much o f the r e a c t i o n remains  obscure.  Mechanisms o f the r e a c t i o n I t i s u s e f u l t o c o n s i d e r some o f the fundamental p r o cesses t h a t may  be o c c u r r i n g i n the r e a c t i o n .  Although no d i r e c t evidence was  obtained f o r the o c c u r -  rence o f f r e e r a d i c a l chains i n the r e a c t i o n , i t seems doubtf u l t h a t the e n t i r e r e a c t i o n c o u l d take p l a c e by a m o l e c u l a r mechanism.  The  complexity o f the r e a c t a n t and the p r o d u c t s ,  the v a r i e t y o f the p r o d u c t s , and the p e c u l i a r r a t e laws f o l l o w e d by the r e a c t i o n s , a l l suggest f r e e r a d i c a l mechanisms. L e t us c o n s i d e r the primary decomposition f i d e i n t o mercaptan and o t h e r p r o d u c t s . m o l e c u l a r process can be SCH / r CH S  following uni-  conceived:  9  +  GHoSH  H  3  The  o f the d i s u l -  +  GH »S 2  p  However such a molecular r e a c t i o n cannot both show an i n d u c t i o n p e r i o d and be f i r s t  o r d e r , i . e . , i t cannot be c a t a l y z e d  by products and be u n i m o l e c u l a r . Consider what may  happen when a f r e e r a d i c a l a t t a c k s the  methyl d i s u l f i d e molecule. (1)  R.  +  GH-,SSGHo  There are t h r e e p o s s i b i l i t i e s : >  RH  +  CH,SSCH . 0  145. (2)  R*  +  CH3SSCH3  *  RCH3  (3)  R.  +  CH3SSCH3  »  RSCH3  +  CHjSS*  +  CH^S*  The t h i r d r e a c t i o n i s o f no consequence; t h e CH3S. f r e e  radi-  c a l could only recombine w i t h i t s e l f , o r r e a c t a g a i n t o g i v e a new molecule  o f d i s u l f i d e p l u s a new CK^S* r a d i c a l .  second r e a c t i o n could be f o l l o w e d by r a d i c a l  The  decomposition  with the formation o f a double bond and a new a t t a c k i n g r a d i cal : (4)  GH SS«  —  3  » • CH^*  +  S=S  The consequence o f r e a c t i o n s (2) and (4) would be the product i o n o f methane, ethane and f r e e s u l f u r , and e v e n t u a l l y o f hydrogen s u l f i d e , ethylene and p o l y s u l f i d e s .  While these  r e a c t i o n s could account f o r t h e t r a c e s o f hydrogen  sulfide  and f r e e s u l f u r formed d u r i n g t h e i n d u c t i o n p e r i o d , they do hot r e s u l t i n mercaptan formation and can not be important. R e a c t i o n (1) can a l s o be f o l l o w e d by a c h a i n propagating dbep w i t h the f o r m a t i o n o f a double bond and a new r a d i c a l . The f o l l o w i n g r a d i c a l c h a i n could r e s u l t :  (1)  R. .+  (5) (6)  CH3SSGH3 . CH SSCH » 3  CH3S.  +  2  CH3SSGH3  .+ RH •  +  CH SSCH2» 3  »  CH3S.  +  GH -S  »  CH3SH  +  C^SSCRg.  2  T h i s r a d i c a l c h a i n not o n l y produces mercaptan i n the observed amount, but a l s o produces t h e p o s t u l a t e d t h i o a l d e h y d e .  Since  the other p o s s i b i l i t y does not g i v e mercaptan, t h i s mechanism seems very h i g h l y p r o b a b l e .  The t h i o a l d e h y d e s could be r e -  moved by p o l y m e r i z a t i o n : (7)  n  CH =S 0  d  *  (-CH_-S-) 2 n  146. T h i s p o l y m e r i z a t i o n would prevent  the  reverse  reaction  from o c c u r r i n g : CH3S.  (8)  +  CH =S  »  2  CH" SSCH . 3  T h i s p r o b a b l e mechanism adds w e i g h t t o criptive  conclusions  about  the  nature  2  our previous of  the  des-  primary  reac-  tion. Consider the These  are:  surface (c)  (a)  becomes  Catalysis  to the Cause  steady (b)  bility  seems  is  poisoned,  (b)  most  Cause  to  lysis  by p r o d u c t m o l e c u l e s  attack.  is  free  to  steady  (d)  has d e f i n i t e l y of  choose  the  length  assume  that  until  the  approach  been  disproved.  reproduciinduction  disulfide  between  may o v e r l a p ,  Slow  o f the  causes  b u t we n o t e  may be d i s t i n g u i s h e d effect  i n t r o d u c e a more e f f i c i e n t  r a d i c a l mechanism.  c a n be  and  s t a t e when t h e  E x a m p l e s w i l l be  We w i l l  (a)  surface  vary w i t h v a r i a t i o n i n the  not p o s s i b l e  the  induction period.  I n h i b i t i o n by an i m p u r i t y ,  i n d u c t i o n p e r i o d ; the  I n some c a s e s t h e y  molecules  the  improbable I n view  (d) .  approach to  of  by p r o d u c t m o l e c u l e s ,  period d i d not It  causes  C h a i n t e r m i n a t i o n on t h e  state.  o f the  used.  possible  fraction (c)  and  that  cata-  from the  of the  process  slow  product of  radical  considered. the  p r i m a r y r e a c t i o n does  Three p o s s i b l e  initiation  conceived: (9)  RSSR  >  RSS«  +  (10)  RSSR  »  2 RS.  (11)  RSSR  »  S:  +  R.  RSR  occur  by  processes  147. In Appendix I I we  have shown on s e m i - t h e o r e t i c a l  t h a t the d i s s o c i a t i o n energy o f t h e S-S f i d e should be l e s s than t h a t o f the it The  i s not  grounds  bond i n methyl d i s u l -  C-S  bond.  Therefore,  o t h e r two  r a d i c a l can  processes are both p o s s i b l e .  conceivably  Since any  free  i n i t i a t e the main r e a c t i o n , i t i s  not p o s s i b l e t o choose between them on the b a s i s o f the mental r e s u l t s .  R e a c t i o n (11)  (11)  t o note t h a t i f r e a c t i o n i n d u c t i o n p e r i o d might be concentration  It i s interesting  i s the i n i t i a t i o n process  a t t r i b u t e d to a slow b u i l d &p  the in  o f s u l f u r atoms.  Hydrogen s u l f i d e could The  expai-  could occur through the i n t e r -  mediate f o r m a t i o n of a branched d i s u l f i d e .  the  (9).  probable t h a t the i n i t i a t i o n occurs by r e a c t i o n  c a t a l y z e the r e a c t i o n i n two  f i r s t p o s s i b i l i t y i s t h a t the hydrogen s u l f i d e might  ways. be  o x i d i z e d t o f r e e s u l f u r atoms by a molecular r e a c t i o n w i t h the d i s u l f i d e ; the f r e e s u l f u r atoms might then i n i t i a t e  the  reaction.  The  (12)  GHjSSCEj  +  HS  (13)  CH SSCH  +  S  3  3  2 CHjSH  2  *  HS.  +  +  S:  CH SSCH . 3  2  second p o s s i b i l i t y i s t h a t hydrogen s u l f i d e might take  p a r t i n a r e a c t i o n which bypasses the normal r e a c t i o n between GH^S* r a d i c a l s and t r a t e d by the termination  the d i s u l f i d e .  f o l l o w i n g r e a c t i o n mechanism i n which the  s t e p has  been chosen without r e g a r d f o r the  bable order of the r e a c t i o n . by  "M".  Such c a t a l y s i s i s i l l u s chain pro-  Methyl d i s u l f i d e i s r e p r e s e n t e d  148. M CH3S.  .fcl  +  M  k ___^ H—+  2  CH S»  +  H S  HS-  + M  C H  3^*  + +  GH3SH  £ 4 — *  2  CH3S.  GH3SH  2  GH3SSGH . 3  2  *  CH3SSGH . 2  C H  2  = S  + HS»  _k£ ^ H S + CH3SSGH. 2  2 GH3S.  k6---_>  The reasonable assumption  2  M  has been made t h a t the HS» r a d i -  c a l s formed w i l l o n l y r e a c t w i t h the d i s u l f i d e . steady s t a t e treatment  The u s u a l  g i v e s the r a t e o f disappearance o f  d i s u l f i d e as: -d(M)/dt  =  d(RSH)/dt  =  (2k /k )*(M)*(k (M) 1  6  2  +• k ( H S ) ) 4  2  Therefore hydrogen s u l f i d e w i l l c a t a l y z e the f o r m a t i o n o f mercaptan i f the CH^S* r a d i c a l s r e a c t w i t h hydrogen s u l f i d e as f a s t o r f a s t e r than w i t h t h e d i s u l f i d e .  Both e n e r g e t i -  c a l l y and. s t e r i c a l l y t h e r e a c t i o n w i t h hydrogen s u l f i d e i s favoured.  The S-H bond i n hydrogen s u l f i d e has a d i s s o -  c i a t i o n energy  o f about 88 k.cal./mole  (see Appendix I I ) ,  whereas t h e C-H bond i n methyl d i s u l f i d e probably has a d i s s o c i a t i o n energy o f the o r d e r o f t h e f i r s t hane, 101 k . c a l s . be g r e a t e r than k  C-H bond i n met-  T h e r e f o r e , from t h i s cause a l o n e , k^ should 2  by a f a c t o r o f approximately  approximately 100 a t 316° G.  e-^/RT^  o  r  Thus a s m a l l c o n c e n t r a t i o n o f  hydrogen s u l f i d e could have a r e l a t i v e l y l a r g e e f f e c t on the r e a c t i o n r a t e .  I t w i l l be r e a l i z e d t h a t one o r o t h e r  r e a c t i o n o f C ^ S ' r a d i c a l s must be predominant i f t h e r e a c t i o n i s t o have a d e f i n i t e a c t i v a t i o n  energy.  Since the observed r e a c t i o n i s probably f i r s t  order,  the chain t e r m i n a t i o n step must be chosen a c c o r d i n g l y . The l a t e r more complex: r e a c t i o n s i n t h e thermal decom-  149.  p o s i t i o n o f methyl d i s u l f i d e , r e a c t i o n s which  produce  hydrogen..sulfide and o t h e r p r o d u c t s , rmay i n v o l v e f r e e r a d i c a l and m o l e c u l a r p r o c e s s e s .  both  At p r e s e n t these  r e a c t i o n s are too obscure t o a l l o w worthwhile  speculation  as t o t h e i r p o s s i b l e mechanisms. I t i s r e a s o n a b l e t o assume t h a t the thermal decomp o s i t i o n o f e t h y l d i s u l f i d e i s a l s o i n i t i a t e d by a s p l i t at t h e S-S bond.  However, one would expect t h e subsequent  r e a c t i o n s t o be d i f f e r e n t from those o f t h e methyl decomposition.  disulfide  F o r example t h e b e l l o w i n g r e a c t i o n s a r e  conceivable: (14)  GH CH S*  (15)  HS»  (16)  CH GHSSGH CH  (17)  HS.  (18)  C.H SS-  3  2  +  3  CH CH SSCH CH3 3  2  +  >  :  •;.  3  CH CH SSCH CH 3  2  2  2  -->  2  2  GH =CH . +  —  3  C  HS  2  CH CHSSCH CH 3  2  3  +  HS 2  .»  CH CH=S  »  C^SH  +  C^SS-  »  C_H -  +  S«=S  3  C  +  ;  C^CHgS-  Reactions such as these would account f o r t h e observed  dif-  f e r e n c e s between t h e thermal decomposition o f methyl and ethyl d i s u l f i d e s . kin,  I t i s i n t e r e s t i n g t h a t F r a n k l i n and Lum-  i n t h e i r e l e c t r o n bombardment s t u d i e s , o b t a i n e d r e s u l t s  which suggest t h a t the C^H^S* r a d i c a l decomposes spontaneously whereas the CR^S* r a d i c a l i s s t a b l e . s t u d i e s o f Thompson, e t a l . , 4 , 2 3 2  a  r  e  also  The p h o t o l y t i c  significant.  Suggestions f o r f u r t h e r work The a n a l y s i s t h a t we have made o f t h e d e s c r i p t i v e nature o f the r e a c t i o n suggests the course t h a t s h o u l d be f o l l o w e d  150.  i n f u r t h e r study. a complete  The i n v e s t i g a t i o n must be d i r e c t e d t o  s e p a r a t i o n of the v a r i o u s processes which com-  pose the o v e r a l l r e a c t i o n and. t o a thorough of each.  understanding  I t i s only necessary t o add a few s p e c i f i c  sug-  g e s t i o n s o f the more important work which should be done. F i r s t l y , and most o b v i o u s l y , the r e a c t i o n  mixture  should be analysed f o r t r i - or p o l y - t h i o f o r m a l d e h y d e s . s o l i d sampling method must be  A  used.  The o r d e r and r a t e constants o f the primary r e a c t i o n should be determined investigation. determined.  at v a r i o u s temperatures  by  analytical  The f a c t o r s which a f f e c t i t s r a t e must be  For example, an a n a l y t i c a l study should be  made of the e f f e c t o f hydrogen s u l f i d e on the primary r e action. A more a c c u r a t e pressure-time curve i s needed f o r the disulfide. improved  The accuracy o f the d i s u l f i d e a n a l y s e s c o u l d be  by c o r r e c t i n g f o r the hydrogen s u l f i d e formed i n  the r e d u c t i o n s .  Another  approach would be t o c a l c u l a t e the  curve f o r d i s u l f i d e decomposed from the curves f o r the products formed, and t o use t h a t curve, t o g e t h e r w i t h a knowledge of the r a t e s o f a d s o r p t i o n and d e s o r p t i o n o f d i s u l f i d e , t o obt a i n the curve f o r d i s u l f i d e p r e s e n t .  The curve obtained  x o u l d be checked a g a i n s t experimental r a t e laws f o r the  de-  composition. Accurate pressure-time curves are needed f o r carbon d i s u l f i d e and f o r hydrocarbons.  Carbon d i s u l f i d e can be d e t e r -  mined e a s i l y and a c c u r a t e l y by one o f the xanthate methods.^-  151. Hydrocarbons £ould be analysed q u a l i t a t i v e l y and q u a n t i t a t i v e l y i f the s u l f u r compounds were removed from the mixture f i r s t .  The s e p a r a t i o n could be done by a  combination  o f gas f r a c t i o n a t i o n and chemical treatment f o r hydrogen sulfide.  With these pressure-time curves i t would be  pos-  s i b l e t o separate the two r e a c t i o n s producing hydrogen s u l f i d e and t o understand them both d e s c r i p t i v e l y . Reactions should be done w i t h other added p r o d u c t s . Free s u l f u r , and t r i t h i o f o r m a l d e h y d e would be the most interesting.  An " i n j e c t i o n run", w i t h the added r e a c t i o n  mixture corresponding t o the end of. the i n d u c t i o n p e r i o d , could p r o v i d e i n f o r m a t i o n about the nature o f the i n d u c t i o n period.  As f a r as p o s s i b l e these experiments  f o l l o w e d by a n a l y s i s .  should be  I f they are not., i t i s i m p o s s i b l e t o  put an u n e q u i v o c a l i n t e r p r e t a t i o n on t h e . r e s u l t s .  Reactions  should a l s o be done i n the presence o f propylene t o g a i n p o s s i b l e evidence f o r f r e e r a d i c a l chains i n the r e a c t i o n . A study o f the p y r o l y s i s o f t r i t h i o f o r m a l d e h y d e c o u l d l e a d t o i n t e r e s t i n g r e s u l t s about the mechanisms o f format i o n of carbon d i s u l f i d e i n the p y r o l y s i s of s u l f u r compounds.  In r e l a t i o n t o the present problem  i t would throw  l i g h t on the r e a c t i o n s p r o d u c i n g hydrogen s u l f i d e .  A kineti  study of the a d d i t i o n of s u l f u r t o d i s u l f i d e s would a l s o be r e l a t e d t o the present i n v e s t i g a t i o n .  Such a study would  be p a r t i c u l a r l y i n t e r e s t i n g i n view of the p r e s e n t sparse knowledge about  polysulfides.  In view of the complexity of the thermal  decomposition  152.  of methyl d i s u l f i d e , i t s f u r t h e r i n v e s t i g a t i o n w i l l have a limited interest.  The primary d i s s o c i a t i o n r e a c t i o n o f the  d i s u l f i d e molecule  can be s t u d i e d more e a s i l y by the t o l u e n e  c a r r i e r gas t e c h n i q u e . the behavior of the S other methods.  x  The f o r m a t i o n o f p o l y s u l f i d e s ,  and  c h a i n can be s t u d i e d more e a s i l y  by  N e v e r t h e l e s s , the f u r t h e r i n v e s t i g a t i o n of  the decomposition  c o u l d prove worthwhile.  The primary r e -  a c t i o n producing mercaptan i s probably not v e r y  complicated.  I f the cause o f the i n d u c t i o n p e r i o d can be understood,  and  i f the dependence o f the r a t e on the v a r i o u s products (probably on hydrogen s u l f i d e and p o s s i b l y on others) can be understood,  i t should be p o s s i b l e t o a s s i g n a f a i r l y  d e f i n i t e mechanism t o t h i s r e a c t i o n .  The knowledge t h a t  >. would f o l l o w o f the fundamental r a t e constants o f r e a c t i o n s between o r g a n i c s u l f u r f r e e r a d i c a l s and molecules g r e a t l y advance our knowledge o f o r g a n i c s u l f u r Again, a d e s c r i p t i v e account  would  chemistry.  o f the p e c u l i a r r e a c t i o n s p r o -  ducing hydrogen s u l f i d e would be o f i n t e r e s t ; i t has been p o i n t e d out t h a t t h i s d e s c r i p t i o n could be obtained  fairly  easily. At the present time t h e r e are a great many simpler s y s tems o f o r g a n i c s u l f u r compounds which could be siidied w i t h profit.  The f o l l o w i n g r e a c t i o n s m e r i t i n v e s t i g a t i o n .  They  might w e l l be l e s s complex than the thermal decomposition methyl d i s u l f i d e . (2)  (1)  The p y r o l y s i s of methyl mercaptan.  The p y r o l y s i s o f higher mercaptans.  of d i s u l f i d e s —  of  (3)  The p h o t o l y s i s  t h i s r e a c t i o n a t low temperatures  should  153. prove l e s s complicated than t h e thermal r e a c t i o n . p y r o l y s i s o f hydrogen d i s u l f i d e —  (4)  The  the study could p o s s i b l y  be done i n quartz t o a v o i d c a t a l y t i c decomposition.  (5)  The  pyrolysis of trithioformaldehyde. I t would be extremely i n t e r e s t i n g t o i n v e s t i g a t e the p o s s i b i l i t y o f producing CH^S• f r e e r a d i c a l s i n q u a n t i t y by the decomposition o f metal mercaptides.  The r e a c t i o n s o f  these f r e e r a d i c a l s with v a r i o u s molecules c o u l d be s t u d i e d . I t i s t o be hoped t h a t t h e p y r o l y t i c and p h o t o l y t i c r e a c t i o n s o f o r g a n i c s u l f u r compounds w i l l be i n v e s t i g a t e d more e x t e n s i v e l y i n the f u t u r e . to  c o r r e l a t e the r e s u l t s o f independent  decide what fundamental At  I t w i l l be p o s s i b l e then investigations to  mechanisms make up these r e a d t i o n s .  present such knowledge i s u n c e r t a i n and fragmentary.  APPENDIX I  THE STRUCTURE OP DISULFIDES AND POLYSULFIDES Two types of structure can be conceived for a chain of sulfur atoms, f i r s t , the covalent straight chain structure -S-S-S-, and second, the branched chain structure -S-S(:S)with coordinate bonding of the side chain atoms.  According-  ly, the question must be asked of whether disulfides (R-S -R) 2  and polysulfides (R-S -R) have the straight chain constitux  tion or whether their chains may be branched i n various degrees. In the case of the disulfides the evidence is completely in accord with the straight chain formulation R.-Sr-S-R. Both the parachor and the molar refraction of hydrogen d i sulfide have been found to agree with the structure H-S-S-H. Similarly, the parachor for ethyl disulfide has been found to agree with the straight chain formulation Et-S-S-Et. ^ 5  In the Raman spectra of methyl d i s u l f i d e ' 5 5  5 6  and ethyl d i -  s u l f i d e * e e r t a i n lines have been attributed to the 57  58  bond. Venkateswaran and Pandya at 506 cm."  1  57  -S-S-  connected the intense lines  (polarized)and 189 cm.~l (depolarized) In the  155.  (  -j  spectrum o f e t h y l d i s u l f i d e w i t h the l i n e s 470 cm."-- "and 150 em.~l  1  o f the same p o l a r i z a t i o n types i n molten s u l f u r ;  they  c o n s i d e r them to be the valence and deformation f r e q u e n c i e s o f the S-S bond.  Gerding and W e s t r i e k  5 5  compared the spectrum  o f methyl d i s u l f i d e w i t h the s p e c t r a o f SOgClg, S 2 G 1 2 * (CHg)gS; they concluded t h a t methyl d i s u l f i d e has the s t r u c ture Me-S-S-Me and e x i s t s i n the c i s form. of d i p h e n y l d i s u l f i d e  (1.81 D.) agrees w i t h the s t r a i g h t c h a i n  s t r u c t u r e i f f r e e r o t a t i o n i s assumed. moments o f methyl d i s u l f i d e and p r o p y l d i s u l f i d e c h a i n formulation.®  The d i p o l e moment  59  S i m i l a r l y the d i p o l e  (1.95 D.), e t h y l d i s u l f i d e  (1.96 U),  (1.96 D.) are i n harmony w i t h the s t r a i g h t The s t r a i g h t c h a i n s t r u c t u r e s have been  0  deduced from c r y s t a l l o g r a p h i c evidence f o r d i p h e n y l and d i b e n zyl disulfides.  F i n a l l y , and most important, the s t r a i g h t  6 1  c h a i n s t r u c t u r e s have been determined  f o r hydrogen d i s u l f i d e ,  methyl d i s u l f i d e , and d i c h l o r o d i s u l f i d e by the e l e c t r o n f r a c t i o n method.  dif-  The S-S i n t e r a t o m i c d i s t a n c e s i n these com-  pounds are s i m i l a r to the S-S i n t e r a t o m i c d i s t a n c e i n rhombic sulfur ( S ) . 8  The  s t r a i g h t c h a i n f o r m u l a t i o n i s a l s o supported by chem-  i e a l evidence.  Consider the f o l l o w i n g t y p i c a l r e a c t i o n s .  (1)  Reduction:  Hg + RSSR  (2)  Oxidation:  3 Qg • RSSR  (3)  Addition:  PhCHCHg + BSSR  2 RSH •>  2 RSO3H  •> Ph-CH(-SR)-CHg (-SR)  I n each o f these r e a c t i o n s the d i s u l f i d e molecule  Is ' s p l i t '  i n t o two fragments.  c o n t a i n R-S  The f a c t t h a t both fragments  156.  bonds suggests t h a t the s t r u c t u r e o f the o r i g i n a l  disulfide  i s R-S:S-fi r a t h e r than R-S(:S)-R. The  evidence  shows c l e a r l y t h a t the common d i s u l f i d e s  have the s t r a i g h t c h a i n s t r u c t u r e .  The methyl d i s u l f i d e (a  s i n g l e pure isomer) which was p y r o l y s e d I n the present t i g a t i o n entered  the r e a c t i o n v e s s e l as  other hand the evidence  On the  CH3-S-S-CH3.  does n o t preclude  Inves-  the e x i s t e n c e o f  isomers o f branched s t r u c t u r e j i t i n d i c a t e s o n l y t h a t  they  must be l e s s s t a b l e than the s t r a i g h t c h a i n s t r u c t u r e s . of the measurements d e s c r i b e d were made on p u r i f i e d  Most  materials  from which t r a c e s o f a second isomer would have been removed. Moreover, the methods themselves would not have shown the presence o f a s m a l l percentage o f a second Isomer. t i c a l l y , o f course,  Theore-  the s t r a i g h t and branched c h a i n isomers  can e x i s t i n e q u i l i b r i u m ; the p r a c t i c a l q u e s t i o n o f the e x i s tence o f branched c h a i n d i s u l f i d e i s r e a l l y a q u e s t i o n , o f the magnitude o f the e q u i l i b r i u m c o n s t a n t .  I t should be empha-  s i z e d t h a t t h i s q u e s t i o n i s open. I t i s p o s s i b l e t h a t a t the e l e v a t e d temperatures o f the present  decomposition experiments the s t r a i g h t c h a i n methyl  d i s u l f i d e i n t r o d u c e d i n t o the r e a c t i o n v e s s e l rearranges a t a s i g n i f i c a n t r a t e to the branched c h a i n d i s u l f i d e . the f o l l o w i n g processes  must be considered  Indeed,  as p o s s i b l e  a t i o n steps f o r a f r e e r a d i c a l decomposition mechanism. CH -S-S-CH 3  3  -  = = = = =  *  R-S-R !• S  ------I  R-S-R  +  S:  initi-  157  Whether or n o t such r e a c t i o n s a c t u a l l y do occur could be determined by a complete  s o l u t i o n o f the d i s u l f i d e p y r o l y s i s  problem. The p o s s i b i l i t y of the i n t e r c o n v e r s i o n o f d i s u l f i d e s and monosulfides i s I n t e r e s t i n g i n i t s e l f .  I t i s w e l l known t h a t  chains o f s u l f u r atoms are l a b i l e , that p o l y s u l f i d e s and d i s u l f i d e s are r e a d i l y I n t e r c o n v e r t i b l e .  However, one would  expect t h e i n t e r c o n v e r s i o n o f d i s u l f i d e s and monosulfides t o occur w i t h more d i f f i c u l t y .  I t would be i n t e r e s t i n g t o heat  methyl monosulfide w i t h f r e e s u l f u r at 300 °C. In the a l k y l d i s u l f i d e s the f r e e r o t a t i o n about the S-S cc  bond appears t o be s e v e r e l y r e s t r i c t e d .  Gerding and W e s t r i c k  concluded from Raman s p e c t r a t h a t methyl d i s u l f i d e e x i s t s i n the c i s form: CH,-S CH -S 3  They found no evidence f o r the e x i s t e n c e even at e l e v a t e d Venkateswaran  temperatures.  and P a n d y a  57  o f the t r a n s  form  I n the case o f e t h y l d i s u l f i d e  s t a t e t h a t the number o f Raman l i n e s  observed can o n l y be e x p l a i n e d  by p o s t u l a t i n g the simulataneous  presence o f c i s and trans forms i n about e q u a l amounts.  Be-  cause they observed no wings accompanying the intense "S-S valence l i n e " they concluded that f r e e r o t a t i o n does not e x i s t In e t h y l d i s u l f i d e .  Prom completely independent d i p o l e moment  evidence, Kushner,Gorin,and  Smyth have concluded that r o t a t i o n  about the S-S bond i s s e v e r e l y r e s t r i c t e d  i n methyl, e t h y l ,  158. and p r o p y l d i s u l f i d e s , and i n methyl t r i s u l f i d e . which has been assigned  The c i s form  to methyl d i s u l f i d e suggests p o s s i b i l -  i t i e s f o r i t s thermal decomposition by a m o l e c u l a r mechanism: / \\  »  H C HJjCH 3  CH3SH  CH =S 2  2  P o l y s u l f i d e s o f an order h i g h e r f o r many y e a r s .  *  than two have been known  They are r e a d i l y i n t e r c o n v e r t i b l e by o v e r a l l  r e a c t i o n s o f the t y p e : R—Sx""*H + 6 5  Sy s^sasssaa-  R—Sx+y~R  In other words, the p o l y s u l f i d e chains  are l a b i l e .  In the ease o f dimethyl t r i s u l f i d e , MeSgMe, the r e c e n t evidence has almost u n i f o r m l y u l a t i o n Me-S-S-S-Me.  favored  Sidgwick,  5 2  the s t r a i g h t c h a i n form-  has pointed  out that i f the  c h a i n were branched the d i p o l e a t t r a c t i o n o f the coordinate l i n k would cause an abnormal r i s e i n b o i l i n g p o i n t along the s e r i e s Me S (38 °C.'), Me2S 2  2  T h i s does not o c c u r .  (109 ° C . ) ,  Butler.and  Maass  3 7  and Me S3 (170 ° C ) . 2  53  assigned  the s t r a i g h t  c h a i n s t r u c t u r e H-S-S-S-H to hydrogen t r i s u l f i d e on the b a s i s of i t s parachor and molar r e f r a c t i o n . assigned  Similarly, Baroni ^ 5  the s t r a i g h t c h a i n s t r u c t u r e t o e t h y l t r i s u l f i d e on  the b a s i s o f i t s p a r a c h o r .  DaW3on and R o b e r t s o n  the s t r a i g h t chain s t r u c t u r e o f b i s ( 2 - i o d o - e t h y l ) from the r e s u l t s o f e l e c t r o n d i f f r a c t i o n . and  Sehomaker  62  6 3  established trisulfide  F i n a l l y , Donohue  e s t a b l i s h e d the s t r a i g h t c h a i n s t r u c t u r e o f  methyl t r i s u l f i d e from the r e s u l t s o f e l e c t r o n d i f f r a c t i o n . Kushner, G o r i n ,  and S m y t h  60  reported  t h a t the d i p o l e moment o f  159. methyl t r i s u l f i d e  (1.66  D.)  unbranched c h a i n s t r u c t u r e The  has  a value  c o n s i s t e n t w i t h the  (upper l i m i t  D.).  2  p h y s i c a l evidence shows c l e a r l y t h a t a l k y l  trisul-  f ides have the s t r a i g h t c h a i n s t r u c t u r e i n the s t a b l e form. the other hand, chemical evidence i n d i c a t e s t h a t branched t r l s u l f i d e s do e x i s t , although t r a n s i e n t l y . R-S-S-R has  +  been shown to o c c u r ,  l a t e an intermediate The tain.  ->  S 6 5  and  branched  s t r u c t u r e s of h i g h e r  Bezzi  has  5  assigned  He  chain  reaction  R-S-S-S-R  I t i s o n l y reasonable to posutrisulfide. p o l y s u l f i d e s are much l e s s c e r - -  branched c h a i n c o n f i g u r a t i o n s to a  number o f p o l y s u l f i d e s , but h i s reasoning able.  The  i s highly  question-  p r o p e r l y s t r e s s e d the p o s s i b i l i t y of branched  and  s t r a i g h t c h a i n isomers e x i s t i n g i n e q u i l i b r i u m and p o i n t e d t h a t most methods would not show the presence of s m a l l t i o n s of a second isomer. ments B a r o n i  5 4  assigned  On  out  frac-  On the b a s i s of parachor measure-  s t r a i g h t c h a i n formulas to e t h y l p o l y -  s u l f i d e s c o n t a i n i n g from 1 to 3 and from 7 to 10 s u l f u r atoms; he  assigned  branched c h a i n formulas to h i s p e n t a s u l f i d e  hexasulfide,  and p o i n t e d  and  out t h a t the ease w i t h which the  l a t t e r l o s e f r e e s u l f u r confirms t h e i r c o n s t i t u t i o n .  The  s u l t s of Dawson, Mathieson and R o b e r t s o n  diffrac-  t i o n I n d i c a t e unbranched chains up bisulfonyl I t has labile  and  6 4  from X-ray  re-  to f i v e s u l f u r atoms i n  trisulfide. long been r e a l i z e d t h a t p o l y s u l f i d e chains interconvertible.  are  Some examples are o f i n t e r e s t s  160. Butler  and  Maass  observed that hydrogen t r i s u l f i d e  f r e e s u l f u r to form a compound  " H 2 S 3 ( 3 S ) " .  also dissolves  i t does not b i n d the  chemically.  f r e e s u l f u r but  Holmburg  66  can be  Hydrogen d i s u l f i d e  ammonia.  Olin  6 7  has  i n a sealed  stated  presence of c a t a l y t i c amounts of a b a s i c Westlake, Laquer, and  f i d e to the  Smyth  65  90 °C.  125  sulfur i n  the  Possibly  disul-  (Identified)  by  an amine at about  °C. f o r 5 h o u r s .  amine c a t a l y s i s i s obscure.  polysulfides  converted e t h y l  straight chain ethyl t r i s u l f i d e  f o r 3 hours and  tube w i t h  c a t a l y s t such as buty-  h e a t i n g the d i s u l f i d e w i t h f r e e s u l f u r and  the  that  produced by h e a t i n g d i s u l f i d e s w i t h f r e e  lamine.  sulfur  observed that e t h y l d i s u l f i d e g i v e s  h i g h e r order p o l y s u l f i d e s when t r e a t e d f r e e s u l f u r and  dissolves  The  i t has  nature to do  of  with  the f o r m a t i o n of atomic s u l f u r from Sg which would n o r m a l l y occur o n l y s l i g h t l y at 125 that  the  In any  case i t i s probable  same r e a c t i o n eould occur at e l e v a t e d  without the The  °C.  amine c a t a l y s t .  reverse reactions  have a l s o been observed.  obtained d i s u l f i d e s from the sulfides.  temperatures  Baroni  5 4  states  destructive  Twiss ® 6  d i s t i l l a t i o n of p o l y -  that h i s e t h y l pentasulfide  and  52  hexasulfide  lose f r e e s u l f u r r e a d i l y .  mercury removes f r e e  Sidgwiek  records  s u l f u r from a t e t r a s u l f i d e l e a v i n g  that  the  disulfide. HS R 4  +  Hg'  »  RS a 2  Thus the f o r m a t i o n of p o l y s u l f i d e s  +  2  HgS  i n the  thermal decom-  p o s i t i o n of methyl d i s u l f i d e i s completely r e a s o n a b l e .  Poly-  161. sulfides  could  be f o r m e d  by a d d i t i o n  to the remaining d i s u l f i d e . of  of the free  sulfur  formed,  The f o r m a t i o n o f HgS o n t r e a t m e n t  t h e g a s s a m p l e d p r o d u c t s w i t h m e t a l l i c m e r c u r y may now be  regarded  as p r o o f o f t h e p r e s e n c e  of polysulfides.  This c l e a r evidence o f the l a b i l e chains suggests that have e x i s t e d  a v e r y complex steady steady s t a t e  i n the methyl d i s u l f i d e  tween p o l y s u l f i d e s various orders.  nature of polysulfide may  decomposition system be-  o f v a r i o u s degrees  o f b r a n c h i n g and o f  R e a c t i o n s s u c h as t h e f o l l o w i n g may be  ima-  gined: (1)  R-S-S-R  (2)  RS -S-S-SyR  +  S  x  -  =  -r==i  x  =  ri  RS  x +  gR  RS -S-S R x  RS -S-S R  y  x  +  y  S:  S (3)  RS R  *  X  S  ^= ==5  2  RS R  ==*  I2 X  S  RS Such r e a c t i o n s  could  produce  atomic  x +  RS . R x4  iR  x  + S:  s u l f u r more  \ RS  X4>  2R  efficiently  than the d i r e c t d i s s o c i a t i o n o f Sg. In conclusion polysulfides this field  i t may be e m p h a s i z e d  i s r e l a t i v e l y unexplored.  would  be b o t h I n t e r e s t i n g  that  the chemistry of  Investigations  and u s e f u l .  i n  APPENDIX I I  THE BONDING AND BOND PROPERTIES OP ORGANIC SULFUR COMPOUNDS In t h i s d i s c u s s i o n  we are concerned p r i m a r i l y  w i t h the  d i s s o c i a t i o n energies i n o r g a n i c s u l f u r compounds.  Most o f  a l l we wish t o know the C-S and S-S bond d i s s o c i a t i o n i n methyl d i s u l f i d e . C-S,  energies  Values f o r the d i s s o c i a t i o n energies o f  S-S, and H-S bonds are almost non e x i s t e n t ,  but an under-  s t a n d i n g o f the bonding i n organic s u l f u r compounds w i l l enable us to draw some u s e f u l  c o n c l u s i o n s from the v a l u e s  that  we do have. We s t a r t by c o n s i d e r i n g X-S bonds from an atomic o r b i t a l p o i n t o f view. configuration  The s u l f u r atom i n i t s ground s t a t e has the ls 2s 2p 3s 3p . 2  2  5  2  When I t i s combined  4  c u l e by two s i n g l e bonds the 3p l e v e l i s f i l l e d . further  bonding i s p o s s i b l e .  i n a mole-  However,  The e x i s t e n c e o f the s u l f o x i d e s  and branched c h a i n p o l y s u l f i d e s  t e s t i f i e s that  the c o v a l e n t l y  bonded s u l f u r atom -S- w i l l form coordinate bonds r e a d i l y w i t h other atoms.  F u r t h e r , i t appears t h a t  s u l f u r i s able t o make  use o f the 3d l e v e l s f o r bonding. The e f f e c t o f the s u l f u r d l e v e l s i s t o give a C-S bond  163.  a p a r t i a l double bond c h a r a c t e r and to give an a c i d i c to the hydrogen atoms attached  to the carbon.  character  A significant  69 example has been d i s c o v e r e d  by R o t h s t e i n .  When H C l i s s p l i t  out from V-chloro-ofot-bis ( e t h y l t h i o ) propane, the double bond appears i n the o( I  3  position:  Cl-CH -CH -CH(-SEt) 2  2  >  2  CH -CH-CH(-SEt,.) 3  2  •  HCl'''  j- .  The  analogous oxygen gives the double bond i n the normal. P V  position.  Any e x p l a n a t i o n o f these  l y f o r the opposite e f f e c t s for  "  results  must account  o f oxygen and s u l f u r ,  first-  and secondly  the t r a n s m i s s i o n of the e f f e c t to the C-H bond.: To e x p l a i n  these  results  on the valence  bond theory  i t i s necessary t o  c o n s i d e r the f o l l o w i n g c a n o n i c a l s t r u c t u r e s : . SEt. .S"Et . C1-CH -CH -CH< C1-CH -CH -C . .' SEt SEt I II H  2  The  2  significant  levels.  2  2  s t r u c t u r e I I must make use o f the s u l f u r d  Since the analogous oxygen compound has no 2d l e v e l s ,  i t could not show t h i s type o f resonance. that the r e l a t i v e l y h i g h energy d l e v e l s  To the o b j e c t i o n could not be i n v o l v e d  i n resonance one can o n l y p o i n t out that the acidic.  Perhaps h y b r i d o r b i t a l s  hydrogen i s  are i n v o l v e d .  In r e p r e s e n t i n g the s t r u c t u r e o f many compounds c o n t a i n i n g sulfur  i t i s necessary  showing =S-. disulfides.  to c o n s i d e r v a r i o u s c a n o n i c a l s t r u c t u r e s  In t h i s l i g h t  l e t us examine the s t r u c t u r e o f  We may w r i t e the f o l l o w i n g types  s t r u c t u r e s f o r methyl d i s u l f i d e :  of canonical  164 .  I.  CH -S-S-CH  II.  CH -S-S-CH  H  +  2  CH §-S CH  H  +  2  H  +  III. IV.  3  3  H  +  2 :  =  3  H*" GHo-SzS-CHo  L.  J  A s i g n i f i c a n t c o n t r i b u t i o n from s t r u c t u r e  I V . would account  f o r the absence o f f r e e r o t a t i o n i n d i s u l f i d e s ; i t would exp l a i n the r e p o r t  that methyl d i s u l f i d e e x i s t s completely i n  the c i s form (see Appendix I ) .  However, we would not expect  the doubly e x c i t e d s t r u c t u r e o f types I I I and I V to be r e l a t i v e l y important.  Any e f f e c t o f the resonance on the d i s s o c i -  a t i o n energies would be expected to a r i s e p r i m a r i l y from the c o n t r i b u t i o n o f the s i n g l y e x c i t e d s t r u c t u r e s o f type I I . A s i g n i f i c a n t c o n t r i b u t i o n would cause a weakening o f the S-S bond r e l a t i v e to what we may c a l l a pure S-S s i n g l e bond due to e l e c t r o s t a t i c r e p u l s i o n o f the d e l t a - n e g a t i v e s u l f u r atoms. The d e r e a l i z a t i o n energy would not o f f s e t t h i s weakening. For hydrogen d i s u l f i d e we may c o n s i d e r the f o l l o w i n g canonical  s t r u c t u r e s which make use o f the d l e v e l s . I.  H-S-S-H  II.  H* S=S-H  III. Structures  H* S=S H*  o f type I I . would be more important than s t r u c t u r e s  of type I I I .  Structures  such as  IV.  H  +  :S-S-H  V.  H  +  :S-S: H  +  165.  are  not considered important.  I n the f i r s t  e l e c t r o n e g a t i v i t i e s o f hydrogen In  t h e second p l a c e , s t r u c t u r e  place the Pauling  and s u l f u r a r e a l m o s t  equal.  7 0  I I . s h o u l d be o f l o w e r e n e r g y  t h a n s t r u c t u r e I V . due t o ( 1 ) e x t r a b o n d i n g , a n d ( 2 ) d e r e a lization.  A p p r e c i a b l e c o n t r i b u t i o n o f s t r u c t u r e s o f type I I .  s t r e n g t h e n t h e S-S b o n d r e l a t i v e  would  t o a " p u r e S-S  single  bond". It w i l l S-S  be r e a l i z e d  t h a t we h a v e b e e n r e f e r r i n g t o a " p u r e  s i n g l e b o n d " s i m p l y a s a n S-S b o n d i n w h i c h t h e r e i s no  r e s o n a n c e o f t h e t y p e we h a v e b e e n c o n s i d e r i n g . s u c h a bond t o e x i s t  We may i m a g i n e  i n a n u n s t r a i n e d r i n g o f s u l f u r atoms b e -  c a u s e i n s u c h a s y s t e m t h e r e w o u l d be no e l e c t r o n s t o e n t e r t h e d orbitals.  ( I t i s assumed t h a t t h e l o n e p a i r e l e c t r o n s a r e  indeed lone p a i r s ) . f o u n d i n Sg.  The c l o s e s t  approach t o such a system i s  We may d e f i n e a " p u r e S-S s i n g l e b o n d  dissoci-  a t i o n e n e r g y " as t h e e n e r g y w h i c h w o u l d be, r e q u i r e d t o d i s s o c i ate  one b o n d i n t h e Sg r i n g i f t h e two u n p a i r e d e l e c t r o n s o n  the  r e s u l t a n t d i r a d i c a l remained  fur  atoms.  tified  sul-  T h i s h y p o t h e t i c a l d i s s o c i a t i o n e n e r g y may be i d e n -  approximately w i t h one-eighth the heat o fformation.of  Ss f r o m atoms, In  l o c a l i z e d on the t e r m i n a l  1/8HJ,(Ss).  a c t u a l f a c t we w o u l d n o t e x p e c t t h e e l e c t r o n s t o r e m a i n  l o c a l i z e d o n t h e Sg d i r a d i c a l .  R a t h e r , we w o u l d  enter a d l e v e l molecular o r b i t a l , large decrease i nenergy.  e x p e c t them t o  some 16 a . u . l o n g w i t h a  T h i s c o n s i d e r a t i o n e x p l a i n s why t h e  27.5 _+ 5 k . c a l . / m o l e a c t i v a t i o n e n e r g y w h i c h h a s b e e n  identi-  166.  71 fled  with  lower  the p r o c e s s  S (ring) 8  l/8 HJ,(Ss),  than  > S  51  (diradical)  8  t o 63 k . c a l . / m o l e .  The  t r a o r d i n a r y d i f f e r e n c e b e t w e e n these, two  values  the  of  3d  levels  do  Consider  It  i n f l u e n c e the b e h a v i o u r  the  i s reasonable  >  2 CH -S«  H-S-S-H  >  2  With t h i s  assumption  t i o n s we  may  the  w i t h the p r e v i o u s  of methyl d i s u l f i d e  of e t h y l d i s u l f i d e . same o r d e r  on  the  the h e a t  sulfur  resonance (1)  s h o u l d be  atoms.  considera-  The less  (2) B o t h d i s s o c i a t i o n e n e r g i e s  as o n e - e i g h t h  bond:  e l e c t r o n s i n the  draw the f o l l o w i n g c o n c l u s i o n s :  d i s s o c i a t i o n energy  S-S  HS-  r a d i c a l s remain l o c a l i z e d and  that  3  t o assume t h a t t h e u n p a i r e d  free  ex-  shows a g a i n  a t the  C H 3 - S - S - C H 3  HS»  otherwise  sulfur.  d i s s o c i a t i o n of d i s u l f i d e s  CH^S* and  i s much  S-S  bond  than  that  s h o u l d be  o f f o r m a t i o n o f Sg  of  from  atoms. U n f o r t u n a t e l y t h e b a s i 3 o f the sonance i s f a r from tion  sound.  o f the g r a d a t i o n o f S-S  I f we  theory of i o n i c - c o v a l e n t  are  to apply  bond e n e r g i e s  i t to the  s h o u l d p r o p e r l y c o n s i d e r a number o f complex f a c t o r s .  it  may  into valid tive  valid  to separate  the  consideration of a  c o n s i d e r a t i o n of c o n s t i t u e n t bonds. to separate and  the  repulsive  amount o f m i x i n g vary  in different  will  affect  both  energy  o f these  components.  between t h e  Third,  lone p a i r  estima-  i n s e v e r a l compounds  we  n o t be  re-  First,  molecule  S e c o n d , i t may  bonds i n t o v a r i o u s  not  be  attrac-  and more s e r i o u s , t h e and  bonding  orbitals  compounds a c c o r d i n g t o t h e bond a n g l e s ; the c o v a l e n t a t t r a c t i o n s  and  the  one  pair  may  this re-  167.  pulsions.  Fourth, the amount o f h y b r i d i z a t i o n may  vary from  one  c a n o n i c a l s t r u c t u r e (of a s i n g l e compound) to another; t h e r e f o r e we  can not a s s i g n the o r b i t a l s to the r e s u l t a n t moleeule.  Fifth  i t can not be assumed t h a t the i o n i c o r b i t a l s are the same as the atomic o r b i t a l s ;  Indeed, the nature o f the i o n i c  orbitals  and the r o l e they may  p l a y i n resonance  These v a -  are obscure.  r i o u s c o n s i d e r a t i o n s e f f e c t i v e l y prevent a q u a n t i t a t i v e use the t h e o r y at the present time.  of  Indeed, they c a s t doubt upon  even q u a l i t a t i v e p r e d i c t i o n s o f the t h e o r y .  However, i t has  been found i n p r a c t i c e that the q u a l i t a t i v e theory, used  as a  set of e m p i r i c a l r u l e s , does make c o r r e c t p r e d i c t i o n s i n m a n y cases.  Therefore we hope t h a t our p r e d i c a t i o n s have some  validity. A second f a c t o r t h a t we S-S  should c o n s i d e r when comparing the  bond energies i n v a r i o u s compounds ,1s the " I n d u c t i v e e f f e c t "  of the groups attached to the s u l f u r atoms.  In the u s u a l i n t e r -  p r e t a t i o n o f t h i s e f f e c t i n a bond X-»S where X tends to "donate e l e c t r o n s , we may  say f i r s t ,  t h a t the e l e c t r o n s i n t h i s bond are  shared u n e q u a l l y , and second, t h a t the r e s u l t a n t e f f e c t on the charge  d i s t r i b u t i o n i s t r a n s m i t t e d In some measure to the r e s t  of the molecule.  (This concept  different molecules).  The  does have meaning when comparing  i n f l u e n c e of the i n d u c t i v e effec.t' on  bond d i s s o c i a t i o n e n e r g i e s i s not at a l l c l e a r . the symmetrical  d i s u l f i d e molecule  opposite e f f e c t s . molecular o r b i t a l s .  In the case of  S->S-S«-X there must be  two  Consider the bonds as composed o f l o c a l i z e d (This i s not i n harmony w i t h the p r e v i o u s  168.  c o n s i d e r a t i o n s of d e r e a l i z a t i o n , lem).  The  X-S  but  m o l e c u l a r o r b i t a l s are  t e r of the molecule by  the  I t i l l u s t r a t e s the distorted  inductive e f f e c t .  atoms should weaken the molecule w i t h no  distorted  two  sulfur  strengthened.  I t i s not  but  i t w i l l be  noted that the  net  e f f e c t should be In the  type we  of the  two  bond i n a molecular mole-  d e n s i t y of bonding  effects  the  S-S  bond w i l l  tend to c a n c e l .  there can  be  no  considered f o r d i s u l f i d e s .  an i n d i c a t i o n  energy of the  i s g r e a t e r than t h a t of any  Therefore  0-0  I t has  of the  the  direction  been e s t a b l i s h e d  bond i n hydrogen perox-  a l k y l p e r o x i d e , and  i t would  i n d u c t i v e e f f e c t tends  make the  S-S  energy of the  i n the  to  bond i n hydrogen d i s u l -  f i d e g r e a t e r than that i n methyl d i s u l f i d e . t i v e e f f e c t should be  The  resonance  seem reasonable to suppose that the dissociation  be  e n e r g i e s between hydrogen peroxide  inductive effect i n d i s u l f i d e s . dissociation  S-S  sulfur  c l e a r which e f f e c t w i l l predominate,  a l k y l peroxides should be  that the  S-S  pre-  small.  gradation i n dissociation the  two  c e n t e r of the  atoms and  organic peroxide s e r i e s  e f f e c t of the  ide  towards the  T h i s d i s t o r t i o n w i l l i n c r e a s e the  e l e c t r o n s between the  and  However the  cen-  resulting  the  bond r e l a t i v e to the  inductive e f f e c t .  o r b i t a l w i l l a l s o be cule.  S-S  towards the  The  sence of antibonding d e l t a - n e g a t i v e charges on  prob-  same d i r e c t i o n  Thus, the as the  induc-  resonance  effect. The equal.  electronegativities I t f o l l o w s that the  of hydrogen and  s u l f u r are  i n d u c t i v e e f f e c t can  not  about  cause  the  169.  d i s s o c i a t i o n energy  of the S-S bond i n hydrogen d i s u l f i d e  to be  v e r y d i f f e r e n t from t h a t o f a "pure S-S s i n g l e bond". We conclude t h a t the d i s s o c i a t i o n energy  o f the S-S bond i n  methyl d i s u l f i d e should be l e s s than that o f the S-S bond i n hydrogen d i s u l f i d e . D(CH S-SCH ) 3  <  3  D(HS-SH)  I f 1/8 H|,(S8) can be i d e n t i f i e d with the "pure S-S s i n g l e bond d i s s o c i a t i o n energy,  then D(CH S-SCH ) and D(HS-SH) should be o f  the order o f l / 8 H J ( S g ) .  3  3  •  We may now draw some c o n c l u s i o n s from the r a t h e r d o u b t f u l experimental values f o r the d i s s o c i a t i o n e n e r g i e s o f bonds containing sulfur. quoted  In t h i s d i s c u s s i o n most o f the energy  r e f e r to 25 °C.  A c t u a l l y we are i n t e r e s t e d i n v a l u e s at  0 °K., but no l a r g e e r r o r i s I n v o l v e d . and H^O  values  The symbols D(), H ^ ( ) ,  r e f e r r e s p e c t i v e l y to-the d i s s o c i a t i o n energy,  the heat  of f o r m a t i o n i n the standard state from elements i n t h e i r s t a n dard s t a t e s , and the heat o f f o r m a t i o n from gaseous atoms. We mention f i r s t o f . a l l the u n c e r t a i n t y i n the d i s s o c i a t i o n energy o f Sg. k. cal./mole ."^  The p r i n c i p a l p o s s i b i l i t i e s The thermochemical /  h i g h value o f 101 k. cal./mole.  t a b l e o f the Bureau o f Stan74  dards use the value 76 k. c a l . / m o l e .  are 76, 81, and 101  7D  P a u l i n g ' has used the u  The evidence up to 1947 f o r  these values has been d i s c u s s e d by Gaydon ^ who f a v o u r s the h i g h 7  rye  value.  I t may be added to h i s d i s c u s s i o n t h a t P o r t e r ' s  recent  s p e c t r o s c o p i c value o f 85 k. cal./mole f o r D(H-S*) a l s o favours the h i g h value o f 101 k. c a l s .  170. The uncertainty i n D(S ) results i n an uncertainty i n the 2  heat of atomization of sulfur, equal i n this case to HI(S ). 8  <s  ,  The p r i n c i p a l p o s s i b i l i t i e s are 51, 53 and 63 k. cal./mole. This uncertainty makes i t impossible to calculate the true heats of formation from atoms of radicals and molecules containIng s u l f u r .  It i s not possible, f o r example, to determine  D(H-S») from a knowledge of D(HS-H).  "  '-'  t  The values of some C-S, S-S and S-H bond energies have been 3 reported i n a recent paper by Franklin and Lumpkin. tron Impact method was used to determine of the HS-,  The elec-  the heats of formation  CH S-, and C H S- radicals,;. and from these values 3  2  5  and known thermodynamic data a number of i n t e r e s t i n g . d i s s o c i ation energies were calculated.  Some of t h e i r values are repro-  duced i n Table XVII . The value of 80.4 k.eal. given by Franklin and Lumpkin f o r D(HS-SH) was not i n accordance with t h e i r data.  Apparently they  neglected to include the latent heat of vaporization i n the standard heat of formation of gaseous hydrogen d i s u l f i d e . have recalculated D(HS-SH) from t h e i r data to be 72  We  approximately  k.cal./mole. Franklin and Lumpkin stated that t h e i r data favours the high  value of 101k.cal. f o r D ( S ) . 2  Curiously, however, they c a l c u l a -  ted D(H-S) and D(CH -S») on the basis of D(S ) 3  cal./mole.  2  equal to 76 k.  I f f o r consistency, they had used the high value,  they would have obtained the values: D(CH3-S')  -  63 k. cal./mole  D(H-S«)  =  78 k. cal./mole  171  which are 11 k. c a l . h i g h e r than the v a l u e s they r e p o r t e d . A l l o w i n g f o r the i n a c c u r a c i e s o f the e l e c t r o n impact method, t h i s value o f D(H-S») i s i n f a i r of  85 k. c a l .  agreement w i t h P o r t e r ' s value  Therefore i t i s d i f f i c u l t  t o comprehend  view t h a t P o r t e r ' s value must be l a r g e l y i n e r r o r .  their  Apparently  t h e i r o n l y reason f o r t h i s view i s t h e i r i n c o n s i s t e n t  assum-  p t i o n o f D(S2)= 76 k, cal./mole.  TABLE XVII D i s s o c i a t i o n E n e r g i e s i n S u l f u r Compounds.  Bond Type  Compound  S-H  H S  '.  2  D i s s o c i a t i o n Enea k.cal./mole .  95.3  CH SH  88.8  C H SH  86.8  CH SH  74.2  C H SH  73.4  CH3SCH3  73.2  CH3SSCH3  73.2^  3  2  C-S  5  3  2  •  S-S  5  C H SSC H 2  5  2  5  70.0  HSSH  72  S  76, 81, 101  2  (80.4)  S-H  HS- r a d i c a l  67.0  C-S  CH S r a d i c a l  52.4  # - estimated x - recalculated  3  X  172.  F r a n k l i n and Lumpkin d i d not o b t a i n d a t a f o r the  direct  c a l c u l a t i o n of the C-3' bond d i s s o c i a t i o n energy i n methyl d i sulfide.  However they p o i n t e d out t h a t the C-S  bond d i s s o c i -  a t i o n energies that were c a l c u l a t e d d i d not seem to be much a f f e c t e d by s t r u c t u r e . that the C-S  Therefore  i t was  reasonable  d i s s o c i a t i o n energy i n methyl d i s u l f i d e i s about  the same as t h a t i n methyl s u l f i d e , about 73 k. Although  to assume  cal./mole.  these r e s u l t s of F r a n k l i n and Lumpkin are u s e f u l  as a b a s i s f o r e s t i m a t i n g the r e l a t i v e magnitudes o f v a r i o u s d i s s o c i a t i o n e n e r g i e s , the r e s u l t s themselves can not be ed as a c c u r a t e .  regard  I t i s w e l l known t h a t the e l e c t r o n impact  me-  thod g i v e s o n l y an upper l i m i t to the bond d i s s o c i a t i o n energy; even when the r e l e v a n t p o t e n t i a l curves  are known the  possibi-  l i t y of n o n - v e r t i c a l e x c i t a t i o n makes the i n a c c u r a c y i n d e t e r minate.  I t i s t r u e , as F r a n k l i n and Lumpkin s t a t e , t h a t i n  p r a c t i s e the e l e c t r o n . i m p a c t method has g i v e n r e s u l t s which agree w e l l w i t h the r e s u l t s o f other methods.  However I t must  be pointed, out t h a t such " s a t i s f a c t o r y r e s u l t s " have been obt a i n e d mainly w i t h carbon compounds; there i s no b a s i s f o r assuming t h a t they w i l l a l s o be obtained w i t h s u l f u r compounds. F o r t u n a t e l y some thermal d a t a i s a v a i l a b l e on the heat f o r m a t i o n o f the HS«  radical.  From a study of the p y r o l y s i s  methyl mercaptan In t o l u e n e , Darwent and S e h o n value H (•SH) - 32 ± 2 k. cal./mole. f  89 i  2 k. cal./mole.  Although  of  7 6  obtained  of  the  T h i s leads to D(HS-H) -  there were c o m p l i c a t i o n s i n the  k i n e t i c s , Darwent b e l i e v e s t h a t t h i s value i s c l o s e r to the  173.  t r u t h than t h a t o f F r a n k l i n and Lumpkin (95 k. c a l . ) .  In I t s  support we would p o i n t out t h a t 32 ± 2 f o r Hf(«SH) leads t o a value o f 86 ± 2 k. c a l s . f o r D ( S ) .  T h i s value o f 86 k. e a l s .  2  Is I n e x c e l l e n t agreement w i t h the 85 k. c a l s . found s p e c t r o s c o p i c a l l y by P o r t e r . From the value Hf('SH) = 32 k. c a l s . , t o g e t h e r w i t h the standard heat o f f o r m a t i o n o f HgSg?  4  the d i s s o c i a t i o n energy o f  the S-S bond i n hydrogen d i s u l f i d e may be c a l c u l a t e d t o be  D(HS-SH) = 59 i 4 k. cal./mole. In our p r e v i o u s c o n s i d e r a t i o n s o f the bonding  i n hydrogen  d i s u l f i d e and methyl d i s u l f i d e we p o i n t e d out t h a t D(CH 3 S-SCH 3 ) should be l e s s than D(HS-SH).  A c c o r d i n g l y we w i l l  that the S-S bond d i s s o c i a t i o n energy about 55 k. cal./mole.  estimate  i n methyl d i s u l f i d e i s  T h i s value Is o f the same order as the  p o s s i b l e values o f 51, 53 or 63 k. cal./mole, f o r l / 8 H£(Ss) i n accordance  w i t h our requirements.  I t i s s l i g h t l y l e s s than the  h i g h value 63 k. c a l . which seems t o be favoured by r e c e n t evidence. The  e r r o r estimated as 18 k. c a l . , i n the e l e c t r o n Impact  value f o r D(CH S-SCH ) a r i s e s from an e r r o r i n H (GH S.) o f 9 3  k. c a l s .  3  f  3  The value o f H (CH «) which was used f o r the c a l c u l a f  3  t i o n o f D(CH 3 S-CH 3 ) was the value which i s g e n e r a l l y c o n s i d e r e d to be c o r r e c t .  Therefore the e r r o r i n D(CH 3 -SCH 3 ) must ,be e s -  timated as 9 k. c a l s . and we estimate the c o r r e c t value as 64 k. cal./mole.  Remembering t h a t G-S d i s s o c i a t i o n e n e r g i e s are  o n l y s l i g h t l y a f f e c t e d by s t r u c t u r e , we would estimate t h a t  174  D(CH SS-CH ) i s a l s o about 64 k. c a l . 3  3  4  The estimates we have made f o r d i s s o c i a t i o n energies of methyl d i s u l f i d e a r e : D(CH S-SCH ) »  55 k. cal./mole.  D(CH SS-CH ) =  64 k. cal./mole.  3  3  3  3  C l e a r l y we have a r r i v e d  at the important c o n c l u s i o n that the  d i s s o c i a t i o n energy o f the S-S bond i n methyl d i s u l f i d e should be l e s s than the d i s s o c i a t i o n energy  of the C-S bond.  Since  these d i s s o c i a t i o n energies are e q u i v a l e n t ,to the a c t i v a t i o n energies of the r e s p e c t i v e  d i s s o c i a t i o n p r o c e s s e s , we may say  that methyl d i s u l f i d e w i l l d i s s o c i a t e more r e a d i l y at the S-S bond than at the C-S bond. CH SSCH 3  is a possible  *  3  2'CH S. 3  i n i t i a t i o n step f o r the thermal decomposition of  methyl d i s u l f i d e .  The process CH SSCH 3  is  Therefore the process  >  3  CH  3  + CH SS. 3  unlikely. Although the d i s s o c i a t i o n energies that we have  f o r methyl d i s u l f i d e can not be regarded  suggested  as a c c u r a t e , they are  probably c l o s e r to the t r u t h than the values of F r a n k l i n and Lumpkin.  In any case the c o n c l u s i o n that D(-S-S-)  i s independent  <  D(-C-S-)  o f the accuracy of our estimate f o r D(CH S-SCH ). 3  For example, i f we had chosen 61 k. c a l . f o r D(CH S-SCH ) 3  3  3  (i.e.  a value which i s g r e a t e r than D(HS-SH) :>, we would have obtained i  67 k. c a l . f o r D(CH SS-CH ). 3  3  The 6 k. c a l . d i f f e r e n c e  is s t i l l  175.  significant. In conclusion i t may be mentioned that much Important work must be done before we s h a l l have a reasonable understanding of molecules containing s u l f u r . to predict their behaviour.  Present theory i s much too crude Reliable values of their d i s s o c i -  ation energies are almost non-existant.  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