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The induced decomposition of diethyl ether Lovell, Edwin Lester 1937

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THE INDUCED DECOMPOSITION OF DIETHYL ETHER by EDWIN LISTER LOVELL, B.A. A Thesis submitted f o r the degree of MASTER OP ARTS i n the Department of Chemistry A p r i l , 1937 INDEX PART I Introduction - - - - - - - - - - - - - - - - Page 1 General D e s c r i p t i o n of Apparatus - - - - - - 9 The C l i c k e r Gage - - - - - - - - - - - - - - 12 Temperature Control - - - - - - - - - - - - - 14 Preparation of Azomethane - - - - - - - - - - 17 Preparation of D i e t h y l Ether - - - - - - - - 22 Preparation of Gas Mixtures - - - - - - - - 23 Experimental Procedure - - - - - - - - - - - 24 Treatment of Data - - - - - - - - - - - - - - 29 The Suggested Mechanism - - - - - - - - - - 32 C a l c u l a t i o n of A c t i v a t i o n Energies - - - - - 36 Di s c u s s i o n of the Mechanism - - - - - - - - - 36 Products of the Reaction - - - - - - - - - - 38 E f f e c t s on the Chain-length - - - - - - - - 40 Further D i s c u s s i o n of Results - - - - - - - 41 PART II Pr e l i m i n a r y Results - - - - - - - - - - - - - 43 A New Apparatus - - - - - - - - - - - - - - - 45 Experiments - - - - - - - - - - - - - - - - - 47 SUMMARY - -. - - - - - - -.- - - - - - - Page 48. REFERENCES - - - - - - - - - - ; .w - Page 50- TABLES OF DATA • Page 53-THE INDUCED DECOMPOSITION OF DIETHYL ETHER The purpose of t h i s r e s e a r c h has been to e s t a b l i s h the e x i s t  ence or non-existence of long r e a c t i o n chains i n v o l v i n g f r e e r a d i c a l s , i n the normal p y r o l y s i s of d i e t h y l ether - by attem p t i n g to induce such chains at a much lower temperature than the normal decomposition r e q u i r e s , with r a d i c a l s produced from another source (azomethane) and f u r t h e r , determining the leng th and nature of such chains ( i f any) by a d e t a i l e d study of the f a c t o r s such as temperature and concentration, a f f e c t i n g them. The r e l a t i o n of such an i n v e s t i g a t i o n to current the o r i e s i n the f i e l d of Reaction K i n e t i c s , i s r e l a t e d i n the f o l l o w i n g paragraphs. Int r o d u c t i o n One of the f i r s t studies made on the thermal decomposit ion of d i e t h y l ether, was by Be r t h e l o t In 1863 1. The f i r s t modern i n v e s t i g a t i o n however was that of N e f 2 (1901), who showed that below 500°C the ether i s r e l a t i v e l y s t a b l e , but decomposes v i g o r o u s l y at 550° to give l a r g e amounts of acet aldehyde, together with ethylene, ethane, methane, carbon mon- oxide and hydrogen. A few years l a t e r , Nef attempted to e x p l a i n the p y r o l y s i s mechanism 4 on the basi s of h i s th e o r i e s concerning d i v a l e n t carbon. This mechanism in v o l v e d the p r e l i m i n a r y d i s s o c i a t i o n of the ether i n t o water and f r e e ethylidene r a d i c a l s C H 3 C H - -which then i n t e r a c t e d to give ethane, acetaldehyde and e t h y l - ene. As evidence f o r h i s theory, Nef quoted the f a c t u that ether decomposes q u a n t i t a t i v e l y into ethylene when passed over P2O5 even below the d i s s o c i a t i o n temperature; f u r t h e r , ether burns very slowly at room temperature i n dry oxygen, g i v i n g acetaldehyde - presumeably from the ethylidene r a d i c a l s pres ent i n e q u i l i b r i u m . The r a t e of thermal decomposition of d i e t h y l ether was s f i r s t s t u d i e d by Hinshelwood 6 i n 1927. His experiments, made i n the temperature range 525-588°C, i n d i c a t e d that, the re a c t  ion was homogeneous or n e a r l y so, and f i r s t - o r d e r over a wide range of pressure, the s p e c i f i c r a t e however f a l l i n g o f f i n value at the lower p r e s s u r e s . Further, he n o t i c e d that t h i s f a l l i n g - o f f d i d not occur i n the presence of excess hydrogen, altho other i n e r t gases f a i l e d to maintain the r a t e . In order to e x p l a i n h i s r e s u l t s , i n view of gas analyses on the re a c t  ion products, Hinshelwood suggested the f o l l o w i n g mechanism: (0 2 H 6 ) 2 0 • CO + i 2CH 4 + CH2-- • CO +• 2CH 4 + iC2H4 This i t may be noted also i n v o l v e d a d i s s o c i a t i o n of the ether i n t o f r e e r a d i c a l s , altho no evidence was given to support the Idea. It was at t h i s stage that Paneth and Hofeditz' 7 announ ced the p r e p a r a t i o n of f r e e methyl and l a t e r f r e e e t h y l r a d i o -a l s , f o r the f i r s t time° Previous to t h i s time, a not Inconsiderable amount of physico-chemical ( p a r t i c u l a r l y photochemical) evidence had been accumulated which p o i n t e d to the momentary existence of f r e e a l i p h a t i c r a d i c a l s i n chemical r e a c t i o n s ? Now however d i r e c t chemical evidence was a v a i l a b l e . P a n e t h ' 3 technique, i n b r i e f , c o n s i s t e d of sweeping metal a l k y l a at a very f a s t pumping speed thru a hot tube and then over a m e t a l l i c m i r r o r deposited i n the c o l d part of the tube. The experiments est a b l i s h e d beyond doubt that the metal a l k y l s when heated, d i s  s o c i a t e i n t o metal atoms and f r e e radicals,' e.g. ZnEtg = Zn + 2Et. These r a d i c a l s then u n i t e with the c o l d m i r r o r and the r e s u l t i n g products may be i d e n t i f i e d by chemical methods. Two years l a t e r , the Paneth m i r r o r technique was used by P.O.Rice and h i s co-workers to show that a great many organic compounds, when heated at 800-1000°C and a few mm pressure, d i s s o c i a t e into f r e e methyl and e t h y l r a d i c a l s ; among the com pounds so te s t e d was d i e t h y l ether. On the basi s of h i s exper iments, and those of Paneth i n which the h a l f - l i f e , temperat ure c o e f f i c i e n t and other p r o p e r t i e s of f r e e r a d i c a l s were i n  v e s t i g a t e d In some d e t a i l , Rice proposed the theory 9»10 that i n general the p y r o l y s i s of organic compounds proceeds t h r u a fr e e r a d i c a l mechanism, and not by i n t e r n a l rearrangements i n the substrate molecule followed by the s p l i t t i n g o f f of prod uct molecules as such. According to Rice, when an organic com pound i s heated I t breaks up i n t o r a d i c a l s , the k i n d of r a d i c  a l s being determined by the nature of the compound and the r e l -a t i v e strengths of i t s various bonds. A r a d i c a l thus formed may undergo e i t h e r of two types of r e a c t i o n ; i t may react with a molecule of the o r i g i n a l compound, forming a compound i t s e l f and l e a v i n g a new (heavy) r a d i c a l ; or i t may spontaneously de compose into a smaller r a d i c a l and a new compound. In t h i s manner a r e a c t i o n "chain" i s set up, which i s only broken when two f r e e r a d i c a l s u n i t e . Such mechanisms, Rice submits, form a more reasonable p i c t u r e now that the existence of f r e e e t h y l and methyl i s d e f i n i t e l y e s t a b l i s h e d , than the somewhat fantas t i c i n t e r n a l rearrangements which are the only a l t e r n a t i v e . U n f o r t u n a t e l y however the Paneth technique cannot be used to e s t a b l i s h the presence of f r e e r a d i c a l s except at high temper atures and low pressures, as otherwise the l i f e of the p a r t i c  l e s i s too short and t h e i r concentration too low f o r de t e c t i o n by t h i s method. Rice has r e c e n t l y worked out i n d e t a i l the f r e e r a d i c a l mechanisms to be expected > i n the p y r o l y s i s of many common organic compounds such as the ethers, aldehydes, ketones and hydrocarbons ( a l i p h a t i c ) . These mechanisms were so chosen that the a c t i v a t i o n energies assigned to the various steps would r e s u l t , i n each case, i n a c o r r e c t value f o r the o v e r a l l r a t e , the temperature c o e f f i c i e n t and the r e l a t i v e proportions of products produced i n the I n i t i a l stages. The a c t i v a t i o n energies i n many cases were also c o r r e l a t e d with thermochemic a l estimates of the various bond strengths i n v o l v e d . The mechanism proposed^ by Rice f o r the normal p y r o l y s i s of d i e t h y l ether, i s as f o l l o w s . Two chains are p o s s i b l e , C 2 H 5 0 C 2 H 5 >G2H50CH2 CH 3 CH3CH0 CH 3 C2H 50G 2H 5 R *RH C 2H 50G2H 4 CH3GHO + G 2 H 5 HCHO -*- C 2H 4 * G H 3 G 2H 50G2H 5 *G 2H 50+ O2H5 CH3CH0 * H This i s followed by a chain decomposition of the acetaldehyde formed, thus The formaldehyde probably decomposes d i r e c t l y i n t o hydrogen and carbon monoxide. These chains account f o r a l l the obser ved products (as w e l l as the observed intermediate aldehydes) and Rice has p r e d i c t e d the r e l a t i v e p r o p o r t i o n s i n which they should be found i n the i n i t i a l stages of the r e a c t i o n 9 but no data i s a v a i l a b l e to check h i s p r e d i c t i o n . One of the fi r s t - 1 - * - experiments c a r r i e d out to t e s t d i r e c t  l y f o r the presence of f r e e r a d i c a l r e a c t i o n chains i n a pyro- genie decomposition, was the work of A l l e n and SIckman i n 1934. According to these authors, the r e a c t i o n R+M should have a lower a c t i v a t i o n energy than the d i s s o c i a t i o n M-*radic a l s . Hence i f f r e e r a d i c a l s from an extraneous source are CH3CH0 + R RH + CH 3C0 GH 3 +• CO -6- mixed with a compound, chemical decomposition of the compound should he induced at a temperature lower than the normal decom p o s i t i o n temperature. This p r e d i c t i o n was t e s t e d by introduc ing methyl r a d i c a l s from azomethane i n t o acetaldehyde, at 300° C and lower. Chains were a c t u a l l y found to be produced under these c o n d i t i o n s , lengths as long as 500 u n i t s being obtained. The experiments were h i g h l y s a t i s f a c t o r y from the p o i n t of view of the chain theory since by mathematical treatment of the expected mechanism, these authors were able to c o r r e c t l y pre d i c t the manner i n which the t o t a l r e a c t i o n r a t e ( i n the i n i t  i a l stages) would vary with the aldehyde and azomethane pres sures r e s p e c t i v e l y ; f u r t h e r , a determination of the temperat ure c o e f f i c i e n t of the r e a c t i o n gave a value which agreed very c l o s e l y with the p r e d i c t e d value based on F.O.Rice's estimat ions of the various a c t i v a t i o n energies i n v o l v e d . In the same year (1934) two other induced chain decompos i t i o n s were rep o r t e d . Frey-^ decomposed c e r t a i n hydrocarbons i n the presence of mercury a l k i d e s . Leermakers- 5-6 r e l e a s e d methyl r a d i c a l s i n dimethyl ether by the p h o t o l y s i s of acetone vapour at 270-400°C, and obtained l a r g e quantum y i e l d s . The high-temperature p h o t o l y s i s of acetone alone showed that meth y l r a d i c a l s do not, below 400°C, i n i t i a t e chains In t h i s com pound. This conclusion was confirmed by an experiment of Rice i n which"'-''' methyl r a d i c a l s from dimethyl mercury were released i n acetone at 350-400°. Only C H 3 C O C H 2 . C H 2 C O C H 3 was obtained, equivalent to the H g M e 2 added. Above 400° however the chain Me 2C0^Me » CH4 + CH3COCH2—*'Me. •+- C H 2 : CO -7- was i n i t i a t e d . Below 300° the acetone was not attacked by the r a d i c a l s . It was suggested by A l l e n 1 ^ that s i n c e the chain-breaking step (the recombination of two r a d i c a l s ) i s a w a l l reaction?, by packing the r e a c t i o n v e s s e l i t should be p o s s i b l e to prod uce a n o t i c e a b l e decrease i n the rate of a r e a c t i o n , provided i t proceeds t h r u a f r e e r a d i c a l chain mechanism. T h i s sugges t i o n was f i r s t u t i l i z e d by Hinshelwood, i n a reinvestigation 1® of the normal p y r o l y s i s of acetone . The rate was very care f u l l y determined i n an empty s i l i c a bulb, then again with the bulb packed with s i l i c a spheres. The author reports a small but d e f i n i t e r e t a r d a t i o n of the rate i n the l a t t e r case, i n d i c  a t i n g that short chains may be present (540-600°C). S i m i l a r but more accurate experiments on acetone, at 460°, were c a r r i e d out by Allen 2*-* who was able to reduce the measur ed i n i t i a l r a t e to n e a r l y h a l f i t s normal value, by using a la r g e r e a c t i o n chamber packed with t u b i n g . These r e s u l t s seem ed to prove that acetone decomposes by a chain mechanism. Experiments with packed bulbs were also made by Hinshel wood on a c e t a l d e h y d e 2 1 at 526°C, but a b s o l u t e l y no r e t a r d a t i o n could be detected - i n d i c a t i n g the absence of chains i n the normal p y r o l y s i s . However, P.O.Rice was able i n the case of acetaldehyde to account on the bas i s of a chain mechanism, f o r pp pc the experimentally observed ^"^^ order of 1.5, an order which cannot apparently be explained by any other mechanism. A t h i r d method of t e s t i n g f o r r e a c t i o n chains was r e c e n t l y developed^ 0 by Hinshelwood and h i s co-workers. I t was found that n i t r i c o x i d e 3 3 reduces the quantum y i e l d i n the photo chemical decomposition of acetaldehyde from hundreds to u n i t y - hence NO appears to be a chain-breaker. As expected, when NO was added to p r o p a l d e h y d e 3 2 , the r e a c t i o n was d e f i n i t e l y i n h i b i t e d presumeably by shortening of the r e a c t i o n chains. For a c e t a l d e h y d e 3 1 and acetone2''1' however, no i n h i b i t i o n was observed. Hinshelwood s t a t e s i n t h i s paper that acetaldehyde must be a non-chain r e a c t i o n and that f u r t h e r the a c c e l e r a t i o n by azomethane observed by A l l e n 1 3 may be explained by homogen eous c a t a l y s i s . A f o u r t h and more d i r e c t method of t e s t i n g f o r r e a c t i o n - chains has been used by Patat and co-workers 17 i n Germany. By means of a deuterium exchange experiment, the hydrogen-atom concentration present i n various decomposing organic compounds was estimated: the substances studied included ethane, acetone, acetaldehyde and propaldehyde. Patat concludes from t h i s work that r e a c t i o n chains do a c t u a l l y e x i s t i n these cases, but such chains are f a r too few i n number to account f o r the t o t a l de composition as r e q u i r e d by the Rice theory. Recently, the p y r o l y s i s of ethylene o x i d e 1 2 has been the subject of much research i n t h i s f i e l d . This substance decora 's!: 7 p o s e s 0 ' to give acetaldehyde and f r e e methyl r a d i c a l s , which then induce a chain decomposition of the aldehyde. I t has t h e r e f o r e been used i n the study of Induced r e a c t i o n s 4 1 , i n p a r t i c u l a r with gaseous p a r a f f i n s 4 ^ , a c e t a l d e h y d e 3 8 > 3 9 and others The preceeding b r i e f summary of the present status of the R i c e - H e r z f e l d chain r e a c t i o n theory, r e v e a l s much evidence which i s seemingly c o n t r a d i c t o r y and which c e r t a i n l y cannot be explained u n t i l much more experimental data concerning the nature of chain r e a c t i o n s , has accumulated. The r e s u l t s of the present i n v e s t i g a t i o n on d i e t h y l ether together with the previous work on t h i s compound, are d i s c u s s e d In a l a t e r para graph. EXPERIMENTAL PART In the present experiments, the r e a c t i o n rates were measured by f o l l o w i n g the pressure i n c r e a s e with time, when the gases were decomposed at constant volume. This method was supplem ented by a s e r i e s of analyses of the r e a c t i o n products. The apparatus was designed to be capable of a c c u r a t e l y measureing rates i n the i n i t i a l pressure range of from 1 to 25 cm. In t h i s range the high-pressure r a t e of many gas r e a c t i o n s begins to e x h i b i t the pronounced decrease which Is of i n t e r e s t theor e t i c a l l y ^ . Further, the design was such that a v a r i e t y of gaseous mixtures c o u l d be r e a d i l y prepared and used f o r r a t e study. General D e s c r i p t i o n of the Apparatus The p y r o l y s i s chamber c o n s i s t e d of a 100 cc pyrex v e s s e l (3.3 by 14 cm), which was connected thru t h i c k c a p i l l a r y Fit. 2 -10- tubing (2 mm bore) to a small c a p i l l a r y stopcock. The press ure i n t h i s chamber was measured by means of a glass-membrane or " c l i c k e r " type of gage, f u n c t i o n i n g as a n u l l - p o i n t i n s t r u  ment. This gage was connected to the chamber thru a c a p i l l a r y side-tube sealed i n j u s t below the stopcock. The t o t a l volume of these connections o u t s i d e the r e a c t i o n chamber was probably not greater than 4cc. The balancing pressures outside the gage membrane were obtained by admitting or withdrawing a i r as necessary t h r u a s i n g l e three-way stopcock (S7). A i r was admitted slowly thru a c o n t r o l l e d leak (L2), and withdrawn by expansion i n t o a large previously-evacuated b o t t l e . The volume of the system outside the membrane was increased to n e a r l y a l i t r e , by i n t r o d u c i n g a l a r g e bulb (v) between the gage and the c o n t r o l l i n g stopcock. A i r pressures were then more e a s i l y adjusted. These pressures were measured e i t h e r on an ordinary c o n s t a n t - l e v e l mercury manometer (M2), or on a s p e c i a l l y - d e s i g n e d type of McLeod Gage pressures of from 5 to 18 cm. Azomethane was s t o r e d i n a 250-ml bulb (A), connected to the 500-ml mixing bulb thru a long mercury c u t - o f f (M3) which served also as a manometer f o r i n d i c a t i n g azomethane pressures and pressures of gas mixtures. L i q u i d d i e t h y l ether (about 5cc) was store d i n a tube (E) connected to the mixing bulb a mercury c u t - o f f f i t t e d with a c a p i l l a r y stopcock. Evacuation of the apparatus was c a r r i e d out by a mercury d i f f u s i o n pump, backed by a Hyvac r o t a r y o i l pump. The high--11- vacuum l i n e was f i t t e d with a McLeod Gage ( a l l g l a s s ) which permitted estimation of gas pressures down to 10"^ mm. The vacuum side of the various manometers of the pressure gage, were connected to t h i s vacuum l i n e t h r u stopcocks. The reaction-chamber stopcock was connected to the a i r - p r e s s u r e s i d e of the c l i c k e r gage t h r u the vacuum l i n e , as the pressures on e i t h e r s i d e of the membrane could not d i f f e r g r e a t l y without danger of r u p t u r i n g i t ; consequently both sides had to be evacuated simultaneously when pumping out from atmos p h e r i c pressure. T h i s stopcock a l s o l e d to a Topler Pump which formed part of a g a s - a n a l y s i s apparatus. For the t h i r d s e r i e s of experiments (Runs 30 et seq) the arrangement f o r s t o r i n g the gas mixtures was a l t e r e d somewhat. The 500cc mixing chamber (M) was p r o v i d e d with a s i n g l e stop cock (S12): the stopcock (S13) l e a d i n g from i t to the r e a c t i o n chamber was removed. A new chamber, c o n s i s t i n g of a long c y l  i n d r i c a l v e s s e l ( c a p a c i t y 425 ml) was connected to the r e a c t i o n chamber t h r u a mercury c u t - o f f , a l s o provided with a stopcock. This storage c y l i n d e r was f i t t e d with a mercury l e v e l i n g bulb, which permitted the gas pressure to be v a r i e d , w i t h i n l i m i t s . The change was made to remedy two defects i n the previous ar rangement - the i r r e p r o d u c i b i l i t y of a given i n i t i a l pressure of gas mixture (the pressure i n the storage chamber n a t u r a l l y f e l l as gas samples were withdrawn f o r each experiment) - and the i m p o s s i b i l i t y of making an experiment with e i t h e r ether or azomethane alone while a mixture was present i n the storage bulb. -12- The C l i c k e r Gage and Pressure-compensated McLeod Gage The glass-memhrane or " c l i c k e r " g a g e 5 0 was constructed of 2-mm bore pyrex tubing, by blowing a bulb of moderate t h i c k  ness and f l a t t e n i n g the end i n a hot flame, i n such a manner that a "wrinkle" r e s u l t e d on the f l a t s u r f a c e . The pressure d i f f e r e n c e on such a membrane necessary to produce a c l i c k sound i s constant over a wide range of temper atures and p r e s s u r e s 5 1 ; i n general the accuracy of the device often exceeds 0.001 cm, i f the z e r o - c o r r e c t i o n i s small. The z e r o - c o r r e c t i o n f o r the membrane used (corresponding to the a i r pressure at which the membrane c l i c k e d out, when the reac t i o n chamber was completely evacuated) was measured a c c u r a t e l y with a cathetometer. The value of t h i s c l i c k - c o n s t a n t was found to be n e a r l y 7 cm. As i t was d e s i r e d to make rate meas urements with i n i t i a l pressures of from 20cm down to one cm, a McLeod Gage was attached f o r use i n t h i s range. For higher pressures an o r d i n a r y mercury manometer (M2) c o u l d be used. The McLeod Gage was constructed of widest-bore heavy c a p i l l a r y tubing: a s e r i e s of bulbs gave magnifying r a t i o s of about 5, 8, 10 and 30, the gage being used as a c o n s t a n t - r a t i o instrument. The mercury head necessary to g i v e readings of s u i t a b l e accuracy i n the lowest range of pressures used, corres ponded to n e a r l y two atmospheres: to eliminate the n e c e s s i t y f o r such an unwieldly column of mercury, the usual form of Mc Leod gage was modified as shown i n the f i g u r e ( 3 ) . The a i r pressure to be measured was introduced thru a stopcock (S5) -13- and a mercury f l o a t valve (F) Into the compression volume of the gage (G). The l a t t e r was provided with three paper marks, e n c i r c l i n g the tube to avoid p a r a l l a x e r r o r s : with a backgr ound of black paper i t was found that t h i s arrangement gave by f a r the best v i s i b i l i t y of the mercury meniscus, i f s t r o n g l y i l l u m i n a t e d from high to one s i d e . Adjustment of the mercury l e v e l to the appropiate mark on the gage, was accomplished by manipulation of the three-way stopcock (S3) and the screw c l i p (C) by means of which the pressure over the mercury r e s e r v o i r (R) could be v a r i e d from a few centimeters to two atmospheres. One arm of t h i s stopcock was connected to a large evacuated b o t t l e , and the other to an oxygen tank g i v i n g 15 l b s / i n s 2 p r e s s u r e . The r e s u l t i n g head of mercury was read from a tube (H) of the same bore and t h i c k  ness as the gage i t s e l f , connected thru a l a r g e stopcock (S4) to one end of a mercury U-trap, the other end of which was sealed i n between the f l o a t valve and compression volume. The top of the mercury-head tube (H) was connected to a 300cc bulb i n which the a i r pressure could be v a r i e d up to one atmosphere by using the three-way stopcock (S2) attached. This pressure i n t u r n could be read o f f the attached manometer (Ml); the t o t a l head t h e r e f o r e was the sum of the pressures observed on t h i s manometer and the other mercury column (H). C a l i b r a t i o n of the m a g n i f i c a t i o n r a t i o s of the McLeod Gage was c a r r i e d out with the cathetometer, the pressures be ing read o f f the manometer (M2). C o r r e c t i o n was made f o r cap i l l a r y depression of the mercury l e v e l , by m e a s u r i n g t h i s sep--14- a r a t e l y . The c l i c k constant, as already mentioned, was also determined with the cathetometer. To set the c l i c k pressure, the vacuum r e s e r v o i r b o t t l e ( B ) was used at a pressure not much l e s s , so that the a i r i n the volume outside the c l i c k e r was withdrawn very slowly. When the c l i c k was heard, the stopcock (S7) was immediately c l o s e d . The e r r o r Involved here, c a l c u l a t e d from the observed r a t e of pressure f a l l (on M2) was n e g l i g i b l e as there was never more than one second delay i n c l o s i n g the stopcock; the s e t t i n g s , furthermore, were e n t i r e l y r e p r o d u c i b l e . Temperature C o n t r o l and Measurement The furnace used i n these experiments c o n s i s t e d of an a l - undum tube, wound c l o s e l y with heavy nichrome wire which was f i x e d i n p l a c e with alundum cement. This was surrounded by a metal case f i l l e d with i n f u s o r i a l e a rth and wrapped on the out side with sheet asbestos paper. To decrease the temperature gr a d i e n t , a somewhat smaller brass tube was p l a c e d i n s i d e the alundum one, and cemented i n t o p l a c e . Further, the r e a c t i o n chamber i t s e l f was wrapped c l o s e l y with a t h i n sheet of alum inum f o i l , and the mouth of the furnace was c l o s e d with four inches of cement p l u g . Inserted t h r u t h i s p l u g , i n t o the space beside the react ion chamber i n the furnace tube, was the stem of a quartz p l a t  inum r e s i s t a n c e thermometer. This was connected thru heavy copper wire to form one arm of a box bridge, f o r reading the r e s i s t a n c e . A s e n s i t i v e b a l l i s t i c galvanometer was used - i t was found by d i r e c t measurement that one centimeter on the •QfflfcJ • v h A l\ or =fl: or K — o -15- galvanometer s c a l e corresponded to about -^°C f o r a l l temper atures between zero and 500°C. C a l i b r a t i o n of the platinum r e s i s t a n c e thermometer was made according to U.S. Bureau of Standards Methods'-" . As r e f  erence p o i n t s , melting i c e , b o i l i n g s u l f u r , f r e e z i n g t i n and f r e e z i n g zinc were used; the c o o l i n g curves were r e a d i l y repro duced. The r e s u l t s are expressed by the equation f o l l o w i n g : Rrp s 50.36(1-*-3.8749xl0~ 5 - 6 .1086xlO" 7T 2 ) In p r a c t i c e , temperatures were read o f f a l a r g e - s c a l e graph of t h i s equation (1mm graph = -|-0C) . The temperature of the furnace was c l o s e l y adjusted by the use of a v a r i a b l e transformer ("Variac") u t i l i z i n g the l l O v l i n e and g i v i n g secondary voltages of from zero to 130v. An ammeter Indicated the current being used - about 2.7 amperes f o r a temperature of 300°C. Owing to the larg e f l u c t u a t i o n s i n the l i n e current supply i t was found necessary to devise a scheme to keep the current e f f e c t i v e l y constant. This was accomplished with the c i r c u i t shown i n f i g u r e (4). The furnace was connected i n s e r i e s with a h e a t i n g c o i l of nichrome ribbon, which was wrapped around the long (pyrex) mercury r e s e r v o i r of a thermoregulator. The l a t  t e r was provided with another r e s e r v o i r f i t t e d with a stopcock f o r s t o r i n g excess mercury r e s u l t i n g from expansion when the c o i l i s f i r s t heated. The whole was mounted i n a small wooden box, l i n e d with asbestos paper, and provided with binding posts o u t s i d e . A small, hole i n the top of t h i s box, Immediately over the c a p i l l a r y tube opening of the r e g u l a t o r , contained a t i g h t - f i t t i n g cork supporting a length of tungsten wire a c t i n g as an -16- e l e c t r i c a l contact with the mercury i n the c a p i l l a r y . Adjust ment of the height of .the contact p o i n t could then he made by simply s l i d i n g t h i s wire down t h r u the cork, to the required p o s i t i o n . To p r o t e c t the r e g u l a t o r as f a r as p o s s i b l e from room temperature changes, the box was placed i n a l a r g e earth enware crock with a s i m i l a r crock i n v e r t e d over i t . The r e l a y c i r c u i t of the thermoregulator c o n s i s t e d of an ordinary magnetic r e l a y provided with s i l v e r contacts, whose a c t i o n opened and c l o s e d a r e s i s t a n c e c i r c u i t In p a r a l l e l with a v a r i a b l e rheostat, which i n turn was i n s e r i e s with the f u r  nace. Current f o r operating the r e l a y was obtained from a 6v D.C. source, which when used i n s e r i e s with a 40-w lamp a c t i n g as a r e s i s t a n c e , was found completely s a t i s f a c t o r y i n regard to lack o f serious sparking across the tungsten-mercury gap, and power to overcome any s t i c k i n g tendency i n the r e l a y I t s e l f . The working of the device depended upon the large temper ature l a g i n the furnace, compared with the extremely low l a g In the thermoregulator r e a c t i o n . To obtain a constant average current t h r u the furnce, the p a r a l l e l rheostats were so adjus ted that when the r e l a y contacts were clo s e d the current was 0.25 amperes above, and when open 0.25 amperes below, the de s i r e d c u r r e n t . This spread was found ample to accommodate the f l u c t u a t i o n s i n l i n e voltage i n t h i s l a b o r a t o r y . In p r a c t i c e the arrangement was found to work very w e l l (the usual v a r i a t i o n during a three-hour run being 0.0 to 0.5°) provided the room temperature was kept quite constant. Bide f l u c t u a t i o n s were found to occur however (up to ±10°) when t h i s -17- was not the case, owing to temperature e f f e c t s both on the furnace and the r e g u l a t o r oven i t s e l f - Further, f o r exact work the method of s e t t i n g the device f o r a d e s i r e d temperat ure ( t h i s i s described under Experimental Procedure) was not s u f f i c i e n t l y p r e c i s e , depending as i t d i d upon adjustment of the furnace to t h i s temperature f o r a p e r i o d of at l e a s t sever a l hours, to e s t a b l i s h proper thermal e q u i l i b r i u m i n the oven. P r e p a r a t i o n and P u r i f i c a t i o n of Azomethane In these experiments some three samples of azomethane were used. The f i r s t (Runs 1-9) and the second (Runs 10-29) were prepared according to the method of Ramsperger^ 6 and of T h i e l e ^ ' . The t h i r d sample was generated using sym-hydrazome- thane d i h y d r o c h l o r i d e prepared from the method of Hatt 5®. The l a t t e r p r e p a r a t i o n proceeded through sym-dibenzoyl hydrazine, from hydrazine s u l f a t e and benzoyl c h l o r i d e , 2C 6H 6C0Cl + N 2 H 4 « H 2 S 0 4 CgH 5 ) 2 C 2 0 2 N 2 H 2 to dibenzoyldimethylhydrazine, using methyl s u l f a t e as the me- t h y l a t i n g agent ( C 6H 5C0) 2N 2H 2+ 2Me 2S0 4+ 2Na0H s (C 6H 5C0) 2N 2Me 2 +• 2MeS04'Na + 2H 20 which i s then hydrolysed to give the d e s i r e d product - (C 6H 5C0) 2N 2Me 2 + 2HG1 + 2H 20 s>MeNHNHMe '2HG1 +• 2C 6H 5C0 2H The benzoic a c i d was extracted with a 1:1 benzene-ether mixture. The y i e l d obtained, using q u a n t i t i e s s t a t e d i n the reference, was about 7 grams of pure dry m a t e r i a l . The method of T h i e l e and of Ramsperger f o r preparing hydr- azomethane was analogous to the above, but was found very unsat--18- i s f a c t o r y , owing to lack of s u f f i c i e n t d e t a i l i n the references quoted. The s t a r t i n g product was syra-diformylhydrazine 2HC00Na +•NgH4«H-2S04-i* HGO.NHMH.HOO *NagS04* 2H 20 According to B e i l s t e i n , when hydrazine s u l f a t e and sodium f o r  mate are heated together dry at 100°C, the above product i s formed. The f r e e z i n g p o i n t i s given as 159-160° and the solu b i l i t y as small i n a l c o h o l , but s o l u b l e i n water, i n s o l u b l e i n ether. No other data on the r e a c t i o n could be found i n the av a i l a b l e j o u r n a l s . In the p r e p a r a t i o n i t was assumed that the equation above represents the main r e a c t i o n ; equivalent q u a n t i t i e s of the two reactants were t h e r e f o r e heated together i n a corked f l a s k at 100° f o r 24 hours. The mixture was then of s e m i - f l u i d consis tency, and gave o f f a strong sharp odour, not recognized. At tempts to p u r i f y the product were not s u c c e s s f u l , so the e n t i r e mass was used i n the next step, assuming a r b i t r a r i l y about 80% y i e l d . Dimethyl s u l f a t e (96cc) and 2§N NaOH (-400cc) were added slowly from separate dropping funnels, to 44g of di f o r m y l hy drazine suspended i n 125cc of water, with continuous s t i r r i n g . The r e a c t i o n mixture was maintained at 20° by gen t l e c o o l i n g i n running water. The reagents were added i n four separate por t i o n s over a p e r i o d of eight hours, the r e a c t i o n mixture being n e u t r a l before each new a d d i t i o n was made. HCONHMGHO + 2 M e S 0 4 ( H G 0 ) 2 N 2 M e 2 + MeHS04 A f t e r complete methylation the mixture (neutral) was evaporated s t r o n g l y , then evaporated f u r t h e r as s t r o n g l y as p o s s i b l e with -19 two volumes of concentrated h y d r o c h l o r i c a c i d almost to dry ness. The residue was taken up i n a small amount of water and v i g o r o u s l y s t e a m - d i s t i l l e d with an excess of concentrated sod ium hydroxide s o l u t i o n (using a 1 2 - l i t r e f l a s k ) u n t i l the res idue no longer r e a c t e d a l k a l i n e . This d i s t i l l a t e was c o l l e c t e d d i r e c t l y i n one mol of d i l u t e h y d r o c h l o r i c a c i d ( 2 - l i t r e f l a s k ) thus (HG.0.')21N2M^ 2> HC1 + ''2HgO — ^ 2HCGGH* MelJHHHMe .HC1 The weakly a c i d l i q u i d was evaporated j u s t to dryness on the water-bath and the residue taken up i n absolute a l c o h o l . This s o l u t i o n was then completely saturated with dry hydrogen ch l o r  ide gas, cooled, and the f i n e white c r y s t a l s o f dihydrochlor ide f i l t e r e d . A f t e r washing with alcohol-HCl (anhydrous) and then absolute ether, t h i s product was d r i e d completely on a porous p l a t e at 110°, i n a vigorous stream of dry hydrogen c h l  o r i d e gas. Y i e l d , about two grams. Prom sym-hydrazomethane d i h y d r o c h l o r i d e , azomethane was prepared i n each case by the f o l l o w i n g method. The apparatus c o n s i s t e d f i r s t of a lOOcc pyrex f l a s k with a tube, le a d i n g from a small dropping-funnel, sealed into the neck. A side-tube from the neck l e d to two drying-tubes ( i n s e r i e s ) : the f i r s t contained pure anhydrous calcium c h l o r i d e and was about 25cm long; the second, c o n t a i n i n g calcium c h l o r  ide and a c e n t r a l l a y e r of soda-lime, was some 50cm long. The tubes were of 2cm diameter. This p u r i f i c a t i o n t r a i n l e d to a cooled trap (carbon dioxide-ether at -78°) from which i t was separated by a s i n g l e r i g h t - a n g l e stopcock, forming one arm of a mercury c u t - o f f . The trap was connected d i r e c t l y to the -20- 250-ml azomethane storage bulb. Azomethane was generated by o x i d a t i o n of the hydrazometh- ane s a l t with a c o l d , saturated s o l u t i o n of n e u t r a l potassium chromate. For the t h i r d sample of azomethane generated, lOg of potassium chromate d i s s o l v e d i n 20cc of water were p l a c e d i n the par t l y - e v a c u a t e d g e n e r a t i n g - f l a s k . In the dropping- funnel was p l a c e d 4g of sym-hydrazomethane d i h y d r o c h l o r i d e d i s s o l v e d i n 8-ml of water. The generator was cooled i n an ice-bath and the pressure over the chromate s o l u t i o n reduced to 25 centimeters. With the carbon di o x i d e - e t h e r f r e e z i n g mixture i n p l a c e , the s o l u t i o n i n the dropping-funnel was added In very small p o r t i o n s over a p e r i o d of f i v e hours. A f t e r standing f o r two hours longer, the pressure i n the system (which had r i s e n c onsiderably owing to the accumulation of a gas not condensed i n the trap) was reduced extremely slowly (to permit complete drying of the gases i n the p u r i f y i n g t r a i n ) to about 3cm. A f t e r standing f o r f i v e hours longer, the c u t - o f f between the generator and the trap was r a i s e d , completing the p r e p a r a t i o n . The product, on removing the freezing-mixture from the trap, was about 3cc of very l i g h t y ellow l i q u i d , which r a p i d l y vapourized when allowed to reach room temperature. D i s s o l v e d gases (which were present i n considerable amount) were removed by pumping o f f the r e - f r o z e n azomethane ( l i q u i d a i r on the trap) with a d i f f u s i o n pump, then allowing i t to vapourize com p l e t e l y by warming to room temperature with the c u t - o f f (M3) r a i s e d , and r e f r e e z i n g and repumping f u r t h e r - the process be ing repeated (about 9-10 times) u n t i l no gas residue remained -21- aft.er f r e e z i n g . When t h i s had been done the above q u a n t i t i e s gave a y i e l d of 51cm pressure i n the storage apparatus d e s c r i  bed. The r e s i d u a l gas pressure over the frozen azomethane was 10* 5mm. Azomethane i s described by T h i e l e as a c o l o r l e s s explosive gas, or p a l e yellow l i q u i d , b o i l i n g p o i n t 1.8° at 756mm. It explodes v i g o r o u s l y when heated i n the absence of a i r . I t s thermal and photochemical decomposition has been studied by R a m s perger 5 6' 6 0, O.K.Rice and Sickman 6 9, P a t a t 6 1 , G o l d f i n g e r 6 3 and Heidt & F o r b e s 6 2 . The f i r s t sample of azomethane prepared as described above was found to decompose slowly i n the storage chamber. A f t e r one month, the sample was f r o z e n with l i q u i d a i r , but a pres sure of-35cm of an uncondensed gas remained, the condensable residue being no more than 10cm. Furthermore, a r a t e determin a t i o n showed that even t h i s residue was not pure azomethane. The second sample was p r o t e c t e d from s u n l i g h t by a black card board screen; no yellow o i l appeared as had In the f i r s t case, even a f t e r two months storage. Some f i n e white l u s t r o u s need le s were observed i n the storage bulb however, the presence of which could not be explained. At the end of some s i x months., these had disappeared. The t h i r d sample showed none pf these e f f e c t s . The i n i t i a l p u r i t y of the azomethane samples was t e s t e d by rate determinations (see Tables of Data) and found to be s a t i s  f a c t o r y i n g e n e r a l . Sample 3 was f u r t h e r checked by an analy s i s of the r e a c t i o n products f o r ethylene (using a c t i v a t e d s u l --22- f u r l c a c i d , q.v.): the r e s u l t showed 1.1% i n exact agreement with Ramsperger's a n a l y s i s ^ 6 . P r e p a r a t i o n of the D i e t h y l Ether.Samples A l l samples of ether used were prepared i n e s s e n t i a l l y the same manner from B.D.H. Aether P u r i s s . anaesthetic ether of h i g h p u r i t y . Sample I (Runs 1-10) and sample II (Runs 10- 29) were allowed to stand over sodium wire f o r three weeks, a f t e r which s e v e r a l f r a c t i o n a l d i s t i l l a t i o n s were c a r r i e d out. Sample I I I (Runs 30 &c) stood over sodium-potassium amalgam (1:2 by weight) f o r one month before f r a c t i o n a t i o n . Sample I I was also d i s t i l l e d s e v e r a l times over f r e s h l y - c u t sodium. In every case, i t was found impossible to detect any ran ge of b o i l i n g - p o i n t during the f r a c t i o n a t i o n s , using a therm ometer reading to tenths of a degree. F i r s t and l a s t runnings from each d i s t i l l a t i o n however were r e j e c t e d . This was taken as i n d i c a t i n g a h i g h degree of p u r i t y i n the ether samples, and no f u r t h e r chemical treatment was considered necessary. Ether samples (about 5cc) were p l a c e d i n the apparatus t h r u the top of the storage tube (E) which was opened with a blowtorch f o r the purpose. Immediately a f t e r the ether had been added, i t was f r o z e n i n the tube with l i q u i d a i r . The open top was then s e a l e d over (with the blowtorch) j u s t as soon as the operation was deemed safe. To remove d i s s o l v e d a i r (which was always considerable) the sample was repeatedly frozen back and f o r t h between the arms of the ether storage tubes, using l i q u i d a i r f o r the pur*- pose, and pumping o f f the s o l i d ether to a very low pressure -23- each time. When a n e g l i g i b l e amount of r e s i d u a l gas remained a f t e r a recondensation of the ether (about 8 or 9 times) the system was completely evacuated and the mercury c u t - o f f r a i s e d . As d i e t h y l ether i s a f f e c t e d by s u n l i g h t , the l i q u i d was s t o r e d always i n the dark. A Dewar f l a s k , containing c o l d water to prevent condensation i n the connecting tubing, served t h i s purpose i A short tube containing K O H p e l l e t s was i n s e r  ted between the ether storage tube and the c u t - o f f . This ser ved to remove i n c i d e n t a l water-vapour, a r i s i n g from the blow t o r c h or other sources. Rate determinations were also made on the ether alone, to check the p u r i t y of the samples. Preparation of the Gas Mixtures The procedure f o r preparing mixtures I to V i n c l u s i v e was as f o l l o w s . The azomethane gas was f i r s t f r o z e n ( l i q u i d a i r ) to permit lowering of the mercury c u t - o f f (M3) connecting the azomethane storage bulb with the mixing chamber. A f t e r the d e s i r e d amount of the gas had been admitted, by removing the l i q u i d a i r , the c u t - o f f was r a i s e d . The exact amount of azo methane admitted was then measured by reading with a cathetom- e t e r the d i f f e r e n c e i n l e v e l between the two mercury columns comprising the c u t - o f f , with the azomethane trap f r o z e n . The vacuum over the frozen azomethane was always p r e v i o u s l y tested by comparing the two l e v e l s when the mixing chamber a l s o was evacuated. To admit ether to the mixing chamber, the mercury c u t - o f f l e a d i n g to the ether storage bulbs was lowered, with the stop--24- cock, however, s t i l l c l o s e d . This stopcock (S14) was then op ened j u s t enough to permit n e a r l y a l l the mercury column above i t to pass through, when i t was again c l o s e d . The mercury lev e l below was then adjusted to a height of about 2cm above the cr u t c h of the c u t - o f f . On again opening the stopcock, and per m i t t i n g the ether vapour to stream out slowly, t h i s arrangement functioned as a v a l v e , allowing ether to pass Into the mixing chamber, but preventing the azomethane already present there from d i f f u s i n g back i n t o the ether storage tubes. When about the r i g h t amount of ether had been admitted i n t h i s manner, the c u t - o f f was r a i s e d above the stopcock (S14) by c o o l i n g the l i  q uid ether i n a c o o l i n g mixture. To determine the p r o p o r t i o n of ether i n the mixing chamber the new t o t a l pressure of both gases was read on the cathetom- et e r , care being taken to have the temperature the same as be f o r e . The volume of the manometer tubing (which of course was increased a f t e r the ether was added) was neglected i n the c a l  c u l a t i o n of percentage azomethane, as being n e g l i g i b l e compared with the t o t a l volume (500cc or more) or amount of gas present. Experimental Procedure i n  Making Rate Measurements. Before an experiment, the furnace was allowed to heat up to about the r i g h t temperature, some 12 hours being r e q u i r e d to a t t a i n proper thermal e q u i l i b r i u m . The r e g u l a t o r oven was then switched i n t o the furnace c i r c u i t (by opening K ) at the same current, and allowed to heat f o r a f u r t h e r 12 hours. Be fore t u r n i n g on the r e l a y c i r c u i t of the r e g u l a t o r , the cur rent was kept at exactly the d e s i r e d value, by manually adjust--25 ing the transformer, f o r a p e r i o d of t h i r t y minutes. The stop cock connecting the mercury r e s e r v o i r on the r e g u l a t o r was then c l o s e d , the l i d on the box being replaced as q u i c k l y as p o s s i b l e to avoid c o o l i n g the oven. The tungsten wire contact, f i t t i n g through the top of the box as described elsewhere, was then pushed down u n t i l the r e l a y c l i c k e d , i n d i c a t i n g contact with the mercury i n the r e g u l a t o r . The current was then i n c r  eased to a value 0.25 amperes below the d e s i r e d e q u i l i b r i u m value: then when the oven cooled and the r e l a y c i r c u i t was broken, the current was a u t o m a t i c a l l y increased to 0.25 amps, above the e q u i l i b r i u m value, i f the rheostats were adjusted to give a t o t a l current change of h a l f an ampere. A greater or les s t o t a l current change than t h i s was found to give l e s s ac curate temperature c o n t r o l . A f t e r at l e a s t s i x hours on the r e g u l a t o r , the furnace e q u i l i b r i u m temperature was record- ed. To completely evacuate the r e a c t i o n chamber and connect ions before a run, the d i f f u s i o n pump was used f o r s e v e r a l hours while the chamber was hot. U s u a l l y the r e s i d u a l gas pre ssure was l e s s than 10" 4 mm, as measured on the McLeod Gage. Several^methods of ob t a i n i n g an i n i t i a l gas pressure were used. I f the pressure d e s i r e d was small (1-10 cm) the connect ing tubing was simply f i l l e d with the gas at a higher pressure, the storage chamber closed, and the r e a c t i o n chamber then open ed to the connections f o r a few seconds. A l t e r n a t i v e l y , i f the a i r pressure outside the c l i c k gage was adjusted to correspond to the d e s i r e d i n i t i a l pressure (Including of course the c l i c k constant) the r e a c t i o n chamber could be opened c a r e f u l l y to - 26- th e storage chamber u n t i l the c l i c k was heard. Neither of these methods enabled a predetermined i n i t i a l pressure to be obtained with any accuracy. For most of the experiments rec orded i n t h i s t h e s i s , the storage chamber was a 500cc bulb. For a high pressure run therefore (10-20cm), the gas pressure which happened to e x i s t i n the bulb at the time of the exper iment, had to be used. Time was measured on a larg e watch f i t t e d with a seconds hand. This watch was checked, before using, against an elec t r i c clock of known accuracy. Time readings were probably accurate to ± 2 seconds. Before allowing the gas into the r e a c t i o n chamber, the a i r pressure on the c l i c k e r gage was increased to such a p o i n t that the maximum p o s s i b l e pressure In the chamber would not exceed i t by more than a few centimeters. The maximum a i r pressure ever used on the p a r t i c u l a r gage of these experiments, with the r e a c t i o n chamber evacuated, was 20cm or s l i g h t l y l e s s . The maximum i n i t i a l gas pressure i t has been p o s s i b l e to use there f o r e , i s about 20cm. When t h i s adjustment had been made, the stopcock l e a d i n g from the r e a c t i o n chamber to the gas sour ce was opened f o r j u s t f i v e seconds, then closed - the l a t t e r a c t i o n marking "zero time". Using the c o n t r o l l e d leak, the a i r pressure i s then increased u n t i l a c l i c k i s heard, i n d i c  a t i n g that the g l a s s membrane has been pushed i n , to i t s un sta b l e p o s i t i o n . This pressure i s 2cm greater than the pres sure at which the membrane " c l i c k s out" to the s t a b l e p o s i t i o n again; the pressure i s t h e r e f o r e reduced to a value about 1mm greater than the c l i c k - o u t c r i t i c a l pressure. Yfhen the gas pressure In the r e a c t i o n chamber r i s e s a m i l l i m e t e r , a c l i c k i s heard, and the time immediately noted. The stopcock connecting i n the McLeod Gage (S5) i s l e f t open during these manipulations; the a i r pressure over the mer cury r e s e r v o i r of t h i s gage must be adjusted, using the three- way stopcock (S3), so that when the c r i t i c a l c l i c k - p r e s s u r e has been set the mercury l e v e l r e s t s j u s t below the t e e - j o i n t on the compression volume (G). I f any dead-space i s allowed above t h i s mercury l e v e l , the a i r pressure being measured w i l l be m a t e r i a l l y changed when the l e v e l i s r a i s e d to take a read i n g . When time has been recorded, and the manometer (M2) also read and recorded ( t h i s may be done before the c l i c k i s heard) the mercury i n the McLeod Gage i s allowed to r i s e quite slowly when the mercury l e v e l has passed the T - j o i n t mentioned above, the stopcock (S5) i s c l o s e d . The l e v e l i s never allowed to r i s e r a p i d l y , as the compression of the a i r must be completely isothermal i f serious err o r s are to be avoided. To obtain ac curate c o n t r o l of the r i s i n g mercury l e v e l , the screw c l i p (C) was i n v a r i a b l y used to r e g u l a t e the flow of compressed a i r or oxygen i n t o the mercury r e s e r v o i r (and not the stopcock). When the l a t t e r i s f i r s t opened the c l i p i s kept t i g h t , and us u a l l y a s u f f i c i e n t pressure of a i r remains above the c l o s e d c l i p to r a i s e the mercury l e v e l above the T - j o i n t as j u s t des c r i b e d . The stopcock (S3) on the r e s e r v o i r i s then opened f u l  l y , and f u r t h e r adjustment of the mercury l e v e l made with the screw c l i p alone. Just before the r i s i n g mercury l e v e l reach es the mark (on the compression volume) to which adjustment i s -28- being made, the l a r g e stopcock (S4) below the mercury-head sc a l e i s opened. The a i r pressure on t h i s mercury head was u s u a l l y kept at one atmosphere by l e a v i n g the stopcock (S2) open. When the mercury l e v e l i n the compression volume reach es the mark (a reading-glass was always used f o r t h i s observ ation) the stopcock on the r e s e r v o i r i s c l o s e d , the mercury- head tube tapped sharply, and the l a r g e stopcock (S4) c l o s e d , i f the f i r s t mercury l e v e l has not then s h i f t e d from the mark. Otherwise, I f the mercury l e v e l has f a l l e n somewhat below the mark (as often happens) i t may be r a i s e d by simply warming the compressed a i r above the mercury r e s e r v o i r very s l i g h t l y , with the hand: the tapping i s then repeated, the mercury l e v e l re- checked and the l a r g e stopcock c l o s e d . The mercury i s now drawn i n t o the r e s e r v o i r (using S3, co nnected to the vacuum b o t t l e ) u n t i l the l e v e l i s again j u s t he- low the T - j o i n t ; the stopcock (S5) above the f l o a t valve i s then opened, and the a i r pressure on the c l i c k e r readjusted to a c r i t i c a l value as before. While w a i t i n g f o r the c l i c k - o u t , the two manometer readings (Ml and H) are recorded. The e n t i r e operation of measuring a pressure as described above, requires about f i v e minutes time. During a two-hour ex periment, a dozen experimental p o i n t s were u s u a l l y obtained, which i s ample f o r graphing a smooth pressure-time curve. P i n a l pressures, taken as soon as the pressure i n the re a c t i o n chamber no longer rose appreciably over a p e r i o d of sev e r a l hours, were measured with the cathetometer and manometer (M2).„ -29- Treatment of Data In order to c a l c u l a t e the pressure of gas In the r e a c t i o n chamber from the McLeod Gage readings r e s c r i b e d above, several f a c t o r s had to be taken into account. F i r s t , the pressure ob t a i n e d on the Gage in c l u d e d of course the " c l i c k constant": t h i s t h e r e f o r e had to be subtracted. The head of mercury i n the gage i t s e l f was read o f f an a r b i t r a r i l y f i x e d s c a l e , hence the s c a l e reading corresponding to zero pressure i n the Gage ( c a l l e d H 0 here) was subtracted from the observed reading (H)» The pressures used on the second manometer (Ml) of the Gage, were va r i o u s , hence a c o r r e c t i o n f o r corresponding changes i n the lower mercury l e v e l of t h i s manometer, was a p p l i e d . This turned out to be 0.02 cm f a l l per centimeter r i s e above zero of the s c a l e ; the l a t t e r corresponded to e x a c t l y 25.40 cm pres sure as measured with the cathetometer. The true mercury head, c a l c u l a t e d with these c o r r e c t i o n s , could then be d i v i d e d by a f a c t o r corresponding to the m a g n i f i c a t i o n r a t i o used i n the ex periment. A l t e r n a t i v e l y , a head of mercury corresponding to the value of the c l i c k - c o n s t a n t could be subtracted before div i d i n g by the f a c t o r , the r e s u l t then representing d i r e c t l y the gas pressure i n the r e a c t i o n chamber. Using the l a t t e r method, the formula f o r c a l c u l a t i n g gas pressures was - P = [lvl(1.02) + (H-H 0) + 25.40 - 33.42] f 4.967 • = [ffl(1.02)+ (H-H 0) - 8.02] 4 4.967 f o r m a g n i f i c a t i o n r a t i o I (March, 1937). Other c a l i b r a t i o n s used f o r the low pressure experiments are given i n the Tables of Data. The C l i c k Constant i n t h i s formula was 6.727 cm, -30- somewhat g r e a t e r than the value used i n e a r l i e r experiments 6^ t (6.700cm). Apparently the constant tends to increase very slowly but appreciably over a p e r i o d of months. The value used however was always redetermined at the beginning and end of any s e r i e s of experiments. When the pressure values ( f o r a run) were c a l c u l a t e d , the r e s u l t s were graphed on a lar g e s c a l e (1mm s c a l e = 10 sees and 0.01 cm p r e s s u r e ) . A smooth curve was drawn t h r u the points by using a s p l i n e ; t h i s also permitted a f a i r l y accurate extrapol a t i o n of the data back to "zero time", g i v i n g the I n i t i a l pres sure. In order to determine the r a t e of decomposition of substr ate molecules, from measurements of the r i s e of t o t a l pressure with time, i t i s necessary to know how many mols of product are being produced from a mol of s u b s t r a t e . I f then i t i s assumed that t h i s number i s constant thruout a given experiment ( i . e . i f the products themselves are not decomposing at a measurable r a t e ) then a simple c a l c u l a t i o n w i l l give the number of mols of substrate, s t i l l undecomposed, at any time from the t o t a l pressure at that time, the i n i t i a l pressure of the experiment and the f i n a l p r essure. In the case of the present experiments i t may be assumed (see d i s c u s s i o n of r e s u l t s ) that d i e t h y l ether produces three mols of product from one of ether, while azomethane i s simultaneously producing two mols. Hence at any time, P - P e ^ P a * 3(P 0(3 - P e) 2(P0c\ - p a ) P e = i | p 0 { 3 - ^ ( l + e - k o t ) j - p| When P a<<P e then P a may be neglected, and hence --31- P " 3P 0 - P 0 « - 2P e 2P Q = P0(3-<*) - P and -dP e/dt = £dp/dt (1) In a l l c a l c u l a t i o n s the c o r r e c t i o n of i n i t i a l - f i n a l pressure r a t i o s f o r deadspace outside the furnace, has been neglected. The value of the c o r r e c t i o n has been d e r i v e d by A l l e n 6 5 . In the present experiments, the ether was never complete l y decomposed; the amount remaining depended on the percentage of azomethane added, the temperature and the t o t a l pressure. In order to determine the number of mols of ether disappearing per mol of azomethane added (i.e.. the t o t a l chain-length A ) measurements of the i n i t i a l - f i n a l pressure r a t i o were used. Thus P - P 0 ( 3 - c * ) - 2P e - P a P f = P 0(3-d) - 2 ( P e ) f N s 3 ~ c A - 2 ( P e ) f / P 0 = 3 - c* - 2{f3 - cXA) whence A = (N - 1 - <*)/2<* (2) This equation v/as used to c a l c u l a t e the chain-length values given i n the Tables of Data. The i n i t i a l s p e c i f i c r a t e of decomposition of the d i e t h y l ether may be c a l c u l a t e d from the i n i t i a l slopes of the press ure-time curves, i f a f i r s t - o r d e r equation i s assumed. Thus dp/dt = -2dP e/dt - dPa/dt At the very beginning of the r e a c t i o n , the l a s t term i s given by k 0P 0<* and the p a r t i a l pressure of ether by P 0 (1- <*), and so P e/P e i s r e a d i l y found. These r e s u l t s are also included i n -32- the Tables of Data. In a l l cases where i t has been necessary to use k 0, the s p e c i f i c r e a c t i o n r a t e constant of azomethane, i n the c a l c u l  a t i o n s , i t has been assumed 2 5 that the c r o s s - a c t i v a t i o n a l ef f i c i e n c y between the ether and the azomethane i s u n i t y . In view of the l a r g e excess of ether present, t h i s seems a reason able assumption. The values of k Q have t h e r e f o r e been c a l c u l  ated on the b a s i s of the t o t a l pressure of gas mixture; the data on values of kj/ko at various pressures was obtained from a paper by K a s s e l 6 6 ' 6 7 . For k ^ the e q u a t i o n 2 5 ^16o-52440/RT was used. K*- 3 ' 1 3 x 10 l be" THEORETICAL PART The Induction of decomposition i n d i e t h y l ether at low temperatures (300°C) by small amounts of azomethane was pre d i c  ted, i n t h i s l a b o r a t o r y , on the bas i s of P.O.Rice and H e r z f e l d f r e e r a d i c a l theory. The mechanism of the normal ether decom p o s i t i o n as p o s t u l a t e d by t h i s author on the b a s i s of h i s the ory, has already been described ( q . v . ) . The mechanism of the induced decomposition w i l l be worked out on s i m i l a r l i n e s , and attempts made to c o r r e l a t e t h e o r e t i c a l r e s u l t s with the exper iments . The Suggested Mechanism The most reasonable mechanism, from c o n s i d e r a t i o n of the known p r o p e r t i e s of f r e e r a d i c a l s and t h e i r r eactions with mol--33- ecules, has seemed to be the f o l l o w I n g i - CR^Ni-NCHs + 2Me + N 2 k G CH3GH20GH2GH3-»-Me—-*CH4-»- CH 5CH 20Cg H k j I 3 4> kg Et + CH3CH0 t CH 3CH 20CH 2CH 3 + Et » C 2 H g + CHgCHgOC^ kg CH3CH0 Me *CH 4 CHgCO k I M e * - C O •4 2Me • G 2H 6 k 5 2Et > C4H-^Q k« Et Me i»G 3H 8 k 6. I f k D &c r e f e r to the r e s p e c t i v e s p e c i f i c r e a c t i o n rates of the various steps of the mechanism; i f f u r t h e r x Q Is the azomethane concentration, X]_ the ether concentration, x 2 that of methyl r a d i c a l s , x 3 that o f CH 3CH 20CHCH 3 r a d i c a l s , x 4 of e t h y l r a d i c  a l s , then at the very beginning of the induced r e a c t i o n , when a steady s t a t e of f r e e r a d i c a l concentration may be assumed, we have 2 dx 2/dt - 2 k 0 x 0 - k^xnx 2 - k5X 2 - k s x 2 x 4 =0 (1 dx 3/dt = k^XjXp •*• k^x^x 4 - kgX^ 0 (2 2 dx 4/dt = k g X 3 - kgX^x 4 - k g X 2 x 4 - kr/,x4 0 (3 - dx-^/dt = k-LX^x2 -+• k3x-j_x4 (4 From equations (2) and (3) we ob t a i n equation (5); then (6) at once fo l l o w s from (1); s u b t r a c t i o n of (5) and (6) gives (7):- k l x l x 2 = k g x 2 x 4 + ky x 4 (5 k l x l x 2 * 2 k o x o - k 5 x 2 2 " k 6 x 2 x 4 ( 6 -34- 2 k 0 x 0 - k 5 x g 2 - k 6 x 2 x 4 - k 7 x 4 2 - 0 (7 x 2 = krjTL^/ik^ - k 6 x 4 ) (8 The l a t t e r follows from equation ( 5 ) . It shows that x 2 <.< x 4, which i s to be expected from the f a c t that x 4 represents the concentration o f the c h a i n - c a r r i e r ( e t h y l ) and hence i t w i l l be l a r g e r than x 2 (methyl c o n c e n t r a t i o n ) . Then, from (7) 2 k Q x 0 - k 7 x 4 2 = 0 x 4 s ( 2 * 0 x 0 / k 7 ) * (9 This value of x 4 may be s u b s t i t u t e d i n equation (8) above; i f then the r e s u l t i s combined with equations (9) and (4) we have -dx-j/dt = k g x ^ g (2k Q/k 7 ) B + 2 k i x i k o x o ( k l x l " X ^ ( H I. where X i s equal to k g ( 2 k Q x 0 / k 7 ) 8 ; since t h i s term may be neg l e c t e d i n comparison with k^x^, as described l a t e r , the f i n a l equation i s -dXi/dt = k 3 ( 2 k 0 / k 7 ) * x l X o * + 2 k o x 0 (12 Considering the concentration terms f o r the moment as pressure, equation 12 may be w r i t t e n i n terms of the i n i t i a l slope of the experimental pressure-time curve, by using a previous equn, page 31. - ( d P / d t ) Q = 2 K ( 2 k 0 ) 2 x 1 x 0 e •*- 5 k 0 x 0 (13* At the very beginning of the r e a c t i o n , where t h i s r e l a t i o n should h o l d , we may p l a c e *o 5 p o * (14 In equation 13 t h e r e f o r e a l l the q u a n t i t i e s with the exception -35- of K, are known or can be measured d i r e c t l y . At low pressures however, i n the region of 15-20 cm or lower, the value of k Q begins to drop o f f , i n accordance with the accepted theory of unimolecular r e a c t i o n s . T h i s means that i n the pressure range i n which the present experiments were c a r r i e d out, the s p e c i f  i c azomethane r a t e k D i s a f u n c t i o n o f the pressure. I f equn 13 i s to be t e s t e d t h e r e f o r e , t h i s must be taken i n t o account. According to the experimental r e s u l t s of Ramsperger on azomethane at low pressures, the r e l a t i o n s h i p of l o g k^/ko to lo g P - where i s the l i m i t i n g high-pressure r a t e constant at a gi v e n temperature, k Q the r a t e constant at a lower pres sure P - Is roughly l i n e a r up to about 100 mm. Hence i n t h i s range we may w r i t e - l o g k Q s l o g k^ - C i l o g P l o g Gg l / k 0 = c/Pk^ k 0 s Constant.P (15 This constant i s of course a f u n c t i o n of the temperature. I f t h i s i s a p p l i e d to equation 13, P Q = 5CP 0 2«* + 2(0<*)^P o 2K = Constant x P 0 2 (16 It t h e r e f o r e f o l l o w s that the v a r i a t i o n of i n i t i a l slope with i n i t i a l pressure f o r a given mixture i s such that P c / P Q s constant f o r a l l I n i t i a l pressures l e s s than 10cm. At pressures great er than t h i s , but l e s s than 200 mm, no such simple r e l a t i o n s h i p can be deduced. For experiments 25 and 26 r e s p e c t i v e l y , the c a l c u l a t e d values of the above quotient are 1.05 x 10" 6 and 1.00 x 10" 6 -36- mm sec™ 1 mm-*'. More data i s needed to check t h i s agreement. A c t i v a t i o n Energies of Elementary Reactions The suggested mechanism f o r the Induced r e a c t i o n , i f shown to be c o r r e c t , may be used to determine experimentally the a c t i v a t i o n energies of the various r e a c t i o n s between f r e e r a d i c a l s and molecules which are involved? The estimation of such energies, as already mentioned, i s extremely important from the point of view of the R i c e - H e r z f e l d theory (qv) as the nature of the f r e e r a d i c a l chains i n other- decompositions may be p r e d i c t e d from these values. The t h e o r e t i c a l equation may be w r i t t e n as follows - X J L s Kk 0^A *• Bk 0 where A and B are constants independent of temperature. By l o g a r i z i n g a f t e r making the s u b s t i t u t i o n s k 0 s Qe'^l/^ and K = C x e " 2 2 / ™ , we o b t a i n In (-Xj-Bko) s -Ei/2RT -E 2/RT -C 3 By d i f f e r e n t i a t i n g with respect to l/T i t i s seen that the value of Eg may be obtained by a p l o t of the experimental data. D i s c u s s i o n of the Suggested Mechanism The thermal decomposition of azomethane has been known f o r some time as a s t r a i g h t d i s s o c i a t i o n Into f r e e methyl r a d i c a l s and n i t r o g e n 2 5 . This simultaneous s p l i t t i n g of two bonds i s explained by the formation of the enormously s t a b l e n i t r o g e n molecule. The f i r s t step of the suggested mechanism, t h e r e f o r e seems secure. The second step i s based on P.O.Rice's mechanism f o r the normal p y r o l y s i s of the ether. The a l t e r n a t i v e step, -37- GH 3GH 20CH 2GH 5 + Me -^CH4 CH 3CH 2O0H 2CE 2 would g i v e , C 2 H 4 f HCHO + Me and ECHO >H 2+C0 so that equivalent amounts of hydrogen and carbon monoxide would be formed. To determine i f t h i s step took p l a c e , an a n a l y s i s of the f i n a l products o f Run 15, and also of Run 28, was made. In both cases, n e g l i g i b l e amounts of hydrogen were found. Ethylene, which should also be formed here i n equlmol- ar amounts with the carbon monoxide, was s i m i l a r l y shown to be absent. With t h i s evidence, i t was assumed that, the r e a c t i o n of a methyl r a d i c a l with the a-carbon atom of the ether molec u l e , r e q u i r e s a s e n s i b l y higher a c t i v a t i o n energy than the re a c t i o n with a b-carbon atom, and hence could be neglected. The t h i r d step, the spontaneous decomposition of the heavy r a d i c a l EtOCHMe i n t o acetaldehyde and f r e e e t h y l , follows from the known i n s t a b i l i t y of heavy r a d i c a l s . An assumption has been made In the d e r i v a t i o n of equation 12 that the chain decomposition of the acetaldehyde i s f a s t compared with the other rate-determining steps. This assump t i o n i s based on the r e s u l t s of A l l e n and Sickman^ 3 who studied the decomposition of acetaldehyde with azomethane, and obtained long chain-lengths i n the temperature range of the present ex periments. In the presence of an excess of azomethane and hen ce f r e e methyl r a d i c a l s , the acetaldehyde would not therefore be expected to appreciably accumulate. Further stages i n the proposed mechanism co n s i s t of the chain decomposition of the d i e t h y l ether, with e t h y l r a d i c a l s -38- as the c h a i n - c a r r i e r . These chains are u l t i m a t e l y broken, pro bably at the w a l l s , by the recombination of two free r a d i c a l s to form a satu r a t e d hydrocarbon. In the mathematical treatment of these processes, i t was assumed that k^x]_ >> ks( 2k 0x Q/k7 ) g . Wow the values of the s p e c i f i c r a t e constants f o r various types of re a c t i o n s i n v o l v  ing f r e e r a d i c a l s , may be estimated i f the a c t i v a t i o n energies are known. By semi-empirical methods, F.O.Rice and co-workers have assigned values to these a c t i v a t i o n energies, which f i t s a t i s f a c t o r i l y many of the r e a c t i o n s thus f a r st u d i e d from the f r e e - r a d i c a l p o i n t of view. I f we represent a l i g h t f r e e rad i c a l by "R" and a heavy r a d i c a l by "Rj" the various simple types of i n t e r a c t i o n of these with molecules (M) are as f o l  lows : R + M-—>RH+ Rn E s. 15 Gal R + R»—••RR' 8 R l !>R'+- M 25-50 R]_-s- M——^R]_H + Rj ' 15-25 Now i n general i t may be w r i t t e n very approximately f o r second- order processes, that k" = 10 9e~ E/ R I*'; f o r unimolecular proces- 5 ses the constant i s 10 , i n u n i t s of mois per 24 l i t r e s . When k j , k5 and ky were c a l c u l a t e d from t h i s data, i t was found that the i n e q u a l i t y s t a t e d above, was j u s t i f i e d . F i nal'Products o f the Reaction I f the f i n a l products r e s u l t i n g from the suggested mech anism of the r e a c t i o n are considered, i t i s seen that f o r the various chain-lengths we may write, f o r example, Me 2N 2 + E t 2 0 - — » 2CH 4 + G3H8+- GO + N 2 M e 2 N 2 + 2 E t 2 0 — * 3CH 4+ C 5H 8+ 2G0+ N 2+ G 2H 6 -39- Me 2N 2 + 5 E t 2 0 » 6 C H 4 +• C 3H 8 + 5C0 + N 2 * 4C 2H 6 Then f o r each extra molecule of ether consumed, the f i n a l prod ucts should be increased by three mols - and by su b t r a c t i o n of the equations, these three mols are C 2Hg, GO and CH 4. Hence In e f f e c t , the induced decomposition Is proceeding thus C 2H 50C 2H 5 >CH 4+ C 2H g+ GO This leaves the azomethane e f f e c t i v e l y producing two mols of products, per mol of azomethane decomposed. This i s the basis of equations used i n a previous s e c t i o n (qv). To t e s t these conclusions, the gas an a l y s i s of Run 15 may be considered. According to the above mechanism, one mol of CO should be produced per mol of ether decomposed. Further, each mol of ether should produce three mols of products. The confirmation o f the r e q u i r e d absence o f hydrogen and ethylene i n appreciable amounts, has already been mentioned. In Exper iment 15, the i n i t i a l pressure of ether was 61.35 mm, and of azomethane was 7.7 mm. The r e s i d u a l ether pressure i n the pro ducts was measured and found to be 21.05 mm (14.1$, of the t o t a l f i n a l p r e s s u r e ) . Hence the chain length In t h i s case was - 40.30/7.7 « 5.2 u n i t s . The carbon monoxide found was 8b% of the decomposed ether. The measured residue of unabsorbed n i t r  ogen and saturated hydrocarbons was 187% of the decomposed ether plus azomethane ( t h e o r e t i c a l l y 200%). Further, i f i t i s assumed that azomethane produces two mols of products, then t o t a l pressure of products 128.2 mm azomethane products 15.4 ether products 112,8 mm Hence one mol of ether produced 112.8/40.3 e 2.8 mols of -40- products. The agreement i s seen to be good. The E f f e c t of Various Factors on the Chain-Length The chain-length of t h i s induced r e a c t i o n i s given by the r a t i o i i / x 0 hence from equation 12, A „ * K(2/k o)«xi/x 0e+ 2 gives the t h e o r e t i c a l I n i t i a l chain-length. The minimum value i s apparently two. T h i s equation may be w r i t t e n A 0 = K(2/k 0pp 0ir(l-c()/oi!- + 2 It i s seen that the chain-length should, q u a l i t a t i v e l y at l e a s t i ) i n c r ease with i n i t i a l pressure i i ) decrease with i n c r e a s i n g percent of azomethane i n the mixture i i i ) decrease with temper atu r e . Chain-length r e s u l t s quoted i n the Tables show that t h i s i s true i n the present experiments. An estimation of the a c t u a l value of the i n i t i a l chain- length, may be made from the experimental i n i t i a l P-t slopes. Thus i f we c a l l k e the s p e c i f i c r a t e constant of the induced ether decomposition, then X 0 » d x i / d x 0 = k e x i / k 0 x 0 = k e^/k 0°(. The values of k e have been c a l c u l a t e d as already described, so the f o l l o w i n g t a b l e was obtained Expt A 5 / \ 19 0.8 5.5 26 1.2 12.3 27 1.6 8 .4 28 2.5 36.7 In t h i s t a b l e , i s the t o t a l chain-length, c a l c u l a t e d from N. It i s seen from these examples, that 1) the i n i t i a l "chains" are seemingly l e s s than two u n i t s long i i ) there i s no apparent connection between the t o t a l and i n i t i a l chain-lengths, at l e a s t when c a l c u l a t e d i n t h i s manner. ~41~ Further D i s c u s s i o n of Results In a d d i t i o n to the c l a s s i c work of Hinshelwood 6 on d i e t h y l ether, t h i s compound has been s t u d i e d by O.K.Rice and Sickman 4 4 S t e a c i e 4 5 ' 4 6 ' 4 7 , K a s s e l 4 8 , Wewitt and V e r n o n 4 9 , F l e t c h e r 4 2 . These authors were i n t e r e s t e d i n the ether decomposition reac t i o n as an experimental example of the current theory of u n i - 25 molecular r e a c t i o n s , which p r e d i c t s a f a l l i n g - o f f i n s p e c i f  i c r a t e of such r e a c t i o n s , at low p r e s s u r e s . When the Rice- H e r z f e l d theory of chain r e a c t i o n s was proposed however, i t 13 became apparent that here was another explanation of the ef f e c t of pressure on r a t e - i . e . the e f f e c t on the r e a c t i o n chains. I t t h e r e f o r e became of very great importance to deter mine whether or not t h i s r e a c t i o n (and others s i m i l a r l y ) i n v o l  ves a chain mechanism, and i f so whether the chains are of suf f i c i e n t importance i n the r e a c t i o n to account f o r the observed dropping-off i n r a t e , with pressure. The various methods of t e s t i n g f o r r e a c t i o n chains have already been de s c r i b e d . Of these, one has r e c e n t l y been appl- l e d by Hinshelwood * , who mixed d i e t h y l ether with n i t r i c oxide and redetermined the r a t e . He found a d e f i n i t e i n h i b i t  i o n , but c a l c u l a t e d from h i s r e s u l t s that the chain decomposit ion represented only a small p a r t of the t o t a l r e a c t i o n . About the same time however S t e a c i e 4 5 , i n v e s t i g a t i n g the normal ether decomposition at pressures up to 20,000 cm, concluded q u a l i t a t  i v e l y that the high pressure r e a c t i o n (observed order 1,4) ag rees with the f r e e r a d i c a l theory. The very recent work of F l e t c h e r 4 2 * 4 5 has shown that a chain decomposition of acetaldehyde i s induced (490-550°C) -42- when t h i s compound Is heated with d i e t h y l ether; presumably these chains o r i g i n a t e with f r e e r a d i c a l s from the ether. In the present i n v e s t i g a t i o n , i t has been shown that methyl r a d i c a l s , produced thermally from azomethane, i n i t i a t e decomposition of the ether at as low as 300°C. The p o s s i b i l i t y that the azomethane i n t h i s r e a c t i o n i s merely a c t i n g as a homogeneous c a t a l y s t (see page 8) does not seem l i k e l y In view of the experimental r e s u l t s . An experim ent was c a r r i e d out i n which the ether-azomethane mixture was heated to 230°C f o r about 24 hours; the t o t a l pressure however remained unchanged. At t h i s temperature the azomethane does not decompose, hence there are no methyl r a d i c a l s present to act on the ether and i n the absence of homogeneous c a t a l y s i s , no decomposition would be expected. The c a l c u l a t i o n s of t o t a l and i n i t i a l chain-lengths, des c r i b e d above, r e v e a l that the induced chains are very short, even 10% of azomethane f a i l i n g to. decompose a l l the ether. Furthermore, the chains become shorter as the temperature i s r a i s e d , and longer as the pressure i s r a i s e d . S t e a c i e 1 s con c l u s i o n s on the high pressure ether decomposition have j u s t been mentioned; the present r e s u l t s are thus i n agreement with h i s i d e a s . The shortening of the chains with temperature i s however more s i g n i f i c a n t : i t seems only reasonable to suppose that at the h i g h temperature of the normal p y r o l y s i s (and a l l o r d i n a r y pressures) the chains must be so short as to form only a very small and unimportant p a r t of the t o t a l decompos i t i o n r e a c t i o n . PART 12 A NEW METHOD OF SEMI-MICRO GAS ANALYSIS -43- AN APPARATUS FOR SEMI-MICRO GAS ANALYSIS In the study of the mechanism of gas r e a c t i o n s by the react ion r a t e method, i t i s apparent that an accurate method of gas a n a l y s i s , which may be used when only moderately small quantit i e s are a v a i l a b l e , and which i s p a r t i c u l a r l y s u i t e d to the de termination of those c o n s t i t u e n t s u s u a l l y found i n the products of organic p y r o l y s e s , i s very necessary. The methods at pres ent a v a i l a b l e are In general f a r from s a t i s f a c t o r y from the p o i n t of view of accuracy and speed; f u r t h e r , many substances 9 which i t i s of great importance i n r e a c t i o n k i n e t i c s to be able to estimate a c c u r a t e l y , cannot be determined at a l l In the mixtures u s u a l l y found In such cases. The work about to be described, forms the f i r s t stage of what i s hoped w i l l form a comprehensive study of t h i s problem, to be undertaken by t h i s l a b o r a t o r y . Perhaps the most import ant p a r t i n the s o l u t i o n of the problem, Is the development of a s u i t a b l e apparatus; t h i s consequently was undertaken f i r s t , as p a r t of a separate research by the present w r i t e r . P r e l i m i n a r y Results The f i r s t apparatus design to be considered, was based on the well-known arrangement of Bone and Wheel e r ^ . The arrange ment i s shown i n the accompanying sketch. The gases to be analyzed were pushed into the constant- volume buret (B) by means of the attached Topler Pump. The connection between the pump and. the buret was a length of quite f i n e c a p i l l a r y tubing, attached to the buret some distance "Topics -44- below the reading marks. The buret i t s e l f was provided with volumes of about 5, 15 and 35 ml; the exact r a t i o between the volumes could be r e a d i l y determined by experiment. The confin ing l i q u i d used was of course, mercury; t h i s was introduced i n  to the buret and the attached manometer-tube side arm (M) by means of a l e v e l i n g bulb. Behind the tube M (of wide-bore but heavy c a p i l l a r y tubing) a m i l l i m e t e r scale was permanently f i x  ed. The top end of the buret comprised one arm of a three-way stopcock t h r u which the gases could be introduced i n t o the gas p i p e t (G) when the mercury r e s e r v o i r (C) was i n p l a c e . Trap ped a i r could be p r e v i o u s l y removed from the pipet by attach ing a Bunsen Pump to the other arm of the three-way stopcock, and c a u t i o u s l y allowing the mercury to r i s e from the r e s e r v o i r . Reagents were introduced into the gas p i p e t by means of a cur ved d e l i v e r y p i p e t ( P ) of 5 ml volume. This apparatus was used i n the a n a l y s i s of Experiment 15 (Part I of t h i s t h e s i s ) . A f t e r some considerable use however the design was f i n a l l y r e j e c t e d , f o r the f o l l o w i n g reasons; 1) The method of i n t r o d u c i n g the gas from the Topler i n  to the buret, was awkward and hard to c o n t r o l ; f u r t h e r , the reverse operation was not p o s s i b l e . 2) Since the reagent had to be p a r t l y drawn i n t o the buret a f t e r an absorption, the buret I t s e l f q u i c k l y became d i r t y with an accumulated mixture of reagents on the w a l l s . The vapour-pressure of these reagents was then a source of er r o r , as i t was not p o s s i b l e to keep a drop of standard a c i d i n the buret to c o r r e c t f o r t h i s . 3) The buret stopcock soon f r o z e i n contact with the re--45- agents used as absorbents. This was l i a b l e to cause a i r leaks during an a n a l y s i s . 4) Cleaning and drying the absorption p i p e t a f t e r each absorption, occupied a very long time, owing to the awkward shape of the p i p e t . 5) I t was also necessary to clean some 200cc of mercury f o r each new reagent used, as well as the g l a s s container. This g r e a t l y increased the time necessary f o r an a n a l y s i s . Several attempts were made to carry out a slow combust ion of hydrocarbons with a platinum s p i r a l heated e l e c t r i c a l  l y I n s e r t e d i n the gas p i p e t . Explosions of serious proport ions r e s u l t e d , however. A New Apparatus In order to eliminate e n t i r e l y any n e c e s s i t y f o r manip u l a t i o n of the reagents, which i s the source of most of the dis advantages of the above apparatus and many others described i n the l i t e r a t u r e , the arrangement i n f i g u r e (5) was devised. Gas samples were introduced i n t o the constant-volume gas buret, f i t t e d with mercury l e v e l i n g bulb and side-tube f o r measuring pressures as before, thru the Topler Pump, To with draw a sample from the buret, the mercury i n the l a t t e r was us u a l l y r a i s e d to a point j u s t above the top of the water-jacket, then h e l d i n p l a c e by clamping the rubber l e v e l i n g - t u b e and pumping on the small dead-space remaining, with the Topler. This allowed the buret to contain permanently about 0.2 ml of 6N s u l f u r i c a c i d , thereby maintaining a constant water vapour pressure during a l l experiments. -46. From the Topler, the gas sample could be pushed, t h r u two three-way stopcocks, i n t o the absorption p i p e t attached at P (diagram) by a ground-glass j o i n t . The p i p e t i t s e l f was mere l y a lOcc g l a s s bulb, sealed to one h a l f of the ground-glass j o i n t . A f t e r absorption had taken p l a c e (using 5 ml of reag ent i n each case) the r e s i d u a l gas v/as replaced i n the buret by pumping with the To p l e r ( u s u a l l y three strokes were ample, as the volume of the Topler was at l e a s t 200cc). To evacuate the p i p e t of a i r a f t e r a reagent had been i n s e r t e d , the Topler pump was also used, and the a i r pushed out t h r u Q ( f i g u r e ) . It i s seen that i n t h i s arrangement an important change has been made from orthodox p r a c t i c e with l i q u i d reagents, i . e . the gas Is removed from the pi p e t by pumping. In t h i s manner, the reagent has no opportunity of contaminating the measuring p o r t i o n of the apparatus, as i t only comes i n contact with the gas sample i t s e l f . A c t u a l l y of course the l i q u i d reagents are not "pumped o f f " i n the usual meaning of the term, but instead the amount of gas or a i r above the reagent i s s u c c e s s i v e l y red uced by p a r t i t i o n with the Topler volume. The space above the reagent i s then always at the p a r t i a l pressure of the reagent i t s e l f (at l e a s t ) , hence there i s no cause f o r i t to b o i l at ord i n a r y temperatures. Other advantages of the arrangement are apparent from the f i g u r e . The p i p e t , being detachable, can be r e a d i l y cleaned and r e f i l l e d with another reagent. The shape of the pi p e t per mits the contents during absorption to be cooled i n a Dewar f l a s k , as i s r e q u i r e d f o r example i n the absorption of ethers. A mercury trough and small test-tube seated over the o u t l e t at Rf-ter p. 1-6 F.Xv 5 -47- Q permits gas samples to be st o r e d i f necessary. An explosion p i p e t may be attached thru a rubber connection at P. As the reagents do not come i n contact with mercury, s o l u t i o n s such as palladous c h l o r i d e which are attacked by mercury, may be used. The important feature however i s the ease and s i m p l i c i t y of manipulation. The e n t i r e process of measuring, absorbing and remeasuring the gas sample, Is c a r r i e d out merely by r a i s  ing and lowering a mercury l e v e l i n g bulb and tu r n i n g two stop cocks. T h i s reduces the p o s s i b l e sources of e r r o r , involved when more complex manipulations are necessary, and also i n c r  eases the speed of oper a t i o n . The disadvantages of the arrangement may be mentioned. Since the gases are removed from the p i p e t by s u c c e s s i v e l y reducing t h e i r p a r t i a l pressure, reagents which depend on phys i c a l absorption (Henry's Law s o l u t i o n ) cannot be used; however these are very seldom met. A l s o , I t i s d i f f i c u l t with the type of p i p e t used, to have a s u f f i c i e n t surface of reagent av a i l a b l e to the gas to permit r a p i d absorption. This i s a more serious problem. Experiments with the Apparatus P r e l i m i n a r y experiments on the apparatus ( J u l y , 1936) i n  cluded a determination of the oxygen i n an a i r sample (sodium hyposulphite reagent), the absorption of a sample of pure hy drogen by palladous c h l o r i d e reagent (at room temperature), and the a n a l y s i s of Experiment 28 (Part I of t h e s i s ) . The f i r s t determination gave j u s t 20.0%' by volume; the second showed that 5 ml PdCl2 reagent was capable of absorbing 3cc of pure hydrogen (1 atm). A l l three experiments demonstrated that -48- none of the common reagents used i n gas a n a l y s i s , show any manent tendency to b o i l , when used i n the new apparatus. SUMMARY (1) The decomposition of d i e t h y l ether has been induced at temperatures between 300-340°G, by i n t r o d u c i n g a few percent of azomethane. (2) A chain mechanism f o r the process has been po s t u l a t e d , and evidence presented to support i t . (3) The experiments i n d i c a t e that the chains are very short; the normal ether decomposition has been discussed i n view of these r e s u l t s . (4) An apparatus f o r accurate r e a c t i o n r a t e measurements has been described. (5) An apparatus f o r the exact determination of small q u a n t i t i e s of gases has been designed. - 50- LITERATURE CITED (I) B e r t h e l o t , Ann chim phys (3) 67, 52 1929 .(2) Nef, Ann 318, 198 1901 (3) Hurd " P y r o l y s i s of Carbon Compounds" Chem Cat Go 1929 (4( Nef, J.A.C.S. 26, 1549 1904 i b i d 30, 645 1908 (5) Henrich "Theories of Organic Chemistry" John Wiley 1922 (6) Hinshelwood, Proc Roy Soc (Lon) 114A, 84 1927 (7) Paneth & Ho f e d i t z , Nature 124, 161 1929 (8) Paneth & H o f e d i t z , i b i d 125, 564 1930 (9) Rice & Rice "The A l i p h a t i c Free R a d i c a l s " Johns Hopkins (10) F.O.Rice, J.A.C.S. 53, 1959 1931 (II) F.O.Rice & H e r z f e l d , i b i d 56, 284 1934 (12) T a y l o r & Jones, i b i d 52, 1111 1930 Heckert & Mack, i b i d 51, 2706 1929 (13) A l l e n & Sickraan, i b i d 56, 2031-1251 1934 (14) F l e t c h e r , i b i d 58, 534 1936 (15) Frey, Ind Eng Chem 26, 200 1934 (16) Leermakers, J.A.C.S. 56, 1899 1934 (17) Rice, Rodowskas & Lewis, i b i d 56, 2497 1934 (18) Hinshelwood & Hutchison, Proc Roy Soc (Lon) 111A, 245 126 (19) Winkler & Hinshelwood, i b i d 149A, 340 1935 (20) A l l e n , J.A.C.S. 58, 1052 1936 (21) Winkler & Hinshelwood, Proc Roy Soc (Lon) 149A, 355 1935 (22) L e t o r t , Gomp Ren 199, 351 1934 i b i d 202, 491 1936 51- (23) Hinshelwood & Hutchison, Proc Roy Soc (Lon) 111A, 380 (24) Kassel, J.A.G.S. 50, 1344 1928 J Phys Ghem 34, 1166 1930 (25) Kassel " K i n e t i c s of Homogeneous Gas Reactions" 1932 (26) Staveley & Hinshelwood, Nature 137, 29 1936 (27) " J Chem Soc 1936, 812 (28) " i b i d 1936, 818 (29) Sachsse, Z Phy Ghem 31B, 79 1935 (30) Patat & Sachsse, i b i d 31B, 105 1935 (31) F l e t c h e r &c Hinshelwood, Proc Roy Soc 141A, 41 1933 (32) Winkler, F l e t c h e r & Hinshelwood, i b i d 146A,' 345 1934 (33) Musgrave & Hinshelwood, i b i d 135A, 23 1932 Hunter i b i d 144A, 386 1934 (34) Symposium on Free R a d i c a l s , J Far Soc 1933 (35) Travers, Nature 138, 27 1936 (36) " I b i d 136, 909 1935 (37) Sickman, J Chem Physics 4, 297 1936 (38) F l e t c h e r , J.A.G.S 58, 1317 1936 (39) Travers & Seddon, Nature 137, 906 1936 (40) Echols & Pease, J.A.G.S. 58, 1317 1936 (41) F l e t c h e r & R o l l e f s o n , i b i d 58, 2135 1936 (42) " i b i d 58, 2129 1936 (43) Sickman, J Ghem Physics 4, 297 1936 (44) O.K.Rice & Sickman, J.A.C.S. 56, 1444 1934 (45) S t e a c i e , Hatcher & Rosenberg, J Ghem Physics 4, 220 1936 (46) S t e a c i e & Solomon, i b i d 2, 503 1934 (47) S t e a c i e , i b i d 1, 313 1933 4 -52- (48) K a s s e l , J.A.G.S. 54, 3641 1932 (49) Hewitt & vernon, Proc Roy Soc 135A, 307 1932 (50) Smith & t a y l o r , J.A.G.S. 46, 1393 1924 i b i d 53, 1811 1931 (51) Ramsperger & Waddington, I b i d 55, 214 1933 (52) Ramsperger & Leermakers, I b i d 53, 2061 1931 (53) U.S. Bureau of Standards Tech. 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(Lon) 154A 335 1936 (72) Ramsperger, J Phys Chem 34, 669 1930 TABLE I Summary of A l l Experiments Expt %&zo P 0 mm 5 9.6 22.20 6 9.6 20 .45 7 1«2 49.70 8 1.2 47.95 13 1 jL e 2 27.60 14 11.2 16.70 15 11.2 69 .05 17 11.2 29.60 18 11.2 28.45 19 6.34 3 2 © V 5 20 1.13 21.90 21 1.13 V1 © 15 22 1.13 30 .95 23 1.13 54.35 24 1.13 33.90 25 1.13 30 .85 26 1.13 57 .50 27 1 # 3.15 53 .65 28 1.13 161.05 31 1 6 24 181 .50 32 1.24 150.15 N T P Q mm/sec 1.97 3.13 • 0 4.1 x 10" 1.62 329 .0 4.8 1.38 306.8 1.4 - 317.5 2.0 2.29 307.2 2.2 307 .4 1.2 2.16 307.7 8.3 - 290 e 5 1.6 1.83 326.2 8,0 1.76 309 .6 2.1 - 309.5 0 s 2 1.35 310.0 2.0 1 .14 325.9 1.2 1.24 343.9 11.0 - 324.4 1.4 - 322*2 1.0 1»29 322»2 2.5 1.20 321.5 2«5 1.84 321.8 13*£3 1,59 325 * 5 15.0 - 313.0 5,0 TABLE I I e Expt • 10 4k o 2.48 13 3*15 2.26 14 2,82 1.14 25 8 .75 1.34 26 9.85 7.65 23 45.0 1.15 21 4.66 1.96 17 1.71 8.10 18 11 »Q 4.32 15 3.88 2.11 19 3.88 1.79 27 9.85 3.48 28 13.2 TABLE I I I T o t a l Chain-Lengths X Expt 15.33 7 5.21 13 8.36 27 36.68 28 4.68 15 12.33 26 3.20 18 5.70 22 4,55 5 2.72 6 10,12 23 14.98 21 5.49 19 TABLE IV Constituent Scale Reading Pressure Temperature sample 49,68 46.40 ©ther 43.15 39.87 ethylene 42.85 39.57 oxygen 42.40 39.12 carbon monoxide 31.67 28.39 hydrogen 31.63 28.35 24.9° 25.0 26.4 25.3 24.8 24.8 Constituent ether ethylene oxygen CO hydrogen r e s i d u e Reagent c o l d cone H 2 S O 4 a c t i v a t e d H 2 S O 4 sod. hyposulf. anira. CuCl PdCl2 @ 20°C % vol 14.1 1.2 0.9 23.1 0.0 60.6 TABLE V Pure Azomethane Sample (I) (Experiment 4) TIME sec PRESSURE zero 22.70 mm 1000 29«35 2000 30.65 2500 31.80 3000 32*95 3500 34.05 4000 35 a 15 4500 36.20 5000 37.10 5500 37.90 6000 38.60 6500 39 © 25 7000 39,90 44.45 Temperature 307.4°C P f / P 0 1.95 TABLE VI Pure Ether Sample (I) (Experiment 3) TIME sec PRESSURE zero 23*65 mm 250 23 o 9 5 500 24 © 25 1000 24.85 1500 25.40 2000 26.00 2500 26.60 3000 27.10 6000 30.20 6500 30.65 7000 31.05 Resistance 136.5 P f/P 0 4.59 dP/dt i n i t i a l l y 1.2 x 10" 3 mm/sec Experiment 12 Pure Ether Sample ( I I ) TIME sec PRESSURE zero 68.45 mm 185 72.73 672 74.52 1087 78.01 1490 81.27 1902 84.47 3185 94.52 i n f i n i t y 264.4 Temperature (139.3 ohms) P f / P 0 3.86 TABLE VII TIME sec RUN 5 RUN 6 RUN 7 RUN 8 RUN 13 zero 22.20 20 .45 49.70 47 .95 27 .60 250 23.30 21.60 50.00 48 .45 28 ©IS 500 24.25 22.80 50.35 48 .90 28 .65 1500 25.90 24.90 51.05 49.80 29.70 1000 27.20 26.90 51.75 50 .65 30.70 2000 28 .35 28 .60 52.45 51 .40 31.60 2500 29 .50 29 .90 53.05 52.10 32.55 3000 30.65 30.80 £33 • *3*3 52.80 33.50 3500 31.75 31,40 54.05 53.50 34.50 4000 32.65 32.00 • 54.55 35.50 4500 33.50 36 .45 5000 34.25 37 .35 5500 34.95 38 .10 6000 35.70 ' 38 .85 i n f i n i t y 43,80 33.15 68 .90 53 • 15 T h i s t a b l e gives t o t a l pressures i n m i l l i m e t e r s TABLE VIII TTME sec RUN 14 RUN 15 RUN 17 RUN 18 RUN 19 zero 16.70 69.05 29 .60 28.45 32.75 250 17.05 71.15 30 .00 30.35 33.25 500 17.30 73.20 30.40 32.10 33.80 1000 17.85 77.35 31.25 35.35 34.85 1500 18.35 81.50 32.05 37.85 35.90 2000 18 .85 85.55 32.85 39.95 36.90 2500 19 .30 89.45 33.65 41.65 37.90 SOOO 19.80 93.15 34 .45 43.15 38 .85 3500 20 .20 96.85 35.25 44.45 39.75 4000 20 .65 100.20 36.00 45.55 40.70 4500 21.10 103.35 36.75 46.40 41.60 5000 21.55 106.25 37.45 47.15 42.45 5500 21.95 109.05 38 .20 47.75 43.25 6000 22.30 111.60 39.00 48 .25 -43.95 I n f i n i t y 149.25 52.10 57 .60 TABLE IX TIME sec RUN 21 RUN 22 RUN 23 RUN 24 RUN 25 zero 71.15 30.95 54 .35 33.90 30 .85 250 71.65 31.25 56.80 34.25 31.15 500 72.15 31.55 58 .55 34 .60 31.40 1000 73.10 32.15 60.80 35.25 31.95 1500 74.05 32.70 62.15 35.90 32.50 2000 75.05 33.15 63.00 36 .35 32.85 2500 76.00 33.50 63.50 36.75 33.15 3000 76.90 33.85 63.90 37.15 33.50 3500 77.85 34.20 64 .25 37 .50 33.85 4000 78 .70 34.55 64 .55 37.90 34.20 4500 79.40 34.80 64.80 38.25 34.50 5000 80 .10 35.00 65.05 38.45 34.80 5500 35.10 65.25 38.70 6000 35.20 38 .95 I n f i n i t y 81.91 35.25 67.85 TABLE X TIME sees RUN 26 RUN 27 RUN 28 zero 57.50 53 .65 161.00 250 58.15 54 .30 164.35 500 58.80 54 .95 167.55 1000 60 .00 56.05 173.65 1500 61.25 56.90 179.15 2000 62.35 57.70 183.95 25000 63 .45 58.40 188.35 3000 64 .35 59.00 192 92 5 3500 65.10 59 • 55 195.40 4000 65.90 60 .05 198.00 4500 66.60 60 .50 200.15 5000 67 .25 60.90 202«. 15 5500 67.80 61.20 204.00 6000 68 .30 61.50 I n f i n i t y 73.95 64 .45 29*7 ® V EXPERIMENT 30 Pure Azomethane (III) TIME sec PRESSURE zero 107.30 mm 370 136.12 705 156.78 1219 177.78 1695 190.26 2160 197.99 2978 206.83 6000 i n f i n i t y 224.2 Temperature 326°C P f / P 0 = 2.09 Gra p h i c a l h a l f - l i f e , 885 seconds ACKNOWLEDGEMENT My most g r a t e f u l thanks are due t Dr. W i l l i a m Ure, with whom t h i s r search was conducted, f o r h i s con stant encouragement and ma t e r i a l a s s i s t a n c e with much of the work done. 

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