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Investigation of the thermal decomposition of methyl and ethyl disulfides Patrick, William Nicholas 1952

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LIT J ^ /  Is £f>.  INVESTIGATION OF THE THERMAL DECOMPOSITION OF METHYL AND ETHYL DISULFIDES by WILLIAM NICHOLAS PATRICK  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ARTS i n the Department of CHEMISTRY  We Accept t h i s t h e s i s as conforming t o the standard required from candidates f o r the degree o f MASTER OF ARTS.  Members of the Department o f Chemistry  THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1952  r  t  ABSTRACT The k i n e t i c s of the thermal decomposition of methyl and e t h y l d i s u l f i d e s were studied i n a quartz r e a c t i o n v e s s e l . The r e a c t i o n s were f o l l o w e d by means of a mercury manometer and chemical analyses f o r r e a c t i o n products were c a r r i e d out at v a r i o u s stages of the r e a c t i o n s ,  A mechanism f o r the de-  composition process i s proposed on t h e b a s i s of t h e r e s u l t s obtained.  T h i s i n v e s t i g a t i o n was c a r r i e d out under the s u p e r v i s i o n of D r . W.  A.  B r y c e to w h o m the author i s v e r y g r e a t l y indebted.  CONTENTS page INTRODUCTION O x i d a t i o n of H y d r o c a r b o n s  1 ..  " The t h e r m a l d e c o m p o s i t i o n of h y d r o c a r b o n s  1 ..  3  The b a s i s of the p r e s e n t i n v e s t i g a t i o n  4  S u r v e y of the l i t e r a t u r e  5  EXPERIMENTAL  9  Reagents .  9  D e s c r i p t i o n of the apparatus  9  D e s c r i p t i o n of a t y p i c a l e x p e r i m e n t  12  P r e s s u r e - t i m e curve for a typical experiment  13  Dependence of rate of r e a c t i o n on i n i t i a l p r e s s u r e of m e t h y l disulfide  .. 15  Dependence of rate of d e c o m p o s i t i o n of m e t h y l d i s u l f i d e on temperature  17  A n a l y s e s f o r the products of the r e a c t i o n .  '. 18  Analysis for mercaptans  . 19  A n a l y s i s f o r hydrogen sulfide  20  E f f e c t of N i t r i c Oxide on rate of r e a c t i o n  24  E f f e c t of s u r f a c e on rate of r e a c t i o n  25  T h e r m a l d e c o m p o s i t i o n of ethyl d i s u l f i d e .  26  D e t e r m i n a t i o n of the o r d e r of the r e a c t i o n  26  D e t e r m i n a t i o n of the energy of a c t i v a t i o n  28  1  S u m m a r y of the e x p e r i m e n t a l r e s u l t s . . . . .  30  DISCUSSION D e r i v a t i o n of a rate e x p r e s s i o n  32 . .'. ... . ... ... ..... 36  D e c o m p o s i t i o n of d i s u l f i d e s arid p e r o x i d e s .......... ... . . .... 41 REFERENCES  .........  43  INTRODUCTION The  r e a c t i o n s of h y d r o c a r b o n s p r o v i d e food f o r i n t e r e s t i n g and  challenging study concerning the m e c h a n i s m s of c h e m i c a l change. The m o r e c o m p l e x the h y d r o c a r b o n i s , the m o r e c o m p l i c a t e d  are the r e a c -  tions i t undergoes. The r e s u l t i s that m a n y t h e o r i e s have been advanced in an endeavour to e x p l a i n some of the phenomena o b s e r v e d i n the r e a c tions of even the s i m p l e s t of h y d r o c a r b o n s . In the p r e s e n t i n v e s t i g a t i o n , the compounds, d i m e t h y l d i s u l f i d e and d i e t h y l d i s u l f i d e w e r e chosen f o r study. It i s hoped that the i n f o r m a t i o n gained i n the study of the t h e r m a l d e c o m p o s i t i o n of an R-S-S-R compound, besides the i n t e r e s t i t holds i n its own right, might be of value t o w a r d s a better understanding of the m e c h a n i s m of o x i d a t i o n of h y d r o c a r b o n s i n w h i c h the h i g h l y unstable i n t e r m e d i a t e R-O-O-R i s postulated.  OXIDATION OF HYDROCARBONS One  of the m o s t i m p o r t a n t r e a c t i o n s of h y d r o c a r b o n s i s t h e i r  oxidation. M u c h w o r k has been done on t h i s subject and m u c h e x p e r i m e n t a l data has been c o m p i l e d . T h e t h e o r e t i c a l d i s c u s s i o n s , however, have often been confusing. The r e a s o n f o r this i s that different h y d r o carbons have been studied by different w o r k e r s under conditions w h i c h often w e r e not e a s i l y c o m p a r a b l e . O x i d a t i o n r e a c t i o n s involve unstable i n t e r m e d i a t e s w h i c h are v e r y i n t e r e s t i n g because oxygen i s d i a t o m i c while m o s t of the o x i d a t i o n products r e q u i r e the s e p a r a t i o n of the two oxygen atoms. K i n e t i c studies  2  of the o x i d a t i o n r e a c t i o n s have been c o n c e r n e d w i t h the p r o b l e m of showing the i n d i v i d u a l steps of the m e c h a n i s m w h i c h would i n v o l v e few a t o m i c movements and of the s i m p l e s t c h a r a c t e r i n a c c o r d a n c e w i t h t h e p r i n c i ple of m i n i m u m disturbance of existing s t r u c t u r e s . That i s , the r e a c t i o n s would i n v o l v e only one c h e m i c a l bond at a t i m e . A c c o r d i n g l y , the o x i d a tion of a h y d r o c a r b o n could be shown to o c c u r i n steps by w r i t i n g the r e actions somehow i n the f o l l o w i n g m a n n e r (9): RCH  3  + O  RCH  2  + 0  RCH  z  2  (1)  z  —• R C H 0 0  (2)  2  RCH OO + RCH z  + HO  2  3  —* RCH OOH + R C H 2  2  (.3)  or R C H 0 0 + R H —* R C H O O R + H 2  2  (4)  E a c h of the above steps i n v o l v e s the f o r m a t i o n of d i s r u p t i o n of only one bond. R e a c t i o n s (3) and (4) a r e chain propagating steps i n w h i c h a r a d i c a l i s p r o d u c e d to take the p l a c e of the one e l i m i n a t e d . T h e p e r o x i d e s m a y undergo r e a c t i o n s such as, R O O R —>  2 RO  (5)  w h i c h i s known as a chain branching r e a c t i o n , o r i f the R's a r e s m a l l , the p e r o x i d e f o r m e d m a y be an i n a c t i v e product. R O m a y be w r i t t e n R ' C H 0 w h i c h would b r e a k up thus: 2  R ' C H 0 —*>R' + C H 0 2  (6)  2  If (6) f o l l o w e d (5) d i r e c t l y , the o v e r a l l equation could be w r i t t e n , R O O R — • 2 R' + 2 C H 0 2  R' could then r e a c t with R H to regenerate R, thus,  (7)  3  R'  +  RH  - t  R + R'H  (8)  or, i f oxygen entered into the c y c l e of changes, a new  radical, R , H  of  s t i l l l o w e r c a r b o n content .would be u l t i m a t e l y f o r m e d , R' + O2 —*•  l e s s active products  •  (9)  E x p e r i m e n t a l i n v e s t i g a t i o n has shown that the r a t e of o x i d a t i o n does a c t u a l l y d e c r e a s e w i t h d i m i n i s h i n g n u m b e r of c a r b o n atoms (9). F r o m this i t may  be concluded that step (8) would m a i n t a i n the r a t e w h i l e  the degradation r e a c t i o n (9.) would l e a d to f o r m s of p e r o x i d e s which a r e m u c h l e s s r e a d y to m a i n t a i n the b r a n c h i n g r e a c t i o n s of step (5). f o r e , (9) may  be c o n s i d e r e d as a c h a i n b r e a k i n g step (9,  The T h e r m a l D e c o m p o s i t i o n of  There-  10, 11).  Hydrocarbons.  T h e r m a l d e c o m p o s i t i o n i s another i m p o r t a n t type of h y d r o c a r b o n r e a c t i o n . N i t r i c oxide i n h i b i t i o n (5) has shown that the d e c o m p o s i t i o n  may  involve a chain m e c h a n i s m of the f r e e r a d i c a l type. One m i l l i m e t e r of n i t r i c oxide may  reduce to a f r a c t i o n of i t s original value the r a t e of r e -  action of s e v e r a l hundred t i m e s i t s own amount of the o r g a n i c substance. The n i t r i c oxide i s g r a d u a l l y used up. If i t i s u s e d up before the d e c o m p o s i t i o n i s over, the rate r i s e s r a t h e r a b r u p t l y to n o r m a l again. S i n c e one m o l e c u l e of n i t r i c oxide, by r e a c t i n g w i t h something i n the s y s t e m , can stop the r e a c t i o n of s e v e r a l hundred m o l e c u l e s of an o r g a n i c compound, the something i t r e m o v e s i n i t s own r e a c t i o n m u s t n o r m a l l y have been r e s p o n s i b l e f o r the d e c o m p o s i t i o n of a l a r g e n u m b e r of m o l e c u l e s . T h i s p r o c e s s i s c a l l e d the b r e a k i n g of a chain, and i t i s this m e c h a n i s m of n i t r i c oxide that, i s made use of i n the detection of f r e e r a d i c a l s i n a  4  reaction. Studies with n i t r i c oxide i n h i b i t i o n have r e v e a l e d that the rate, i s often r e d u c e d to a constant f r a c t i o n of its. o r i g i n a l - v a l u e . It i s not known whether the r e s i d u a l r e a c t i o n i s of a chain type unaffected by n i t r i c oxide, o r of a m o l e c u l a r type. A d e t a i l e d study was made of the t h e r m a l d e c o m p o s i t i o n  of hy-  d r o c a r b o n s by R i c e and H e r z f e l d (12). T h e y d e v i s e d a f r e e r a d i c a l m e c h a n i s m f o r the decomposition  of a n u m b e r of compounds and by a  suitable choice of the a c t i v a t i o n energies of the v a r i o u s steps, the apparent a c t i v a t i o n energy of the o v e r a l l r e a c t i o n c o u l d be made to agree w i t h the. e x p e r i m e n t a l value. The B a s i s of the P r e s e n t I n v e s t i g a t i o n . In the d i s c u s s i o n of the o x i d a t i o n of h y d r o c a r b o n s , i t appeared that the key to the m e c h a n i s m f o r the r e a c t i o n s was the decomposition  formation'and  of a l k y l p e r o x i d e s . It so happens, however, that p e r o x i d e s  as a whole are v e r y r e a c t i v e and unstable. T h i s m a k e s i t quite d i f f i c u l t to gain i n f o r m a t i o n of a nature that would s p e c i f y the r o l e p l a y e d by t h e m i n the r e a c t i o n . S u l f u r , on the other hand, f o r m s compounds that are m u c h m o r e stable.It would be i n t e r e s t i n g , t h e r e f o r e , to s t a r t w i t h a d i sulfide i n place of an a l k y l p e r o x i d e i n t e r m e d i a t e and study i t s t h e r m a l decomposition to see i f i n f o r m a t i o n gained r e g a r d i n g activation, energy, effect of s t r u c t u r e on rate and products obtained would a i d i n i n t e r p r e t ing the r o l e of p e r o x i d e s i n o x i d a t i o n r e a c t i o n s . It was stated p r e v i o u s l y that the m e c h a n i s m f o r the r e a c t i o n s of  5  p e r o x i d e s depended on the r u p t u r e of the -O-O- bond w h i c h i s w e a k e r than the -G-O bond. With s u l f u r , a different s i t u a t i o n e x i s t s . T h e -S-Sbond s t r e n g t h i s a p p r o x i m a t e l y of the s a m e o r d e r of magnitude as the -C-S- bond s t r e n g t h (14). The question a r i s e s , does a d i s u l f i d e decompose i n the same m a n n e r as a p e r o x i d e ? B e s i d e s the i n t e r e s t a d i s u l f i d e holds because of i t s s i m i l a r i t y to a p e r o x i d e , the study of the t h e r m a l d e c o m p o s i t i o n of. d i s u l f i d e s i s of value i n i t s own r i g h t s i n c e k i n e t i c studies have not been c a r r i e d out on these compounds p r e v i o u s l y .  There-  f o r e , this i n v e s t i g a t i o n was undertaken to study d i m e t h y l and d i e t h y l d i s u l f i d e s f o r t h e m s e l v e s , as w e l l as w i t h a v i e w to gaining i n f o r m a t i o n on the r o l e of p e r o x i d e s i n h y d r o c a r b o n d e c o m p o s i t i o n and o n the effect that different groups such as m e t h y l and ethyl w o u l d have on v a r i o u s bonds that m i g h t .break i n t h e r m a l d e c o m p o s i t i o n . S u r v e y of the L i t e r a t u r e . A s u r v e y of the l i t e r a t u r e r e v e a l e d no r e p o r t s on the study of the t h e r m a l d e c o m p o s i t i o n of d i s u l f i d e s i n the gas phase. However, other r e p o r t s on i n v e s t i g a t i o n s of a r e l a t i v e nature w e r e of a s s i s t a n c e . One of the l a t e s t v a l u e s of C-S and S-S bond s t r e n g t h s w e r e r e p o r t e d by F r a n k l i n and L u m p k i n (14). T h e y d e t e r m i n e d the heats of f o r m a t i o n of the SH, GH S, and C2H5S r a d i c a l s by the e l e c t r o n i m p a c t method. 3  From  this and known t h e r m o d y n a m i c data, s e v e r a l C-S, H-S, and S-S bond strengths i n a l k y l s u l f u r compounds w e r e c a l u c l a t e d . Some of these a r e as. f o l l o w s .  6  Compound  B o n d Strength kcal./mole  Bond  CH SH  CH -SH  74.2  C H SH  C Hg - S H  73.4  CH3SCH3  CH3 SCH3  73.2  C2 HgSC H3  C H g —SCH3  71.8  CH3SC2H5  CH3-SG2H5  70.6  C2H5SC2H5  C H5-SC H^  69.3  CH S  CH -S  52.4  C H j S  C2H5-S  53.6  H S  H - S H S-H  95.3 67.0 .  CH SH  CH S-H  88.8  C HgSH  C H S-H  86.8  Sz  S-S  3  2  5  3  2  2  3  2  3  2  _  2  2  2  3  3  2  5  • .  76, 81, 101  HS-SH  73.2  CH3SSCH3  CH3S -SCH3  73.2  C2H5SSC2H5  C2tigS - S C 2 H g  70.0  It i s seen i n the above table that the strengths of the S-S bonds i n the a l k y l d i s u l f i d e s a r e about, the same as that of the C-S bonds i n m e r c a p t a n s and t h i o e t h e r s . D a t a was not a v a i l a b l e f o r the c a l c u l a t i o n of C-S bond strengths i n the d i s u l f i d e s . However, i t i s l i k e l y that these would be of about the s a m e value as i n the t h i o e t h e r s and m e r c a p t a n s and, therefore,- of about the s a m e value as the S-S bond i n the d i s u l f i d e s .  7  The s t r u c t u r e of the gas m o l e c u l e  of m e t h y l d i s u l f i d e was i n -  vestigated by the e l e c t r o n d i f f r a c t i o n method by Stevenson and B e a c h (13). T h e y found m e t h y l d i s u l f i d e to be a c h a i n m o l e c u l e w i t h bond d i s t a n c e s of,  •  . S-S  2.03 A °  C-S  1.77 A °  H-C  ----  1.09 A ° , (assumed).  F a r a g h e r , M o r r e l l and C o m a y (8) i n t h e i r w o r k on the d i s t i l l a t i o n of ethyl d i s u l f i d e at 496°C. found the p r o d u c t s to be hydrogen sulfide, sulfur, ethyl mercaptan, a l k y l s u l f i d e s and thiophene. In addition, u n s a t u r a t e d and s a t u r a t e d gaseous p r o d u c t s w e r e also obtained. T r e n n e r and T a y l o r (7) studied the t h e r m a l d e c o m p o s i t i o n of ethyl m e r c a p t a n by the static method taking note of the rate of change of p r e s s u r e w i t h t i m e as was done i n the p r e s e n t i n v e s t i g a t i o n . T h e i r e x p e r i m e n t s w e r e c a r r i e d out at t e m p e r a t u r e s ranging f r o m 380-410 °C. and at p r e s s u r e s of f r o m 50 to 400 mm.  T r e n n e r and T a y l o r found there was an i n d u c t i o n p e r i o d w i t h an  a u t o a c c e l e r a t i n g rate up to a m a x i m u m and a subsequent f a l l i n g off. The volume i n c r e a s e obtained i n t h e i r e x p e r i m e n t s ranged f r o m 63 to 76 p e r cent under v a r y i n g conditions. T o g a i n evidence of an e q u i l i b r i u m being established, they took a m i x t u r e of ethylene and hydrogen sulfide equivalent to the ethyl m e r c a p t a n i n v e s t i g a t e d and subjected this m i x t u r e to the same conditions as the m e r c a p t a n r e a c t i o n . A f t e r 24 hours the m i x ture showed a c o n t r a c t i o n of 3 4 % i n p r e s s u r e . A n equivalent amount of m e r c a p t a n at the same t e m p e r a t u r e gave a percentage change of 69.6.  8  T r e n n e r and T a y l o r (7) also w o r k e d w i t h ethyl sulfide and obtained r e a c tion rates and .decomposition c u r v e s that were s i m i l a r to those of-the mercaptan decomposition. B r y c e and H i n c h e l w o o d ( l ) , i n t h e i r w o r k on the r e a c t i o n s of h y d r o c a r b o n s w i t h s u l f u r , postulated.that the i n i t i a l step was the r e a c tion of a h y d r o c a r b o n m o l e c u l e w i t h a f r e e s u l f u r atom. T h e p r o p o s e d m e c h a n i s m of the r e a c t i o n i s as f o l l o w s : . 'CH CH CH CH 3  2  2  3  6c S  -+  CH3CH CH CH3  6c HS  CH CH CHCH  —»  2  2  3  2  CH CH CHCH 3  2  CH CH  2  3  .H & CH -  —»  S  —»  3  3 :  •-—• C H C H 3  x  -* t a r  CH CH=CH  2  6c S  3  C H C H = C H - 6c C H 3  CH CH CHCH 3  2  2  2  6c H S 2  6c C H 3  6c C H = C H  2  3  (l)  2  2  6c H  (3) (4)  (6) (7)  —> 3  (.2)  (5)  HS 6c S _ i  6c S  3  —* 3  2  2  3  CH CH=CH  CH =CH  6c H S  CH CH CHCH3  C H C H = C H - 6c HS  —*  3  CH CH=CH-CH 3  3  (8) (9)  F r o m this m e c h a n i s m they obtained an e x p r e s s i o n f o r the r e a c t i o n rate w h i c h was f i r s t o r d e r w i t h r e s p e c t to the i n i t i a l h y d r o c a r b o n p r e s s u r e and f i r s t o r d e r with.respect to the s u l f u r atom c o n c e n t r a t i o n . T h e s e i n v e s t i g a t i o n s show that a v a r i e t y of p r o d u c t s m a y be expected f r o m the decomposition of s u l f u r containing hydrocarbons.. T h e w o r k of B r y c e and H i n s h e l w o o d w i t h t h e i r t h e o r y on the r e a c t i v e f o r m of s u l f u r , and the w o r k of T r e n n e r and T a y l o r w h i c h gave an i n d i c a t i o n of what might be expected i n the t h e r m a l d e c o m p o s i t i o n of m e t h y l and .ethyl  d i s u l f i d e s , was of great a s s i s t a n c e i n this i n v e s t i g a t i o n .  EXPERIMENTAL Reagents. The m e t h y l d i s u l f i d e and ethyl d i s u l f i d e u s e d i n this i n v e s t i g a t i o n were of reagent grade obtained f r o m E a s t m a n K o k a k Company, Rochester, New Y o r k and w e r e not subjected to f u r t h e r p u r i f i c a t i o n except to d i s t i l l off a f r a c t i o n of the l i q u i d h e l d i n the storage bulb i n the v a c u u m  appara-  tus before using i t i n the e x p e r i m e n t a l work. D e s c r i p t i o n of the A p p a r a t u s . The apparatus u s e d i n t h i s i n v e s t i g a t i o n was s i m i l a r i n design to that u s e d by B r y c e and H i n s h e l w o o d i n t h e i r w o r k w i t h h y d r o c a r b o n s at the U n i v e r s i t y of O x f o r d (1). The s y s t e m c o n s i s t e d of a q u a r t z r e a c t i o n v e s s e l heated to a known t e m p e r a t u r e i n an e l e c t r i c f u r n a c e and connected to ah evacuating system, a m e r c u r y manometer, and storage v e s s e l s f o r the r e a c t a n t s . A d i a g r a m of the e n t i r e apparatus i s p r e s e n t e d i n F i g . 1 with 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 . The f u r n a c e was of the v e r t i c a l type and was heated by a l t e r n a t ing c u r r e n t . A d i a g r a m of the c o r e of the f u r n a c e i s g i v e n i n F i g . 2 . It c o n s i s t e d of a quartz tube, three inches i n d i a m e t e r and twenty-three incehs long. The q u a r t z tube was wound i n t h r e e s e c t i o n s w i t h a l u m e l r e s i s t a n c e w i r e . The c u r r e n t i n each s e c t i o n was c o n t r o l l e d by means of a v a r i a c . Since heat l o s e s a r e g r e a t e r at the ends of the core than at the m i d d l e , c o n t r o l of the c u r r e n t i n each s e c t i o n by m e a n s of a v a r i a c made .  '•J  TO PUMP "F  <  M  LIQUID AIR TRAP'  L^Q T  7  tjJlQ T  £  \ =0= SAMPLING PIPETTE  D' F i g . 1.  o o C  D  FURNACE  Diagram of apparatus used i n the study of the thermal decomposition of dimethyl and diethyl disulfides.  10  i t p o s s i b l e to obtain a constant t e m p e r a t u r e o v e r the length of the r e a c tion v e s s e l i n s i d e the f u r n a c e . The f u r n a c e was i n s u l a t e d w i t h f o u r i n c h e s of p o w d e r e d asbestos. The c o v e r of the f u r n a c e was made of asbestos board and the opening into the c o r e was i n s u l a t e d w i t h a t h i c k l i d , also made of a s b e s t o s . The t e m p e r a t u r e of the r e a c t i o n v e s s e l w a s m e a s u r e d by means of two c h r o m e l - a l u m e l t h e r m o c o u p l e s connected i n s e r i e s to give a h i g h er s e n s i t i v i t y . The t e m p e r a t u r e i n the f u r n a c e was adjusted by means of a " V a r i a c " v a r i a b l e t r a n s f o r m e r i n the power supply. The t e m p e r a t u r e was c o n t r o l l e d m a n u a l l y by means of a s w i t c h w h i c h cut a r e s i s t a n c e i n and out of the c i r c u i t s e r v i n g the f u r n a c e . The d i a g r a m f o r this i s g i v e n i n F i g . 3. R  x  i s the " V a r i a c " v a r i a b l e t r a n s f o r m e r and R  2  i s the v a r i a b l e  r e s i s t a n c e w h i c h was cut i n and out of the c i r c u i t w i t h the a i d of key, K. The t e m p e r a t u r e was m e a s u r e d by means of a t h e r m o c o u p l e i n a potentiometer, c i r c u i t . V a r i a t i o n s of t e m p e r a t u r e w e r e f o l l o w e d by the d e f l e c t i o n of the g a l v a n o m e t e r needle w h i c h could be kept about z e r o by m a n i p u l a t i o n of the key, K. In this manner, the t e m p e r a t u r e c o u l d be kept constant w i t h i n J10.5 C. o  The apparatus was evacuated by means of a m e r c u r y d i f f u s i o n pump backed by a r o t a r y o i l pump. A t r a p , c o o l e d w i t h l i q u i d a i r , was p l a c e d between the apparatus and the m e r c u r y pump to condense out any h a r m f u l r e a c t i o n v a p o u r s before they could r e a c h the pumping s y s t e m . The upper and l o w e r g a l l e r i e s of the apparatus c o u l d be evacuated s e p a r ately o r s i m u l t a n e o u s l y and through t h e m the reaction v e s s e l could be  F i g . 2.  Diagram of furnace core and heating c o i l s .  5  F i g . 3.  Wiring diagram for c o n t r o l l i n g temperature of furnace.  11  evacuated by suitable adjustment of the i n t e r v e n i n g taps. The p r e s s u r e in the c a p i l l a r y connecting the r e a c t i o n v e s s e l w i t h the s y s t e m was shown by the c a p i l l a r y manometer, M. One end of the m a n o m e t e r was under a v a c u u m w h i c h was p r e s e r v e d by a 100 m l . bulb, V. In this way, outside b a r o m e t r i c p r e s s u r e changes had no effect on the p r e s s u r e m e a s u r e m e n t i n the s y s t e m . A d i s c h a r g e tube, F, was attached to the apparatus as a v a c u u m i n d i c a t o r . A s the p r e s s u r e was reduced, the disappearance of the d i s charge was taken to indicate a s u f f i c i e n t l y complete vacuum. M e t h y l and ethyl d i s u l f i d e s , w h i c h a r e l i q u i d at r o o m t e m p e r a ture, w e r e s t o r e d i n bulbs, C. To f i l l the bulb, the open end of tip, was i m m e r s e d i n the l i q u i d d i s u l f i d e . T  2  D,  was then opened and the v a c u u m  i n the s y s t e m above drew the l i q u i d up the t i p , D, and into bulb, C. Having f i l l e d the bulb, the tip, D, was s e a l e d off. B e f o r e using, the d i s u l f i d e i n the bulb was c o o l e d w i t h c o l d water and thoroughly pumped to r e m o v e t r a c e s of a i r . A f r a c t i o n of i t was a l l o w e d to d i s t i l l off. When ready to be i n t r o d u c e d into the r e a c t i o n v e s s e l , the l i q u i d was v o l a t i l i z e d by w a r m i n g the bulb i n a beaker of hot water. The tubing i n the l o w e r g a l l e r y as w e l l as the c a p i l l a r y connecting it to the r e a c t i o n v e s s e l and the m a n o m e t e r was wound w i t h r e s i s t a n c e w i r e and heated to p r e v e n t the h y d r o c a r b o n d i s u l f i d e f r o m condensing out in the s y s t e m w h i l e the r e a c t i o n v e s s e l was being f i l l e d . In addition to f o l l o w i n g the r e a c t i o n by p r e s s u r e m e a s u r e m e n t s the'reaction m i x t u r e was s a m p l e d and a n a l y s e d at v a r i o u s stages. S a m p l e s  12  w e r e c o l l e c t e d i n the s a m p l i n g pipette, S, w h i c h was attached to the l o w e r g a l l e r y by a ground g l a s s joint and could be connected to the r e a c t i o n v e s s e l by adjustment of taps T j , T , and T . 4  D e s c r i p t i o n of a T y p i c a l E x p e r i m e n t .  5  .  The t e m p e r a t u r e of the f u r n a c e was adjusted to that at w h i c h the e x p e r i m e n t was to be made, and the c u r r e n t was t u r n e d on i n the winding of the c a p i l l a r y tubbing. T h e t e m p e r a t u r e of the tubing was kept just below the  b o i l i n g point of m e r c u r y , as p a r t of the m a n o m e t e r had to be wound  i n o r d e r to p r e v e n t the d i s u l f i d e s and any condensable r e a c t i o n p r o d u c t s f r o m condensing o v e r the m e r c u r y . P r e s s u r e changes, t h e r e f o r e , had to be r e a d on the other a r m of the "U"-shaped m a n o m e t e r . T h e r e a c t i o n v e s s e l and the l o w e r g a l l e r y w e r e t h o r o u g h l y evacuated u n t i l no d i s c h a r g e was noticeable i n the d i s c h a r g e tube. T h e d i s u l f i d e i n the s t o r a g e bulb, C, was w a r m e d by i m m e r s i n g the bulb i n a b e a k e r of hot w a t e r . When r e a d y to i n t r o d u c e the d i s u l f i d e to the r e a c t i o n v e s s e l , taps T w e r e c l o s e d and tap T  2  4  and T  8  was opened. T h e p r e s s u r e 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 was i n d i c a t e d on the m a n o m e t e r . When w o r k i n g w i t h m e t h y l d i s u l f i d e , an i n i t i a l p r e s s u r e up to s i x t y m i l l i m e t e r s could be obtained. M a x i m u m i n i t i a l p r e s s u r e obtainable w i t h ethyl d i s u l f i d e was 1.3.6 mm.  When the d e s i r e d p r e s s u r e was r e a c h e d tap, T  lf  was c l o s e d .  I n c r e a s e s i n p r e s s u r e , as the r e a c t i o n took p l a c e , w e r e noted at t h i r t y second i n t e r v a l s with the a i d of a l a r g e e l e c t r i c G r a y - L a b U n i v e r s a l T i m e r . T h e t i m e r was s t a r t e d at the beginning of _the  disulfide i n -  t r o d u c t i o n into the r e a c t i o n v e s s e l . T h e f i l l i n g of the v e s s e l took f r o m  13  f i v e to f i f t e e n seconds depending on what, i n i t i a l p r e s s u r e was r e q u i r e d . P r e s s u r e - T i m e Curve for a Typical Experiment. The p r e s s u r e - t i m e c u r v e f o r a t y p i c a l e x p e r i m e n t a l r u n i s shown i n F i g . 4. T h i s p a r t i c u l a r run.was made w i t h an i n i t i a l 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 of 46 mm. of m e t h y l d i s u l f i d e and p e r i o d i c i n c r e a s e s of p r e s s u r e w e r e noted f o r a p e r i o d of 30 m i n u t e s . T o check on the  r e p r o d u c i b i l i t y of the runs and to see whether o r not continued runs  in e v e r i n c r e a s i n g n u m b e r would affect the r e s u l t s of an experiment, the run  shown i n F i g . 4 was repeated p e r i o d i c a l l y throughout the e x p e r i m e n -  t a l work. F o r instance, F i g . 5 shows one r u n that was made at the end of a s e r i e s of 14 runs on the apparatus, one at the end of 22 runs, another at the end of 42 runs and s t i l l another at the end of a t o t a l of 62 r u n s . T h e p r e s s u r e - t i m e plot f o r each of these runs f e l l v e r y c l o s e l y along the same curve, i n d i c a t i n g that a l l i n t e r m e d i a t e r u n s c o u l d be c o n s i d e r e d r e l i a b l e . In addition to t h i s , any runs, the r e s u l t s of w h i c h w e r e used i n c a l c u l a t i o n s , w e r e made i n duplicate o r t r i p l i c a t e to ensure that each i n d i v i d u a l r u n could be r e p r o d u c e d . It w i l l be noted that t h e r e i s an i n d u c t i o n p e r i o d of 4 m i n u t e s . No p r e s s u r e i n c r e a s e was noted u n t i l the f i f t h minute, after w h i c h the p r e s s u r e began to i n c r e a s e i n the s y s t e m . The rate i n c r e a s e d u n t i l about 9 minutes h a d e l a p s e d and then f o r about 3 minutes i t r e m a i n e d constant. By the t i m e 12 minutes had elapsed, the rate h a d s l o w e d down c o n s i d e r ably and the i n c r e a s e i n p r e s s u r e f r o m then on was v e r y slow. T h e value taken as the rate of r e a c t i o n was the m a x i m u m rate w h i c h under these  F i g * 4.  Pressure-Time curve f o r a t y p i c a l run i n the thermal decomposition o f  dimethyl d i s u l f i d e .  I n i t i a l pressure, 46 mm.  Temperature, 340°C.  3D -  O O * >< x ^  - a  «A  A  20  P  mm.  Legend A®  Hg.  10  Rim N o .  Init. P.  x.  XIV  46.0  mm.  o  XXII  47.0  mm.  A  XLII  45.2  mm.  "  20  10  30  Time i n minutes, Fig 5 .  Pressure-Time p l o t f o r three runs w i t h dimethyl d i s u l f i d e , showing r e p r o d u c i b i l i t y of r e s u l t s .  14  conditions appeared i n the t i m e i n t e r v a l of f r o m 9 to 12 minutes after the beginning of the e x p e r i m e n t a l run. A t higher t e m p e r a t u r e s ,  the induction p e r i o d was c o n s i d e r a b l y '  shortened. A t 380 °C., t h e r e was an i n d u c t i o n p e r i o d of only half a. minute, after w h i c h the p r e s s u r e i n c r e a s e d v e r y r a p i d l y f o r a p e r i o d of one m i n ute and then slowed down again. When making an e x p e r i m e n t a l run on the apparatus,  pressure  readings w e r e taken f o r a p e r i o d of half an hour as it was found that a l l the u s e f u l i n f o r m a t i o n could be gained i n this t i m e . F r o m a t r i a l r u n at 340 °C. and 29 mm. i n i t i a l p r e s s u r e , i t was found that the p r e s s u r e i n c r e a s e was only 0.4 mm. i n the second h a l f - h o u r and no i n c r e a s e was noted i n the second and t h i r d hours of the r u n . On the c o m p l e t i o n of a run, the r e a c t i o n v e s s e l and the a p p a r a tus i n g e n e r a l w e r e thoroughly evacuated.. It was found that if the r e a c tion v e s s e l was not evacuated but allowed to stand overnight with the r e a c t i o n products i n i t , a fine b l a c k f i l m was deposited on the w a l l s of the v e s s e l . T h i s b l a c k substance would not d i s s o l v e i n either concent r a t e d n i t r i c o r s u l f u r i c a c i d . Water, benzene, actone o r c a r b o n d i s u l f i d e had no effect on i t . It could o n l y be r e m o v e d by s c r u b b i n g . If the r e a c t i o n v e s s e l was evacuated i m m e d i a t e l y after a run, there was no deposit on the w a l l s , even after a great number of s u c c e s s i v e runs had been made. No blackening was seen at the stopcock b o r e s where the products would have to pass and come i n contact with stopcock grease, on evacuation. However, the b l a c k deposit appeared i n the l i q u i d - a i r t r a p , Y, where i t  15  was p r o b a b l y t r a p p e d by the f r o z e n - o u t d i s u l f i d e , some of w h i c h was a l ways pumped off f r o m the storage, bulb, C, at the beginning of the day's e x p e r i m e n t a l . w o r k . S i n c e - v e r y l i t t l e deposit, of the b l a c k substance was made f o r any one p a r t i c u l a r run, it was a l l o w e d to accumulate o v e r a number of about t h i r t y r u n s . T h i s p e r m i t t e d the f i l m to b u i l d up s u f f i c i e n t l y so that some "could be s c r a p e d into a test tube. To t h i s b l a c k substance i n the test tube, some  sodium;carbonate  was added and the test tube heated. A s i l v e r m i r r o r appeared on the w a l l s of the tube. A little, iodine was next p l a c e d on the .bottom of the tube and the tube w a r m e d u n t i l the iodine s u b l i m e d . The r e s u l t was an orange and red  c o l o r a t i o n of the s i l v e r m i r r o r . The above d e s c r i b e d p r o c e d u r e i s a  s t a n d a r d test f o r the detection of a m e r c u r y compound (2).  The a r o m a  present, on r e m o v i n g the l i q u i d air, t r a p s t r o n g l y suggested the p r e s e n c e of m e r c a p t a n s i n the r e a c t i o n p r o d u c t s . A p p a r e n t l y the d i s u l f i d e and i t s r e a c t i o n p r o d u c t s r e a c t e d w i t h the l i t t l e m e r c u r y vapour p r e s e n t to f o r m compounds of m e r c u r y . Dependence of Rate of Reaction, on .Initial P r e s s u r e of M e t h y l D i s u l f i d e . The o r d e r of the r e a c t i o n was determined, by i n v e s t i g a t i n g the dependence of the r e a c t i o n rate on the i n i t i a l p r e s s u r e of m e t h y l d i s u l f i d e admitted to the r e a c t i o n v e s s e l , at a t e m p e r a t u r e of 340 °C. In T a b l e I i s l i s t e d the data for ,the e x p e r i m e n t s i n v o l v e d .  16 r Table I R e a c t i o n r a t e s and rate constants f o r v a r i o u s i n i t i a l p r e s s u r e s of m e t h y l d i s u l f i d e at 340°C.  Run No.  Initial Pressure mm.  P in 8 min. mm.  dp/dt %  Increase  mm/  min.  Rate constant kmin x 10  1  25.4  9.0  35.4  1.96  7.42  2  31.6  10.8  34.2  2.23  7.06  3  34.0  12.5  36.8  2.50  7.35  4  42.0  16.3 .  38.8  3.13  7.45  5  46.0  16.5  35.9  3.29  7.16  6  59.6  22.4  37.6  4.56  7.65  - '  2  The rate constant was c a l c u l a t e d f r o m the equation, dp/ dt = k c where " c " i s the i n i t i a l p r e s s u r e . The tangent to the steepest p o r t i o n of the p r e s s u r e - t i m e curve was taken to r e p r e s e n t the i n i t i a l r a t e of r e a c tion s i n c e the p r e s s u r e i n c r e a s e up to this point was n e g l i g i b l e . F i g . 6 shows the p r e s s u r e - t i m e c u r v e s f o r the runs c o v e r e d by the data g i v e n i n Table I. In F i g u r e 7 l o g dp/dt i s plotted v s . l o g c. The values plotted w e r e c a l c u l a t e d f r o m the data given i n T a b l e I. F r o m the equation dp/dt = k c " which y i e l d s  4  8 4 t i n minutes.  F i g . 6.  Dependence of the r e a c t i o n rate on the i n i t i a l pressure of methyl d i s u l f i d e .  17  d(log dp/dt) d log c  =  n  it i s seen that the slope gives the o r d e r of the r e a c t i o n . The slope of the graph i n F i g . 7 i s unity, i.e., the r e a c t i o n i s of the f i r s t o r d e r . Dependence of Rate of D e c o m p o s i t i o n of M e t h y l D i s u l f i d e i n T e m p e r a t u r e . The dependence of the rate of d e c o m p o s i t i o n of m e t h y l d i s u l f i d e on t e m p e r a t u r e was studied w i t h a v i e w to obtaining a value f o r the e n e r gy of a c t i v a t i o n f o r the r e a c t i o n . E x p e r i m e n t a l runs w e r e made at temperatures ranging i n steps of 10° f r o m 320°C to 380°C. T a b l e II contains the data f o r the runs i n v o l v e d . Two o r t h r e e runs w e r e made at each t e m p e r ature to check on the r e p r o d u c i b i l i t y of the r e s u l t s . F i g . 8 shows two runs made at a t e m p e r a t u r e of 380 °C and an i n i t i a l p r e s s u r e of 32 mm. Hg. The value f o r the rate constant "k" g i v e n i n the table i s an average v a l u e f o r each t e m p e r a t u r e . T able II V a l u e s of the rate constant "k" f o r the t h e r m a l d e c o m p o s i t i o n of m e t h y l d i s u l f i d e at different t e m p e r a t u r e s . Temp. °c. 380  sec  k x 10 175  log.(k x 10 ) 4  ^abs.  x  2.244  1.532  370  81.0  1.909  1.556  360  41.2  1.615  1.580  350  26.3  1.418  1.607  340  12.2  1.088  1.631  330  7.51  .875  1.660  320  3.86  .586  1.688  10  '  .8 -  .6 -  log dp/dt  •4 :  • 2: 1.4  1.6  — r  log p. F i g 7.  P l o t of log dp/dt vs. log ( i n i t i a l pressure) for decomposition of dimethyl d i s u l f i d e , obtained from values given i n Table I . g i v i n g order of r e a c t i o n , i s u n i t y .  Slope,  30  0  F i g * 8.  4  P l o t showing r e p r o d u c i b i l i t y at a temp, o f 380°C.  8 i n minutes;. of runs f o r dimethyl d i s u l f i d e . .  12  16  These two runs were made  18  F i g . 9 shows a group of r e a c t i o n - t i m e graphs f o r the t h e r m a l d e c o m p o s i t i o n of m e t h y l d i s u l f i d e at d i f f e r e n t t e m p e r a t u r e s . F r o m these graphs it m a y be seen how the i n c r e a s e i n p r e s s u r e b e c o m e s m o r e r a p i d as the t e m p e r a t u r e of the r e a c t i o n i s i n c r e a s e d . A l s o , the graphs show the shortening of the i n d u c t i o n p e r i o d w i t h the r i s e i n t e m p e r a t u r e . A point of i n t e r e s t i s the dip i n each c u r v e . It appears that after the rate of r e a c t i o n has d e c r e a s e d f o l l o w i n g an i n i t i a l high value, i t s h a r p l y i n c r e a s e s f o r a b r i e f p e r i o d of t i m e and then l e v e l s off again.> T h i s i s apparently due to a s e c o n d a r y r e a c t i o n of some k i n d . The E n e r g y of A c t i v a t i o n may be obtained by m a k i n g use of the A r r h e n i u s equation . E/RT k = Ae ' B y ploting l o g . k v s . l / T , a s t r a i g h t line i s obtained, the s l o p of w h i c h i s equal to -E/2.303R. T h i s plot i s shown i n F i g . 10 and the data f o r i t was taken f r o m T a b l e II. The slope of the g r a p h t u r n e d out to be -1.11 x 10 . 3  Therefore, Slope =  E/-2.303R  = -1.11 x 10  3  and , E = 50.8 k. c a l . A n a l y s e s f o r the P r o d u c t s of the R e a c t i o n . I n f o r m a t i o n as to the m e c h a n i s m of the c h e m i c a l p r o c e s s i s f r e quently obtained f r o m a qualitative and quantitative analyses f o r the p r o ducts f o r m e d . In this i n v e s t i g a t i o n , the products of the r e a c t i o n of m e t h y l d i s u l f i d e w e r e analysed f o r the amount of m e r c a p t a n s and h y d r o g e n s u l fide p r e s e n t .  Time i n minutes. F i g . '9-  Pressure-time curves for decomposition o f dimethyl d i s u l f i d e at d i f f e r e n t temperatures.  oH  1.52  1—•  1  1  1.60 ^abs.  F i g - 10*  1  1.68 x  1 0 3  Energy of A c t i v a t i o n for the Thermal Decomposition of Dimethyl D i s u l f i d e calculated from slope of graph, 50,8 k . c a l . per mole.  19  Analysis for Mercaptans. The method used was that of B o r g s t r o m and R e i d as m o d i f i e d by B e l l and A g r u s s (3). F i f t y m l . of analar benzene was a d m i t t e d to the gas s a m p l i n g pipette and the m i x t u r e was t h o r o u g h l y shaken f o r f i f t e e n m i n u t e s . The s o l u t i o n was washed w i t h 100 m l . p o r t i o n s of a c i d i f i e d 5 % c a d m i u m c h l o r i d e s o l u t i o n to r e m o v e the H S w i t h w h i c h i t was contaminated. M e r c a p 2  i tans a r e not attacked by this reagent (4). The benzene s o l u t i o n was r u n into a g l a s s - s t o p p e r e d f l a s k and 15 m l . of f e r r i c a l u m i n d i c a t o r w e r e added. T h i s reagent c o n s i s t e d of 6.38 g r a m s of f e r r i c a l u m i n 100 ml., of 1:1 n i t r i c a c i d made up to 500 m l . w i t h ethyl alcohol.. F i f t y m l . of s t a n d a r d (0.0100N) s i l v e r n i t r a t e was a d m i t t e d and the s t o p p e r e d f l a s k was shaken f o r two m i n u t e s . T h e excess s i l v e r n i t r a t e was t i t r a t e d to a s a l m o n - p i n k end-point w i t h .00965 N. a m m o n i u m thiocyanate. F r o m the amount of s i l v e r n i t r a t e r e q u i r e d to r e a c t w i t h the m e r c a p t a n s , the conc e n t r a t i o n of m e r c a p t a n s i n the s a m p l i n g pipette and hence the p a r t i a l 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 was c a l c u l a t e d . A sample c a l c u l a t i o n i s given l a t e r .  •  Gaseous s a m p l e s of the r e a c t i o n p o r d u c t were w i t h d r a w n at different stages of the e x p e r i m e n t and a n a l y s e d . F o r instance, i n one run, the m e t h y l d i s u l f i d e was a l l o w e d to r e m a i n i n the r e a c t i o n v e s s e l f o r 5 m i n u t e s then a sample was w i t h d r a w n and a n a l y s e d . In the next e x p e r i m e n t a l run, the s a m p l e was w i t h d r a w n after 10 minutes h a d elapsed. In other words, a new r u n h a d to be s t a r t e d f o r each a n a l y s e s  20  that was to be made. The i n f o r m a t i o n gained f r o m these analyses, a r e shown i n T a b l e III, and the r e s u l t s show that the percentage of m e r c a p tans i n c r e a s e d r a p i d l y during the p e r i o d f r o m 5 to 10 minutes after beginning the e x p e r i m e n t but did not v a r y m u c h t h e r e a f t e r . T a b l e III R e s u l t s f r o m the m e r c a p t a n analyses of the d e c o m p o s i t i o n of m e t h y l d i s u l f i d e at 340 °C.  Initial MejSj pressure, mm.  Time length of run, min.  Total pressure mm.  .:P •. mm.  48.6  5  49.2  0.6  9.2  .189  48.0  10  61.0  13.0  41.5  .865  42.0  15  61.2  19.2  42.0  48.0  20  69.0  24.4  34.2  .767  48.0  30 . .  76.2  28.2  41.2  .858  RSH mm.  Fraction RSH/Init. P.  1.00.  A point of i n t e r e s t i s that apparently some m e r c a p t a n s a r e f o r m e d before the induction p e r i o d i s o v e r . A n a l y s i i s f o r H S. 2  '  •  The method used f o r the d e t e r m i n a t i o n of h y d r o g e n sulfide i n the d e c o m p o s i t i o n p r o d u c t s was s i m i l a r to that u s e d f o r the e s t i m a t i o n of m e r c a p t a n s (3). A known amount of s i l v e r n i t r a t e was a l l o w e d to r e a c t w i t h the sulfide and then the. e x c e s s s i l v e r n i t r a t e was t i t r a t e d w i t h  21  s t a n d a r d ammonium.thiocyanate. The steps of the a n a l y s i s w i l l be g i v e n here i n d e t a i l , as w e l l as the c a l c u l a t i o n s f o r a t y p i c a l e x p e r i m e n t . F i f t y m l . of a 2 %  s o l u t i o n of s o d i u m h y d r o x i d e w e r e admitted to  ! the s a m p l i n g pipette and the. m i x t u r e was shaken f o r 15 minutes to d i s solve the hydrogen s u l f i d e . T h i s p r o c e d u r e a l s o caused the s o l u t i o n of the m e r c a p t a n s p r e s e n t and so the h y d r o g e n s u l f i d e had to be e s t i m a t e d by the method of d i f f e r e n c e s . The a l k a l i n e s o l u t i o n was emptied into a g l a s s - s t o p p e r e d f l a s k and the pipette was washed w i t h d i s t i l l e d w a t e r . The washings w e r e added to the r e s t of the solution. F i f t y m l . of 0.0100 N. A g N 0  3  w e r e added and  the contents a c i d i f i e d by the addition of 20.0 m l . of 6 N. n i t r i c a c i d .  The  l i b e r a t e d h y d r o g e n sulfide was i m m e d i a t e l y p r e c i p i t a t e d as b l a c k s i l v e r s u l f i d e . The f l a s k was stoppered, t h o r o u g h l y shaken,- and a l l o w e d to stand f o r f i f t e e n minutes. The p r e c i p i t a t e was then f i l t e r e d off through g l a s s w o o l so that its p r e s e n c e w o u l d not i n t e r f e r e w i t h the'observation, of the end-point i n the subsequent t i t r a t i o n . The r e s i d u a l s i l v e r n i t r a t e i n the f i l t r a t e was t i t r a t e d to a s a l m o n - p i n k end-point w i t h 0.00965 N. a m m o n i u m thiocyanate. In a t y p i c a l experiment, m e t h y l d i s u l f i d e was admitted to the r e a c t i o n v e s s e l i n such quantity that the i n i t i a l p r e s s u r e r e a d 49.2 and it was a l l o w e d to r e m a i n t h e r e at a t e m p e r a t u r e of 340 ° C , minutes. P r e s s u r e readings w e r e taken e v e r y h a l f - m i n u t e . The i n c r e a s e i n p r e s s u r e was 22.2 mm.  mm.,  for. 15 resultant  F r o m the a n a l y s i s , it was found that  the a l k a l i n e s o l u t i o n of m e r c a p t a n s and h y d r o g e n sulfide was equivalent to  22  32 m l . of 0.0100 N. A g N 0 . Now, 3  E q u i v a l e n t s of AgNC>3 used  32 x 10" x 0.0100 3  =  3.2 x 10 - 4  (1)  V o l u m e of pipette  =  480 m l .  (2)  V o l u m e of connecting tube  = . 35 m l .  Total  =  515.ml.  (3)  F r o m (2) and (3), F r a c t i o n of gas s a m p l e d contained i n pipette  480/515  =  .932  (4)  T o t a l p r e s s , at end of 15 minutes  71.4 mm. .  P r e s s u r e on s a m p l i n g  9.6  mm.  Difference  61.8  mm.  (5)  (6)  F r o m (5) and (6), . F r a c t i o n sampled  61.8/71.4  = .867  (7)  F r o m (4) and (7), .867 x .932. = .808  F r a c t i o n analysed  (8)  F r o m (1). and (8), T o t a l equiv. of A g N 0 f o r 3  experiment  32 x 10 x 0.0100 .808 3.96 x l O " '  M o l e s of R S H found i n s i m i l a r r u n at end of 15 m i n . when i n i t i a l p r e s s u r e was 42 mm.  •= 2.76 x 10  (9)  23  At  an i n i t i a l p r e s s u r e of 49.2 mm., be  2 , 7 6  X  this should 1 0  x 49.2  4  42  Which for A g N 0  =  3  3.23 x 10"  moles  4  =  3.23 x 10"  .=  (3.96 - 3.23) x 10~*  =  7 . 3 x 10"  4  equivalents  (10)  F r o m (9) and (10), E q u i v a l e n t s u s e d by H S 2  5  The r e a c t i o n i s , HS 2  + 2AgN0  3  .—>  Ag S + 2 . H N O 3 2  E q u i v . of H S p r e s e n t  =  7 . 3 x 10~ /2  =  3 . 7 x 10"  =  250 m l .  =  760 x  =  5.7  i n i t i a l p r e s s u r e o f 5.7/49.2 =  .16  2  Vol.  of R e a c t i o n V e s s e l  5  (11)  5  (12)  F r o m (11) and (12), P r e s s u r e e x e r t e d by  HS 2  + lA-n-T  at 340, C  7An  22410 ^  x  • 613-  '273"  x  •  -  x  5  mm.  and this r e p r e s e n t s a f r a c t i o n of the  The r e s u l t s of a n a l y s i s f o r f i v e e x p e r i m e n t a l runs are shown i n T a b l e IV. U s i n g the value of the f r a c t i o n , H S/.Initial p r e s s u r e , a plot was 2  showing the i n c r e a s e i n p r e s s u r e of H S 2  made  with t i m e f o r a t y p i c a l e x p e r i -  m e n t a l run-. T h i s plot i s shown i n F i g . 11 and i n it i s also shown a plot of the g r o w t h of m e r c a p t a n s f o r the same run.  Total Pressure Curve O  — o  Total P. mm.  Hg.  Time i n minutea. F i g . 11.  Pressure-time p l o t f o r the thermal decomposition of dimethyl d i s u l f i d e , showing growth of mercaptan and HgS pressures w i t h time.  24  T a b l e IV Results f r o m H S 2  a n a l y s i s of the d e c o m p o s i t i o n  of m e t h y l d i s u l f i d e at 340 °C.  Initial pressure, mm.  Time length of run, min.  Total pressure mm.  P mm.  RSH mm.  45.2  5  45.6  0.4  6.2  .139  49.0  10  54.6  15.6  13.1  .268  49.2  15  71.4  22.2  5.7  44.0  20  68.6  24.6  14.3  .326  44.2  30  72.4  28.2  18.9  .427  Me S2 2  •  Fraction RSH/Init. P.'  .16  >  It w i l l be n o t i c e d that the H S, 2  l i k e the m e r c a p t a n s , is p r e s e n t  before the induction p e r i o d i s over, but the amount r e m a i n s m u c h l o w e r than that of the m e r c a p t a n s throughout the e x p e r i m e n t . However, the  HS 2  p r e s s u r e s t e a d i l y r i s e s whereas the m e r c a p t a n p r o d u c t i o n l e v e l s off o r perhaps even d e c r e a s e s a f t e r 12 m i n u t e s . E f f e c t of NO  on Rate of R e a c t i o n .  I n h i b i t i o n of a.reaction by NO  i s a method often used to t e s t for,  the p r e s e n c e of f r e e r a d i c a l s i n the r e a c t i o n m e c h a n i s m (5). In f i g . 12 are shown four r e a c t i o n - t i m e p l o t s . P l o t .I i s a t y p i c a l c u r v e obtained when no N O i s present. Plots 2, 3 and 4 w e r e made w i t h 0.6 mm., and 10.0 mm.  of NO  1.0  mm.  p r e s e n t i n the r e a c t i o n v e s s e l , r e s p e c t i v e l y . It w i l l  be n o t i c e d f r o m plot 2 that when a p p r o x i m a t e l y  0.6 mm.  of NO  had been  0  10  20  Time i n minutes* E i g * 12'.  E f f e c t of NO on r e a c t i o n rate of thermal decomposition of dimethyl d i s u l f i d e at 340°C.  25  introduced, the i n d u c t i o n p e r i o d was longer and the rate of r e a c t i o n s l o w e r u n t i l about ten minutes had elapsed, after w h i c h t i m e the r a t e r o s e r a p i d l y to the same value as i n plot 1, when no N O was p r e s e n t . P l o t 3 i s a r e a c t i o n - r a t e curve when 1 mm. of NO was i n t r o d u c e d into the r e a c t i o n chamber. It appears that this amount of N O has the best i n hibiting effect, as m a y be seen f r o m the graph. A n y amount of N O  over  1 mm. had the effect of reducing the i n d u c t i o n p e r i o d and i n c r e a s i n g the t o t a l p r e s s u r e i n c r e a s e , £ P . P l o t 4 i n F i g . 12 i l l u s t r a t e s this v e r y eff e c t i v e l y . In this run, 10 mm. of N O w e r e admitted to the r e a c t i o n v e s s e l and 42 mm. of m e t h y l d i s u l f i d e . The i n d u c t i o n p e r i o d was shortened to 3 minutes. The m a x i m u m rate, 'however, as shown by the steepest p a r t of the curve, was the same as when no NO was p r e s e n t . E f f e c t of Surface on Rate of R e a c t i o n . The rate of a gaseous r e a c t i o n c a n f r e q u e n t l y be a l t e r e d by v a r y ing the s u r f a c e - t o - v o l u m e  r a t i o of the r e a c t i o n v e s s e l . I n this i n v e s t i g a -  tion, the r e a c t i o n v e s s e l was packed w i t h s h o r t p i e c e s of q u a r t z cut f r o m quartz tubing. A f o u r - f o l d i n c r e a s e i n the s u r f a c e - t o - v o l u m e  r a t i o was  attained. The p r e s s u r e - t i m e c u r v e s f o r the r e a c t i o n a r e g i v e n i n F i g . 13. F o r an e x p e r i m e n t a l r u n i n an unpacked v e s s e l , the m a x i m u m rate of  f  p r e s s u r e i n c r e a s e obtained was 4.2 mm./min. when the i n i t i a l p r e s s u r e •was 45.8 mm.  U s i n g the equation dp/dt = k c , where k i s a constant and  c the i n t i a l p r e s s u r e , the m a x i m u m rate m a y be c a l c u l a t e d f o r a r u n when the i n i t i a l p r e s s u r e would be 39.4 mm.  The c a l c u l a t e d m a x i m u m  rate t u r n s out to be 3.6 mm./min. Two runs w e r e made w i t h the r e a c t i o n  0  10  20  Time i n Minutes* F i g . 13.  The e f f e c t of packing the reaction v e s s e l , on the thermal decomposition of dimethyl d i s u l f i d e at 340°C.  26  v e s s e l packed. The i n i t i a l p r e s s u r e i n both c a s e s being 39.4 mm. of m e t h y l d i s u l f i d e . The m a x i m u m rate f o r each of these runs was found to be 2.1 mm./min. That i s , i n c r e a s i n g the s u r f a c e - t o - v o l u m e r a t i o by a f a c t o r of four, d e c r e a s e d the rate by a f a c t o r of 1.7. The m o s t m a r k e d inhibition, of the r e a c t i o n i s shown by the f i r s t p a r t of the c u r v e . A f t e r the p r e s s u r e began to c l i m b , the rate was v e r y slow f o r the f i r s t f i v e - m i n u t e p e r i o d of the i n c r e a s e . F o l l o w i n g that, however, the rate q u i c k l y r o s e to its m a x i m u m value f o r the r u n . I t ' appears that the r e a c t i o n was of the c h a i n m e c h a n i s m type w i t h the c h a i n being b r o k e n at the s u r f a c e . However, a p p a r e n t l y m o r e chains w e r e i n i t i a t e d i n the c o u r s e of the t h e r m a l d e c o m p o o s i t i o n of m e t h y l d i s u l f i d e than w e r e b r o k e n at the s u r f a c e , w i t h the r e s u l t that the rate b u i l t up to a m a x i m u m and then slowed up again. D u r i n g the l a s t 10 minutes of a 30r minute run, however, the r e a c t i o n p r o c e e d e d at a h i g h e r rate than i n an unpacked v e s s e l but i t d i d not, d u r i n g the 30-minute p e r i o d , approach the t o t a l p r e s s u r e i n c r e a s e of a s i m i l a r r u n made i n an unpacked v e s s e l . T h e r m a l D e c o m p o s i t i o n of E t h y l D i s u l f i d e . The t h e r m a l d e c o m p o s i t i o n of E t h y l D i s u l f i d e was studied w i t h a view to d e t e r m i n i n g what the effect of substituting C H 2  5  for C H  3  would  have on r e a c t i o n rate and energy of a c t i v a t i o n . S e v e r a l e x p e r i m e n t a l , runs were made at one t e m p e r a t u r e s t a r t i n g w i t h d i f f e r e n t i n i t i a l p r e s sures to determine the o r d e r of the r e a c t i o n . Other runs w e r e made at t e m p e r a t u r e s ranging f r o m 330 °C. to 370 °C. i n o r d e r to obtain data f o r the d e t e r m i n a t i o n of the energy of a c t i v a t i o n .  27  The e x p e r i m e n t a l p r o c e d u r e was the same as f o r m e t h y l d i s u l fide. T h e only difference was that the ethyl d i s u l f i d e was contained i n a storage pot of its own .and so the p r o p e r tap had.to be m a n i p u l a t e d to introduce the ethyl.disulfide to the r e a c t i o n vessel,. . D e t e r m i n a t i o n of the O r d e r .of the R e a c t i o n . S e v e r a l e x p e r i m e n t a l runs w e r e f i r s t made on the apparatus at one t e m p e r a t u r e and w i t h the same i n i t i a l p r e s s u r e i n each case to check on the r e p r o d u c i b i l i t y of the r e s u l t s . Having been a s s u r e d the data gathered would not be e r r a t i c , three runs w e r e made w i t h i n i t i a l p r e s s u r e s o f 6.4 mm.,  8 mm. and 12 mm.,  r e s p e c t i v e l y . Since ethyl d i s u l f i d e  has a.lower vapour p r e s s u r e than, m e t h y l d i s u l f i d e , the m a x i m u m p r e s s u r e to which..the r e a c t i o n v e s s e l c o u l d be .filled, was 12 mm.  Conse-  quently, the n u m b e r of e x p e r i m e n t a l runs that c o u l d be made .were fewer than.for a p a r r a l l e l case w i t h m e t h y l disulfide.. The data for.the t h r e e runs i n q u e s t i o n i s shown plotted.in Fig.. 14. In Table V a r e given.the percentage i n c r e a s e s i n p r e s s u r e with r e spect to i n i t i a l p r e s s u r e f o r each r u n at the end of t i m e i n t e r v a l s of 4, 7 and 9 minutes.  ..  28  Table V m  P e r c e n t a g e i n c r e a s e i n p r e s s u r e w i t h r e s p e c t to i n i t i a l p r e s s u r e i n the t h e r m a l d e c o m p o s i t i o n of ethyl d i s u l f i d e at 350 °C.  Time min.  Run No. 1 , R u n No. 2 Init. P. = 6.4 mm. .Init. P. = 8.0 mm. P mm. % I n c r e a s e P mm. % I n c r e a s e  4  3.9  61%  4.4  7  5.8  90%  ?  6.7  96%  R u n No. 3 Init. P. = 12.0 mm. P mm. % I n c r e a s e  . .55%  7.1  60%  6.8 -  85%  1.0.2  85%  7.7  96%  11.4  95%  It w i l l be n o t i c e d that- i r r e s p e c t i v e of the i n i t i a l p r e s s u r e , the percentage i n c r e a s e i s a p p r o x i m a t e l y the same f o r each r u n i n any one t i m e i n t e r v a l . F o r instance, at the end of 4 minutes the p r e s s u r e h a d . i n c r e a s e d by a p p r o x i m a t e l y 6 0 % of the o r i g i n a l p r e s s u r e , at the end of 7 minutes the i n c r e a s e was a p p r o x i m a t e l y 8 5 % and at the end.of 9 minutes the. i n c r e a s e was a p p r o x i m a t e l y 9 6 % . It appears, t h e r e f o r e , that the rate of d e c o m p o s i t i o n i s d i r e c t l y p r o p o r t i o n a l to the i n i t i a l c o n c e n t r a t i o n and so the r e a c t i o n must be of the f i r s t o r d e r . D e t e r m i n a t i o n of the E n e r g y of A c t i v a t i o n f o r the D e c o m p o s i t i o n of E t h y l Disulfide. The method e m p l o y e d f o r the d e t e r m i n a t i o n of the energy of a c t i v a t i o n of ethyl d i s u l f i d e was the same as f o r m e t h y l d i s u l f i d e . E x p e r i m e n t a l runs were made at five d i f f e r e n t t e m p e r a t u r e s ranging f r o m 330 °C to 370 °C., ( F i g . 15). Two o r m o r e runs w e r e made at each t e m p e r a t u r e  Time i n minutes. F i g . 15.  Rate curves f o r the thermal decomposition of D i e t h y l D i s u l f i d e at d i f f e r e n t temperatures.  29  to m a k e s u r e that the r u n c h o s e n f o r u s e i n the c a l c u l a t i o n s c o u l d be reproduced. A n i n t e r e s t i n g f e a t u r e n o t e d w a s that the i n d u c t i o n p e r i o d w a s m u c h s h o r t e r t h a n f o r m e t h y l d i s u l f i d e . A t 330 ° C . ,  ethyl disulfide began  to show a p r e s s u r e i n c r e a s e i n a p p r o x i m a t e l y 2 m i n u t e s .  This induction  p e r i o d w a s s h o r t e n e d u n t i l at 370 ° C . t h e r e w a s b a r e l y t i m e to f i l l the  .  r e a c t i o n v e s s e l b e f o r e the p r e s s u r e b e g a n to r i s e . A n o t h e r p o i n t of i n t e r e s t i s that at the h i g h e r t e m p e r a t u r e s e a c h c u r v e i s s e e n to be m a d e up of t h r e e p a r t s . . T h a t i s ,  t h e r e a r e two; d i s t i n c t d i p s i n e a c h c u r v e .  T h e s e d i p s w e r e a l s o o b s e r v e d at the l o w e r t e m p e r a t u r e s b u t s i n c e t h e r e a c t i o n was s l o w e r ,  the s e c o n d p o i n t w o u l d c o m e to the r i g h t of the  g r a p h w h e n p l o t t e d o n the s c a l e u s e d i n F i g . 15.  The total p r e s s u r e i n -  c r e a s e w a s f o u n d to r a n g e f r o m 100% at the l o w e r t e m p e r a t u r e s to  150%  at the h i g h e r t e m p e r a t u r e s . T h e m a x i m u m r a t e s of d e c o m p o s i t i o n of e t h y l d i s u l f i d e , f r o m the c u r v e s i n F i g . 15,  obtained  w e r e u s e d to c a l c u l a t e the r a t e c o n s t a n t ,  k,  for each t e m p e r a t u r e . T h e r e s u l t i n g data is g i v e n i n T a b l e V I f r o m w h i c h the g r a p h s h o w n i n F i g . 16 w a s p l o t t e d . F r o m the s l o p e of t h i s g r a p h , the e n e r g y of a c t i v a t i o n w a s c a l c u l a t e d i n the o r t h o d o x m a n n e r a n d w a s f o u n d to. be 41.1 k . c a l .  30  Table VI • R a t e d a t a f r o m the t h e r m a l d e c o m p o s i t i o n o f e t h y l d i s u l f i d e at d i f f e r e n t  Temp.  Run  °G.  No.  I  330  II  340  III  350  IV V  Rate mm./min.  k x 10 sec,"  temperatures;  3  1  log.(kx  10 ) 3  -l/T x  1.13  .0518  1.659  2.03  .380  1.631  2.59  3.59  .555  1.607  360  4.21  6.04  .782  1.580  370  6.82  9.78  .991  1.556  .8Z5 "  1.66  10  3  S u m m a r y of the E x p e r i m e n t a l R e s u l t s . T h e m a i n r e s u l t s o b t a i n e d i n t h i s i n v e s t i g a t i o n m a y be s u m m a r i z e d as (1)  follows. M e t h y l and ethyl disulfides decompose with a m e a s u r a b l e  v e l o c i t y at t e m p e r a t u r e s a b o v e 320 ° C .  A t 380 ° C . the r e a c t i o n  i s a l m o s t too r a p i d to f o l l o w o n the p r e s s u r e i n d i c a t o r . (2)  M e t h y l d i s u l f i d e h a s a n i n d u c t i o n p e r i o d of about 5 m i n u t e s at 340 " C . a l t h o u g h the m e r c a p t a n a n d H S a n a l y s i s w o u l d i n - • 2  d i c a t e that t h e s e p r o d u c t s b e g i n to a p p e a r as s o o n as the d i s u l f i d e h a s b e e n i n t r o d u c e d into the r e a c t i o n v e s s e l .  This i n d u c -  t i o n p e r i o d i s s h o r t e n e d to about 45 s e c o n d s at 380 ° C . (3)  T h e i n d u c t i o n p e r i o d f o r e t h y l d i s u l f i d e i s about 2 m i n u t e s  log (k x 10 ) 3  F i g . 16.  Determination of the Energy of A c t i v a t i o n for the thermal decomposition of Diethyl  Disulfide.  at 330 ° C . a n d o n l y 15 s e c o n d s at 370 ° C . T h e r a t e of r e a c t i o n is of the f i r s t o r d e r w i t h r e s p e c t to the i n i t i a l d i s u l f i d e  pressure.  A d d i t i o n of N O to the r e a c t i o n v e s s e l h a s a n i n h i b i t i n g e f f e c t o n the d e c o m p o s i t i o n of m e t h y l d i s u l f i d e w h e n a d m i t t e d i n s m a l l q u a n t i t i e s but s e r v e s as a p o s i t i v e  catalyst when p r e -  s e n t at a p r e s s u r e of 2 m m . , o r m o r e . I n c r e a s e of the s u r f a c e / v o l u m e  r a t i o b y a f a c t o r of 5,  c r e a s e s the m a x i m u m r a t e b y a f a c t o r of 1.7 f o r the  de-  decom-  p o s i t i o n of m e t h y l d i s u l f i d e . M e r c a p t a n s a n d H S a p p a r e n t l y a r e f o r m e d to a l a r g e 2  t e n t i n the d e c o m p o s i t i o n ;  T h e f o r m a t i o n of m e r c a p t a n s  exbeing  m u c h m o r e r a p i d at f i r s t t h a n the f o r m a t i o n of H S . 2  T h e t o t a l p r e s s u r e i n c r e a s e r e l a t i v e to the i n i t i a l p r e s s u r e of the d i s u l f i d e w a s about 70% f o r m e t h y l d i s u l f i d e , f r o m . 100% to 150% f o r e t h y l d i s u l f i d e ,  and r a n g e d  d e p e n d i n g o n the  temperature. A black deposit,  n o t i c e d o n the w a l l s of the r e a c t i o n v e s s e l  if the r e a c t i o n p r o d u c t s w e r e a l l o w e d to r e m a i n i n i t f o r about twelve hours,  or,  c a u g h t i n the l i q u i d - a i r t r a p o n e v a c u a t i o n ,  w a s f o u n d to be a m e r c u r y c o m p o u n d . T h e m e r c u r y a p p a r e n t l y c a m e f r o m the m a n o m e t e r ,  one e n d of w h i c h w a s i n c o n t a c t  w i t h the r e a c t i o n p r o d u c t s . T h e e n e r g y of a c t i v a t i o n f o r the t h e r m a l d e c o m p o s i t i o n of  32  m e t h y l d i s u l f i d e w a s f o u n d to be 50.8 k . c a l . a n d f o r e t h y l d i s u l f i d e 41.1 k . C a l .  DISCUSSION  A m e c h a n i s m f o r the t h e r m a l d e c o m p o s i t i o n of m e t h y l d i s u l f i d e only,  w i l l be c o n s i d e r e d h e r e .  E t h y l d i s u l f i d e w i l l not be d e a l t w i t h i n  the s a m e m a n n e r s i n c e none of the p r o d u c t s * of its d e c o m p o s i t i o n w e r e investigated. B e f o r e p r o p o s i n g a m e c h a n i s m f o r d e c o m p o s i t i o n of m e t h y l d i s u l f i d e it m a y be w e l l to r e v i e w b r i e f l y the o v e r a l l e x p e r i m e n t a l p i c t u r e to the e x p l a n a t i o n of w h i c h the s t e p s i n the m e c h a n i s m m u s t  necessarily  be d e d i c a t e d a n d b y w h i c h t h e y m u s t a l s o be l i m i t e d . First,  it was n o t i c e d t h a t at 340 ° C . t h e r e was a n i n d u c t i o n  p e r i o d w h i c h was not v o i d of r e a c t i o n f o r the e n d of 5 m i n u t e s , the p r e s s u r e h a d s t a r t e d to r i s e ,  b o t h m e r c a p t a n r , a n d H S w e r e f o u n d to  be p r e s e n t i n a p p r e c i a b l e a m o u n t s . of p r e s s u r e i n c r e a s e ,  before  2  Second,  i n t h e ' p e r i o d of h i g h e s t r a t e  the r e a c t i o n was f o u n d to be f i r s t o r d e r w i t h r e -  s p e c t to the i n i t i a l d i s u l f i d e c o n c e n t r a t i o n .  Then,  the t o t a l p r e s s u r e i n -  c r e a s e w a s f o u n d to be a p p r o x i m a t e l y 70% of the i n i t i a l p r e s s u r e ,  and  f r o m F i g . 11 it is e v i d e n t that the s u m of the m e r c a p t a n a n d H S p r e s 2  s u r e s d o e s not e q u a l the t o t a l p r e s s u r e  recorded. Therefore,  either  there  is a n e q u i l i b r i u m e s t a b l i s h e d b e t w e e n p r o d u c t s and p a r e n t s u b s t a n c e , one m o l e c u l e of d e c o m p o s i n g molecules  substance  or,  d o e s not n e c e s s a r i l y y i e l d two  of p r o d u c t , o r b o t h p o s s i b i l i t i e s  a r e a r e a l i t y a n d last but not  33  least,  o t h e r g a s e o u s s p e c i e s m a y b e p r e s e n t . a m o n g the p r o d u c t s . T h a t the . r e a c t i o n . i s of the f r e e r a d i c a l ,  c h a i n - m e c h a n i s m type,  h a s b e e n s h o w n b y the e f f e c t t h a t p a c k i n g the r e a c t i o n v e s s e l a n d i n t r o ducing n i t r i c oxide,  h a d o n the r a t e of d e c o m p o s i t i o n .  P a c k i n g the  s l i g h t l y l e n g t h e n e d the i n d u c t i o n p e r i o d a n d r e d u c e d the r a t e . t i o n of 1 m m . of N O d a m p e n e d the r a t e e f f e c t i v e l y o v e r t h e  vessel  Introduc-  greatest-part  of a 30 m i n u t e p e r i o d . L e s s t h a n 1 m m . of N O s l o w e d d o w n the  reaction  at f i r s t but as s o o n as it h a d b e e n u s e d u p , the r a t e r o s e to the  same  v a l u e as w h e n no N O w a s p r e s e n t . c a t a l y t i c a c t i o n o n the r e a c t i o n , Finally,  M o r e t h a n 1 m m . of N O h a d a p o s i t i v e  as i s o f t e n f o u n d to be the c a s e  o n l y two c o m p o n e n t s w e r e a n a l y s e d f o r i n the  (5). decom-  p o s i t i o n m i x t u r e . T h a t o t h e r p r o d u c t s w e r e f o r m e d , , i s a vpo-s'isibility t h a t cannot,  and i n this c a s e ,  w i l l not be o v e r l o o k e d . T h e i d e n t i t y a n d r o l e of  s u c h o t h e r p r o d u c t s w i l l be left f o r b e t t e r s u i t e d d i s c u s s i o n ,  i n the  de-  v e l o p m e n t of the m e c h a n i s m . In c o n s i d e r i n g the d i f f e r e n t p o s s i b l e reaction,  s t e p s of the  decomposition  one m u s t b e a r i n m i n d the v a l u e of the d i f f e r e n t b o n d s t r e n g t h s .  T h e l a t e s t v a l u e s of the d i f f e r e n t b o n d s t r e n g t h s  a r e as d i s c u s s e d at the  b e g i n n i n g of t h i s p a p e r . T h e v a l u e s c o n c e r n i n g t h i s i n v e s t i g a t i o n a r e follows.  as  34 Bond Strength Compound CH SSCH 3  Bond  kcal./mole  CH S-SCH  3  3  CH3SSCH3  73.2  3  CH3-SSCH3  CH S  73.2  . CH -S  3  .  3  CH SH  CH S-H  H S  H-SH  3  •  52.4 ,  3  2  SH  .  S-H  .  (14).  .  -  100  79 - 89  are those given by F r a n k l i n and L u m p k i n  T h e C - H and C - C bond strengths  F r o m the a b o v e t a b l e , strengths  67.0  90 -  C-C first six values  88.8 95.3  C-H  The  (approx.)  are those given by Steacie  (15).  i t i s s e e n t h a t the C - S a n d the S - S b o n d  i n C H 3 - S - S - C H 3 a r e a p p r o x i m a t e l y equal. T h e i n i t i a l steps in  the d e c o m p o s i t i o n c o u l d be as CH SSCH 3  3  follows.  —>  2CH S  (l)  3  CH S  + CH SSCH 3  3  —v  CH SH  CH S  + CH SSCH  3  —f  CH SCH SSCH  3  3  2  3  3  + CH SSCH 2  2  3  3  (2) (3)  T h e s e t h r e e s t e p s w o u l d a c c o u n t f o r the f o r m a t i o n of m e r c a p t a n w i t h no increase in p r e s s u r e .  Next,  s i n c e the C - S b o n d s t r e n g t h i n C H S ' i s o n l y 3  52.4 k . c a l . w h i l e the C - H b o n d s t r e n g t h i s a p p r o x i m a t e l y 95 k . c a l . , r a d i c a l c o u l d r e a c t , as •CH3S  follows.  —>  CH  S + CH3SSCH3  3  + S —•  (4) SH + CH SSCH 2  3  (5)  the  + CH3SSCH3  SH CH  3  + CH S  —>  2  2  or  .(7) .  CH  3  + CH SSCH 2  —>  3  . (6)  3  CH3SCH3  —>  3  H S + CH SSCH  ,  CH CH SSCH 3  2  . (8)  3  or.  , CH  3  + CH  C H  3  2  (9)  6  T h i s w o u l d a c c o u n t f o r t h e f o r m a t i o n of H S w i t h no r e s u l t a n t p r e s s u r e 2  increase.  A n o t h e r p o s s i b l e s t e p i n the r e a c t i o n i s CH SSCH 2  3  —>  CH S  + CH S  2  (10)  3  T h i s is a chain propagating step. T h e C H S r a d i c a l would result in either, 3  a m o l e c u l e of m e r c a p t a n o r H S . It i s n ' t l i k e l y t h a t the t h i o f o r m a l d e h y d e 2  w o u l d r e m a i n i n t a c t s i n c e i t h a s b e e n s h o w n t h a t f o r m a l d e h y d e f o r m e d as an i n t e r m e d i a t e s t e p i n the d e c o m p o s i t i o n of d i m e t h y l e t h e r at 5 0 4 ° composes  fifteen t i m e s faster than would.be expected f r o m its  r a t e of d e c o m p o s i t i o n a l o n e (15).  de-  normal  T h i s i s p r o b a b l y due to r a d i c a l  sensi-  t i z a t i o n . A s i m i l a r fate m a y b e f a l l a n y t h i o f o r m a l d e h y d e w h i c h m i g h t be f o r m e d in this investigation. CH S 3  CHS H  + CH S  (11)  CS + H  + CH3SSCH3  . H + H + M Here,  CH3SH.+ C H S  —•  2  —>  T h e r e f o r e , f u r t h e r s t e p s c o u l d be,  —>  (12)  —*• H H  2  2  + CH SSCH 2  (13)  3  + M  M i s a t h i r d b o d y . O n e .would t h e r e f o r e e x p e c t to f i n d H  (14)  2  as one of  the p r o d u c t s . . U n f o r t u n a t e l y , - h y d r o g e n w a s not a n a l y s e d f o r as t h e r e insufficinet t i m e .  was  A n o t h e r s p e c i e s that c o u l d be e x p e c t e d i s m e t h a n e if  36  m e t h y l r a d i c a l s a r e f o r m e d i n the r e a c t i o n . CH  3  +  CH  3  + CH S  CH  3  + CH SH  C H 3 S S C H 3  Thus,  + CH SSCH  — >  C H 4  C H 4  + CHS  2  (15)  3  or —>  2  (16)  or  Therefore,  CH4 + CH S  —>  3  (17)  3  it i s s e e n that a t h o r o u g h a n a l y s i s of the p r o d u c t s m u s t be  m a d e i f a s a t i s f a c t o r y m e c h a n i s m i s to be p r o p o s e d . m e c h a n i s m suggested above, reactions  4,  2 ,  5,  pagating steps,  6,  In the  possible  r e a c t i o n ( l ) i s the c h a i n i n i t i a t i n g s t e p , 13,  1 0 , 1 1 , 1 2 ,  w h i l e r e a c t i o n s -3, 7,  15,.- 16 a n d 17 a r e the c h a i n p r o 8,  9 a n d 14 a r e c h a i n t e r m i n a t i n g  steps. D e r i v a t i o n of a R a t e E x p r e s s i o n . The  p r o b a b l e s t e p s i n the m e c h a n i s m m a y be s u m m a r i z e d  as  follows i C H 3 S S C H 3  — •  2 C H 3 S  xi CH S  + CH SSCH  3  x  3  2  - *  3  CH x  2  S + CH SSCH 3  x  7  SH X  8  3  —*  x  3  "  ->•  Xj  3  —>  3  (  2  )  CH SCH SSCH 3  2  3  (3)  5  (4) 7  SH + CH SSCH 2  x  r  + CH SSCH  )  2%  + S  6  3  2  3  x  3  + CH SSCH  3  X4  CH S  x  CH SH x  + CH SSCH  z  x  •—•  3  l  3  1  2  x  2  CH S x  x  (  (5)  3  ^4  8  H S + CH SSCH 2  X9  2  X4  3  (6)  37  GH  4" C H 3 S  3  6  x  CH X  +  3  CH3SCH3  —  CH SSCH 2  CH CH2SSCH 3  3  X4  6  CH  C H 2  3  X6  CH SSCH 2  (9)  6  X12 CH S  -+  3  +  2  CH S  +  3  x  x  3  +  x  H.  + CH3SSCH3  16  x  +  H  i6  x  CH x  +  3  x  x  1  +  2  6  x  CH SSCH 2  (13)  3  4  (14)  M  CH4 +  CH SSCH 2  (15)  3  X4  18  CH4 + C H S  —*•  xis  3  3  4  1 7  i  + CH SH  3  1  x  x  2  X6  CH  H  —»  +  2  1 7  C H 3 S S C H 3 —*  + CH S  3  3  H  x  x  x  6  --r  i6  6  CH  1  x M  (14)  + GHS  (12)  l  +  2  H  X15  w  H x  1  x  CS  CHS x  3  2  2  X CH SH  CH S  (10)  3  Xl3 CH S  (8)  3  n  x  + C H —-  3  X6  x  (7)  z  x  —•  x  CHi  1  4  + CH  xis  3  (16)  x  (17)  S  3  2  A s s u m i n g s t e a d y - s t a t e c o n d i t i o n s we o b t a i n f o r the t i o n of r a d i c a l s a n d o t h e r t r a n s i e n t dCH S 3  _  dx  dt  dt  2  0  species:  = kjxj + kjfp^ + k  -k x X4 3  dCH SSCH dt 2  3  =  ^ 4 , dt  =  concentra-  2  0 = k x 2  - 1*4X2  _  + k x  2 X l  5  1 7  x x  k  6  3  T 6 2 x  k 2l  -  x  "  x  x  2  k  n  x  z  x  + k^gxx + k  7 X l  (18)  l 3  1 3  x  1 6 X l  + k X X ! - k X X4 " 8 6 X 4 - l6X4 15  6  3  2  k  x  k  (19)  38  dCH — —  _ -dx - —  3  6  n  = 0 .=  ., k4X  , - k x x  2  7  6  i i ? r - k x X4 - k x 2 - k  2  - l6X6Xi3 - k k  If  =  l!t~  dSH  0  "  5  dGH S  dx  =  2  t  ^  =  5  k  x  n 0  T  i = lo*4  _  dt  n  0 -= k  1 2  2  x  x x 6  1 5  x  6 X l  (20)  3  l  (  2 1  u  1 4  1 3  --  2  + k  )  (22)  , - k x x  k  n  x  1 7  6  9  - kptgxj  T  •= 0 = k x x  ^El6  dt'  =  1 3  =  From  ****  =  dx, f- = 0 = k x x i dt  dt  d  =  6  8  1 3  l 6 X 6  -  , k  1 3  -  .  x  _-  kijxx&i  t  x  l 6 X 6  k  1 2  _ . „x _.z k 1 4  f 1 3  x  - , ? \ (23)  (24)  1 4  (25)  z  1 4  (18), k  CH S 3  = x  2  =  + k + k x x * k X! + k x4 + k* + k x + 2  l X l  3  l o X 4  1 7  7  6  6  _ (26)  3  k x n  1 3  S t e p s 3 a n d 7 ( r a d i c a l r e c o m b i n a t i o n ) w i l l be s m a l l c o m p a r e d to the and t h e r e f o r e m a y be o m i t t e d f r o m the d e n o m i n a t o r .  others  Step 17 w i l l a l s o be  s m a l l s i n c e the p r o d u c t i s p r e d o m i n a n t l y a m e r c a p t a n f o r the f i r s t p a r t of the r e a c t i o n . S t e p 4 m u s t a l s o be s m a l l s i n c e t h e r e i s l i t t l e H S 2  formed.  T h e a m o u n t of m e r c a p t a n f o r m e d b y s t e p 11 w i l l be s m a l l s i n c e  it i s a c c o m p a n i e d b y the f o r m a t i o n of C S w h i c h c o u l d not a c c o u n t f o r a l a r g e a m o u n t of the p r e s s u r e as R S H i s f o u n d to be the m a j o r p r o d u c t . Therefore, x  2  =  (26) k  becomes + k  l X l  k  From  (19),  2Xi  l o X 4  (27)  39  CH SSCH 2  = X4 =  3  k  2 2*i + k x  +• kfpcgxx  x  5  7 X l  +  k  k x + k X6 + k 3  2  8  x  1 3  1 6 X l  -I- k  I 5  x  ^ ^  6 X l  2Q  1 0  Since the p r e d o m i n a n t product f o r the f i r s t p a r t of the r e a c t i o n is a m e r captan,. steps 5, 6, 13 and 15 would contribute l i t t l e i n that p e r i o d . Steps 3 and 8, ( r e c o m b i n a t i o n of r a d i c a l s ) would a l s o be n e g l i g i b l e . T h e r e f o r e , (20) b e c o m e s X4  _• k X X! 2  (29)  2  kio  F r o m (23), CH S = x 2  =  1 3  " f f k x + k x k  u  .  l  2  l 6  (30)  6  Since the greates percentage of the product i s R S H , step (16) must be s m a l l and (30) bee omes -  =  kjox^ k x n  ( 3 1 )  2  The rate of r e a c t i o n i s a l m o s t the same as the rate of p r o d u c t i o n of m e r c a p t a n s i n c e the m e r c a p t a n i s the m a i n product d u r i n g the p e r i o d when the steepest r i s e i n p r e s s u r e i s noted. rate = d C—HaS H 3  = —d-xi = , k x 3  2  dt  2 X l  +, , k  .x  l l X 2  l 3  Therefore,  - ,k x x 1 7  6  3  dt  Again, since the m e r c a p t a n i s the m a i n product  step (17) w i l l be n e g l i -  gible and the equation becomes ^  = k x 2  2 X l  + k x x n  2  (32)  1 3  F r o m equations (31) and (29) we can substitute f o r the l a s t t e r m , thus, dx 3 = k x X ! + k x X ! = 2k x xx dt 2  2  2  2  2  2  (33)  40  E q u a t i o n ( 2 7 ) g i v e s us the  relation  k x X! = k x i + ki X4 2  We a r e ,  however;  x  2  (34)  0  c o n c e r n e d w i t h the i n i t i a l r a t e of d e c o m p o s i t i o n ,  Xj w o u l d be v e r y s m a l l c o m p a r e d w i t h x j . T h e r e f o r e , the r i g h t w o u l d be n e g l i g i b l e k x x 2  2  r  a n d (34)  rate  Thus,  the s e c o n d t e r m o n  yields  = k x!  (35)  x  so that the r a t e e x p r e s s i o n ,  (33),  = ^S dt 3  when  m a y be w r i t t e n  = 2kjXi  = kxj-  (36)  the m e c h a n i s m p r e s e n t e d y i e l d s a r e a c t i o n r a t e f o r the p r o d u c t i o n  of m e r c a p t a n w h i c h is f i r s t o r d e r w i t h r e s p e c t to the c o n c e n t r a t i o n o f disulfide.  S i n c e the m e r c a p t a n i s p r o d u c e d i n m u c h g r e a t e r  quantity than  the H S , e s p e c i a l l y so d u r i n g the p e r i o d of h i g h e s t r a t e of r e a c t i o n , 2  o v e r a l l r e a c t i o n i s f i r s t o r d e r w i t h r e s p e c t to the  the  the  disulfide.  A l t h o u g h the a b o v e m e c h a n i s m d o e s p r e d i c t the p r o p e r d e p e n d e n c e of r a t e o n the c o n c e n t r a t i o n of the d i s u l f i d e u s e d ,  the  t h e m s e l v e s s h o u l d be c o n s i d e r e d as t o k e n r e a c t i o n s o n l y ,  reactions  representing  the t y p e s of p r o c e s s e s w h i c h c o u l d be o c c u r r i n g . It m a y w e l l be that other reactions C o m a y (8),  a l s o take p l a c e .  F o r instance,  F a r a g h e r , M o r r e l l and  i n t h e i r w o r k o n the d e c o m p o s i t i o n of e t h y l d i s u l f i d e d u r i n g a  distillation process, reaction products.  e s t a b l i s h e d the p r e s e n c e In t h i s i n v e s t i g a t i o n ,  of t h i i o p h e n e a m o n g the  t h i o p h e n e was not a n a l y s e d f o r ,  a n d i s s o m e t h i n g that w o u l d be w o r t h w h i l e d o i n g i n f u r t h e r w o r k o n the problem.  O t h e r s u b s t a n c e s t h a t s h o u l d be a n a l y s e d f o r a r e h y d r o g e n ,  41  methane, ethane and o l e f i n s . F a r a g h e r , M o r r e l l and C o m a y (8), w o r k e d on the d i s t i l l a t i o n of d i s u l f i d e s at 496° and obtained a t a r r y r e s i d u e as p a r t of t h e i r product. O b s e r v a t i o n of any such r e s i d u e i n this i n v e s t i g a t i o n was c o m p l i c a t e d by the appearance of the m e r c u r y compound as stated p r e v i o u s l y . However, t r e a t m e n t of the b l a c k substance f o r m e d , w i t h carbon d i s u l f i d e ,  produced  no c o l o r i n the solvent so that it does not s e e m l i k e l y that any p o l y s u l f i d e t a r s a r e f o r m e d at the t e m p e r a t u r e of the e x p e r i m e n t . D e c o m p o s i t i o n of d i s u l f i d e s and p e r o x i d e s . The p r e s e n t i n v e s t i g a t i o n r e v e a l e d that the i n i t i a l i n c r e a s e i n p r e s s u r e f o l l o w i n g the induction p e r i o d i s due m a i n l y to the f o r m a t i o n of a m e r c a p t a n . T h i s i s p r o b a b l y m e t h y l m e r c a p t a n , the s i m p l e s t one of . the s e r i e s . T o e x p l a i n the r a p i d i n c r e a s e i n the p r e s s u r e of m e r c a p t a n , an i n i t i a l s p l i t of the -S-S- bond i n the d i s u l f i d e has been postulated. The C H S r a d i c a l s f o r m e d would then a b s t r a c t h y d r o g e n f r o m the parent m o l e 3  cule to y i e l d m e t h y l m e r c a p t a n . T h i s appears to be a p l a u s i b l e mechani s m s i n c e a s i m i l a r s p l i t of the -O-O- bond i s p o s t u l a t e d f o r a p e r o x i d e intermediate. In o r d e r to c o m p l e t e l y i l l u c i d a t e the m e c h a n i s m of d i m e t h y l d i s u l f i d e decomposition, however, it i s n e c e s s a r y to make a thorough a n a l y s i s of the products at different stages of the r e a c t i o n . The next step would be to study the d e c o m p o s i t i o n of the i n t e r m e d i a t e s formed.. F o r instance, the d e c o m p o s i t i o n of t h i o f o r m a l d e h y d e , a p o s s i b l e i n t e r mediate, should be studied at different t e m p e r a t u r e s and i n the p r e s e n c e  42  of f r e e r a d i c a l s . T h e s t a b i l i t y of the C H S r a d i c a l s h o u l d a l s o be 3  tigated.  Thus,  inves-  the p r e s e n t i n v e s t i g a t i o n i s o n l y the b e g i n n i n g of the  of a c o m p l i c a t e d but i n t e r e s t i n g r e a c t i o n .  study  REFERENCES  (1)  B r y c e , W. A. and Hinshelwood, C. N. T h e s i s •-• The R e a c t i o n s of H y d r o c a r b o n s  with Sulfur.  (2)  T r e a d w e l l , F. P. and H a l l , W. T. " A n a l y t i c a l C h e m i s t r y " , V o l . I. Q u a l i t a t i v e A n a l y s i s , p. 107 W i l e y and Sons, New Y o r k (1946).  (3)  B e l l , R. T. and A g r u s s , M. S. Ind. Eng. Chem., 13, 297-9 (1941).  (4)  B r y c e , W. A. and Hinshelwood, C . N . T h e s i s - The R e a c t i o n s of H y d r o c a r b o n s  w i t h S u l f u r , p. 70.  (5)  Hinshelwood, C . N . "The K i n e t i c s of C h e m i c a l Change" U n i v e r s i t y P r e s s , O x f o r d (1945).  (6)  P a u l i n g , I_. "The N a t u r e of the C h e m i c a l Bond", C o r n e l l U n i v e r s i t y P r e s s , Ithaca (1945).  (7)  T r e n n e r , N. R. and T a y l o r , H. A. J . Chem. Phys., 1_, 83 (1933).  (8)  F a r a g h e r , M o r r e l l and C o m a y Ind. Eng. C h e m . 20, 529 (1928).  (9)  G u l l i s , Hinshelwood, M u l c a h y and P a r t i n g t o n D i s c u s s i o n F a r a d a y Soc. 2_, 111 (1947).  (10) L e f f l e r , J . E . Chem. R e v i e w s 45_, 385 (1949). (11) B o l l a n d , J . L . Q u a r t e r l y R e v i e w s 3_, 385 (1949). (12) R i c e , F . O. and H e r z f e l d , K. F . J . A m . C h e m . Soc. 60, 284 (1934). (13) Stevenson, D. P. and B e a c h , J . Y. J . Am. Chem. Soc. 60, 2872 (1938).  (14)  F r a n k l i n , J . L . and L u m p k i n , H . E . J . A m . C h e m . S o c . 74,  (15)  Steacie,  1023  (1952).  E . W. R.  " A t o m i c and F r e e R a d i c a l R e a c t i o n s " , Corporation,  New  York.  Reinhold  Publishing  

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