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The thermal decomposition of azomethane in the presence of nitric oxide Shipton, Cuthbert Bernard 1939

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L t b By TEE THERMAL DECOMPOSITION  OF AZOMBTHANE  IN THE PRESENCE OF  NITRIC OXIDE G. BERNARD SHIPTON, B.A. A Thesis submitted f or the degree of MASTER OF ARTS i n the department of CHEMISTRY TEE UNIVERSITY OF BRITISH COLUMBIA April,1939 Vancouver,Canada ACKKOWIEDGEMENT It i s a great pleasure to express thanks and appreciation to Dr. William Ure for h i s i n t e r e s t and encouragement during the course of t h i s investigation. Without his d i r e c t i o n and c o l l a t o r a t i o n much of the i n -s p i r a t i o n would have "been lacking. THE THERMAL KB COMPOS IT? I OK OF AZQMBKPHAHE IN THE PRESENCE OF NITRIC OXIDE INTRODUCTION z " „ 23,25 If we are to believe with. Rice and Herzfeld that vapour phase reactions such as many of the organic decompositions studied today, proceed "by a series of chain processes i n which fragments from a few decomposing molecules attack stable substrate molecules which then decompose, forming stable substrate molecules and more free r a d i c a l s which are then able to i n i t i a t e further decomposition i n the substrate, we must assume either that we have a large number of short chains set up, or else a few long and persistent ones. It i s therefore evident that a free r a d i c a l decomposition mechanism must remain pure hypothesis u n t i l , f i r s t , the presence of chain reactions i s shown, and secondly, the lengths of such chains are established. U n t i l 1936, the only methods of chain length determination of general and accepted ap p l i c a t i o n were the study of induced reactions and the study of quantum yi e l d s i n photochemical processes, but at that time Stavelpyand Einshelwood announced the 29 p a r t i a l i n h i b i t i o n by n i t r i c oxide of several reactions such as the decomposition of dimethyl e t h e r 3 0 1 3 , d i e t h y l e t h e r 3 0 a , propal-37 dehyde, butyraldehyde , and of se\ reral other compounds, among them ethane . Einshelwood and his co-workers have explained the large i n h i b i t -ions observed i n c e r t a i n cases by assuming that n i t r i c oxide i s capable of reacting i n some way with free radicals and thus remov-ing them from active p a r t i c i p a t i o n i n the decomposition mechanism, If this is the case, a reaction which proceeds only "by a free r a d i c a l chain process, should he e n t i r e l y i n h i h i t e d hy the addition of s u f f i c i e n t n i t r i c oxide, while i f the reaction proceeds hy some manner of molecular rearrangement, the rate should he lar g e l y independent of the n i t r i c oxide concentration. If a decomposition proceeds simultaneously hy hoth processes, however, then we may expect only p a r t i a l i n h i b i t i o n hy n i t r i c oxide, and th i s i s what the E n g l i s h ohservers found f o r a l l cases investigated except f o r acetaldehyde, c h l o r a l , acetone and the iodine catalysed decomposit-ions of aldehydes and ethers, where no i n h i h i t i o n was noted. For these cases, then, the Hinshelwood theory would lead us to assume the entire ahsence of chain reactions. (For acetone, see also Spence and Wild, J . Chem. Soc., 1937, p 352 - 361). For the int e r p r e t a t i o n of t h e i r r e s u l t s , Staveley and Hinshelwood have calculated what they term "mean chain lengths", hy taking the r a t i o of the uninhibited r e a c t i o n rate to that of the f u l l y i n h i b i t e d reaction (For a complete summary of t h e i r work see reference # 31), but i t should be noted that t h e i r conclusions have been drawn e n t i r e l y from manometric data, by p l o t t i n g the time for 50 <?o pressure change, at a given temperature and i n i t i a l pressure of substrate, against the n i t r i c oxide pressure. According to the a n a l y t i c a l work of Gay and Travers on d i e t h y l 9 ether and n i t r i c oxide however, p a r t i c u l a r l y with large amounts of n i t r i c oxide, where Hinshelwood had claimed a c a t a l y t i c e f f e c t , must he interpreted with caution as oxidation processes are highly probable. This has been confirmed by the band spectra studies of Thompson and Meissner " who found that prominent n i t r i c oxide bands observed i n the early stages of the reaction were gradually replaced, when working v/ith dimethyl ether by ammonia bands, while with d i e t h y l ether continuous ab-sorption was observed which could not be so r e a d i l y i d e n t i f i e d . Several months after the present investigation had been commenced the e f f e c t of n i t r i c oxide on the photolysis of azomethane was reported by Davis Jahn and Burton 5 who found that below 80°C illumination was followed by a considerable pressure decrease (about 80% of the t o t a l n i t r i c oxide pressure) which took place i n the f i r s t 30 minutes of reaction (approx-imately estimated from graph) af t e r which the pressure increased to a value near the i n i t i a l pressure. At higher temperatures, the pressure change was p o s i t i v e , hut very small. Unfortunately no data for azome-thane alone i s given from which a comparison of i n h i b i t e d and normal rates can be made for the p a r t i c u l a r l i g h t i n t e n s i t y used. Also large percentages of n i t r i c oxide were used and i t has been shown i n t h i s lab-oratory that with such concentrations, the observed reactions i n the p y r o l y s i s at l e a s t , i s not that of the azomethane decomposition. The rate of decomposition of azomethane was f i r s t studied by Eamsperger^O i n 1927, at which time he showed that the process was a hom-ogenious, f i r s t order reaction, v/ith a rate constant sensibly independant of i n i t i a l pressure at pressures greater than 5cm. r7.11 On the basis of Thiele's e a r l y work on azomethane he considered the main reaction to be CH0HHJH3 = N 2 + 0 2 % and as the f i n a l pressure was 2.04 times the i n i t i a l pressure, and as f u r -thermore the' reaction products contained a l i t t l e ethylene, he suggested two possible side reactions ' '.• CHgMCHg. =. '»g+. C 2B£ + Hg 2CH3KNCfi3 = 2H 2 + ZCEg + C"2H4 He la t e r showed that the products, p a r t i c u l a r l y ethane, are e f f e c t i v e i n maintaining the r a t e 2 1 , i n "both p y r o l y s i s and photolysis,, After the development of Paneth's mirror technique „^ t e s t i n g for free r a d i c a l s , Eice and Evering 2*^, and la t e r Aeermakers^ detected free rad-i c a l s i n azomethane while flowing through a hot tube at 450-475°C, though they d i d not i d e n t i f y the fragments, Eeermakers 1 5 also introduced lead t e t r a e t h y l into azomethane' at 275° and concluded that since there was no increased decomposition of the azomethane, ethyl r a d i c a l s at least, do not attack this compound. The free r a d i c a l theory of Bice and Herzfeld coupled with such ev-idence would therefore indicate that th© azomethane decomposition i s a simple, iran chain process involving nothing more complicated than s p l i t -Sing from the molecule two methyl r a d i c a l s which then corahine to form ethane. Azomethane was therefore used as a r a d i c a l source to induce decom-p o s i t i o n i n acetaldehyde at 300°, a temperature at which the aldehyde alone is undecomposed, hut where azomethane decomposes at a measureahle rate. This work "by A l l e n and Sickman was followed "by that of other work-7 ers such as Fletcher and Rollfcfson who, "by the use of ethylene oxide', another free r a d i c a l source, induced decomposition i n acetaldehyde. Ure and L o v e l l 3 5 , of t h i s laboratory showed that azomethane i s capable of inducing short chains i n d i e t h y l ether at temperatures between 500-400°C. Since* the experiments of l e e r makers i n 1935, many extensive measure-ments have been made, notably by Sickman and R i c e ^ , 0 n the thermal de-composition of azomethane, both alone, and i n the presence of what may be assumed inert g a s e s 2 6 , Dg, N g, CH^, CO and C0 2. Their results for pure azomethane are i n good agreement with the work of Ramsperger after dead-space-1- and self-heating corrections are made, although they record a EyPj- r a t i o 3% higher than Ramsperger's. R i b l e t t and R u b i n 2 2 have recently subjected this reaction to a thorough a n a l y t i c a l study and they f i n d that while the reaction does give good rate constants from the a n a l y t i c a l data, such constants are (quite uniformly) -50% higher than those calculated from manometric data. Their analysis, by means of f r a c t i o n a t i o n and combustion of the separate f r a c t i o n s over a heated platinum s p i r a l , show that i n the range from 50 to 80% decomposition only about 70% of the nitrogen of the decomposed azomethane i s evolved as such, and that the ethane y i e l d is f a i r l y con-stant at 0.3 mols per mol of azomethane decomposed. The balance of the product was methane and a l i q u i d which condensed out during f r a c t i o n a t i o n and compression of the gas for analysis. The methane y i e l d was 0.4 to 0.5 mols/mol from 30 to 60% decomposition (by analysis) and 0.6 to 0.8 at complete decomposition. This i s i n agreement with the q u a l i t a t i v e data of Heidt and Forbes^ 2. This concludes the survey of data on the thermal decomposition of azomethane, but i t i s important to consider some of the work on the analagous photochemical reaction as this throws considerable, i f sometimes contradictory, l i g h t on the question of chains i n this decomposition. 9 0 P I ft After Ramsperger*s~ ' early study, Forbes, Heidt and Sickman" established the quantum y i e l d as 1 at low pressures for s i x d i f f e r e n t mon-ochromatic wave lengths. The photolysis was also studied "by Goldfinger^ 0 hut his results are not in agreement with those of other investigators, as he found a considerable increase i n rate at higher pressures, a small acceleration with argon, and retardation with nitrogen or methane (Cf Sickman and Rice ^ ) P a t a t " ^ a concluded on the basis of the quantum y i e l d of one and the absence of a temperature c o e f f i c i e n t that the prim-ary d i s s o c i a t i o n i s CE3K s ICHg + -EeH3 + Bz which i s followed immediately by the reaction 2CH 3 =• C 2H 6 before any chains are started. From the photolysis of azomethane with dimethyl e t h e r x , he concluded that for the process CH 3 + CHgH = NCH3 = CH 4 -* B 2 CH 3 , the energy of act i v a t i o n i s greater than 20cal. For the analagous r e -action with H atoms he measured E - 5.1 c a l . The thermal reactions he claims i s c l o s e l y analagous. Burton, Davis and Taylor^ have recently investigated the pho t o l y t i c products and found that the N 2 production was f a i r l y constant at 1.06 to 1.16 mols/mol of azomethane decomposed (calculated from t h e i r data) the ethane production varies from 0,8 to 0.9 and the methane from 0 to 0.16, when working at 20°. At higher temperatures, the nitrogen production was found to i n -crease f a i r l y evenly to 1.38 mols/mol at 223° while the ethane dropped o f f to less than 0.1 and methane rose to about 0.5, This work w i l l be considered more completely a f t e r the presentation of data from t h i s laboratory. The present investigation r e a l l y was undertaken as a result of an ex-periment made' hy Lo v e l l of th i s laboratory i n which he found that with a mixture containing approximately equal parts of n i t r i c oxide and azo-methane, the pressure increased very ; rapidly during the f i r s t stages of reaction and l a t e r gradually decreased. It was therefore thought that i t would be of considerable interest to study the reaction more completely i n order to determine the extent to which the normal azomethane pyrolysis i s subject to i n h i b i t i o n by n i t r i c oxide, and further to elucidate i f possible the mechanism of the i n h i b i t i o n . Several complicating factors which made themselves apparent at various stages of the work, have been studied as c a r e f u l l y as time permitted. When we examine as we have done, the contradictions between the ev-idence of Patat, Heidt and Forbes and Leermakers on the one hand, f o r a non-chain process,, and on the other the a n a l y t i c a l data of R i b l e t t and Rubin, and Burton, Davis and Taylor, which could he the res u l t of only a complicated decomposition mechanism, we see-at once that the inves t i g a t i o n how i n hand should f u r n i s h information not only about the extent to which chains are a factor i n the decomposition of azomethane, but should also have bearing on the s p e c i f i c i n h i b i t i o n of chains by n i t r i c oxide as postulated by Hinshelwood. Also i t i s necessary to admit that quite pos-i b l y i f this decomposition i s shown not to be larg e l y i n h i b i t e d by n i t r i c oxide, then either the free r a d i c a l theory or Hinshelwood's postulate must be regarded as inadequate to explain the experiments here described. JTEE EXPERIMENTAL PROCEDURE: Preparation of Azomethane: -Dimethyl hydrazine dehydrochloride was prepared and r e c r y s t a l l i z e d a f t e r the method of H a t t 1 1 . From this s a l t azomethane. was prepared hy oxidation at 0°C with a neutral potassium chromate sol u t i o n after the procedure described by Ramsperger 2 0. For the f i r s t preparation 6.5g of the dimethyl hydrazine s a l t i n 15 ml of f r e s h l y b o i l e d d i s t i l l e d HgO Was added drop by drop to 20g of E 2 C r 0 4 i n 40 mo of f r e s h l y b o i l e d d i s t i l l e d HgO i n the a l l glass prep-aration t r a i n described by L o v e l l 1 6 . As the preparation proceeded the a i r pressure i n the apparatus was slowly reduced from 40cm. to 3cm. Hg. From the generator, the gas, was drawn slowly through two drying tubes, the f i r s t , 25cm long f i l l e d with calcium chloride, the second, 50cm long, f i l l e d with a mixture of calcium chloride and soda lime, into a trap sur-rounded with a dry ice-ether mixture at -78°. After 5 hours the mercury cut-off between the trap and preparation t r a i n was r a i s e d and the pale yellow l i q u i d which had c o l l e c t e d was allowed to vapourize into the azo-methane storage bulb, separated from the rest of the apparatus by the azomethane mercury manometer and cutoff (marked M3 i n Figure l ) . To remove dissolved gaseous impurities the azomethane was refrozen with l i q u i d a i r and pumped off with the d i f f u s i o n pump, then vapour!zed with the cutoff raised, after which the product was refrozen and more impur-i t i e s pumped off, the process being repeated 12 times after which the pressure had reached a constant value of less than 1 10" 5 cm of Eg. About 21cm of gas was obtained. In the second preparation, the same procedure was followed except that 8g dimethyl hydrazine was used, and the f i n a l y i e l d was 77cm of azomethane.. " Preparation of M i t r i c Oxide; N i t r i c oxide was prepared by the well knov/n reaction of Cu and d i -lu t e n i t r i c acid according to the equation 5Cu +.8HN03 .•=- 3Cu(K0 3) 2 + 2N0 4H 20. The copper used was Baker's e l e c t r o l y t i c sheet (99.9% pure). This was shredded, covered with d i l u t e hydrochloric acid f or several hours washed repeatedly with d i s t i l l e d water and dried overnight under vacuum. . The n i t r i c a c i d used was prepared by d i s t i l l i n g Baker's C P . acid from a pyrex r e t o r t , the f i r s t and l a s t fractions being discarded. The product was colourless. This a c i d was then d i l u t e d to 4N with b o i l e d d i s t i l l e d water i t having been previously established that t h i s was the lowest concentration which would generate n i t r i c oxide at a convenient rate at 0°. The apparatus used i s shown in f i g u r e 2. B and B' are b a f f l e s to remove any acid spray; G i s a sulphuric acid bubble trap to remove any traces of dioxide; D i s a mercury cutoff and safety valve, SO COT . u s e f u l also as a rough manometer; while E is a tube,, long f i l l e d with KOHpellets to remove any a c i d i c vapours and further dry the gas, and F is the storage bulb, volume 500 ml. .. Before preparation of the gas the generator f i l l e d with shredded copper and the p u r i f i c a t i o n t r a i n stood under vacuum for 6 days to r e -move as much water vapour from the hydroxide p e l l e t s as possible. For the preparation the generator was cooled to -5° i n a s a l t bath and the 4 N n i t r i c acid was run in i n very small quantities over a period of -12-36 hours. Since i t was impossible from the design of the sulphuric acid trap to evacuate the generator to a pressure less than 1cm E g , the whole apparatus excluding the storage bulb, F, was flushed out and pumped'down as low as was f e a s i b l e , 5 times before any gas was collected. Care was exercized to make sure that during c o l l e c t i o n the gas was drawn very slowly through the p u r i f y i n g t r a i n . The product consisted of 59cm of gas at 20°C i n the 500 ml f l a s k . According to M e l l o r 1 8 , such a method of preparation y i e l d s a gas completely absorbed by FeSO^.. Description of Apparatus: The apparatus used was that described by L o v e l l ^ except that a mer-cury "U", tapped with a s t i r r i n g motor was substituted for the c l i c k e r guage. The mercury l e v e l s on each arm of the U were observed with a cathetometer. The procedure used i n making rate measurements w i l l be described l a t e r . Temperature Control and Measurement: For the f i r s t series of runs (prior to June 1958) the thermo-regulator control of L o v e l l was used but for a l l subsequent experiments a photoelectric control designed and b u i l t by the workers of t h i s lab-oratory, was u t i l i z e d . The c i r c u i t i s that shown i n Fig.111. The p l a t -inum resistance thermometer used i n the standard manner to measure furnace temperatures, was also incorporated by means of a double throw switch into a second Wheatstone bridge c i r c u i t . The mirror of the moving c o i l galvanometer connected across t h i s bridge was illuminated by a well f o -cus sed beam from a 15 watt microscope lamp which was r e f l e c t e d by the galvanometer mirror through a converging lensp to the. active .surface of a photoelectric c e l l , the current from which, after amplification, could he used to actuate a relay. When the re l a y closed^a small resistance i n " series with the furnace was short c i r c u i t e d and the current through the furnace was thus increased and the temperature raised. The..geometrical arrangement of the o p t i c a l system is i l l u s t r a t e d i n Fig.111b. The bridge was set to balance f o r a resistance i n the p l a t -inum resistance thermometer corresponding to the desired temperature, and when balanced^the r e f l e c t e d beam f e l l just on the edge of the active sur-face of the photo-cell. As soon as the furnace temperature f e l l s l i g h t l y ; the lowered resistance of the thermometer l e f t the bridge off balance and the beam from the galvanometer mirror was deflected to the photocell sur-face, when the r e l a y closed and the furnace current was raised. Stops were b u i l t into the galvanometer to prevent the angular d e f l e c t i o n of the suspension from becoming so large that the l i g h t beam f e l l beyond the c e l l window. When the furnace was operated by a minimum current 0,25 amps less than the equilibrium value for the desired temperature, with a current change of 0,5 amps, xipon c l o s i n g the relay, furnace temperatures con-stant to 0.5°C were maintained without d i f f i c u l t y f o r periods of from 12 to 24 hours. Factors which must be c a r e f u l l y considered i n such an arrangement are the s e n s i t i v i t y of the photocell, the l a g i n the furnace and the resistance of the thermometer. Extensive tests showed that the l a s t two factors were of great im-portance. A very small change i n the platinum resistance was s u f f i c i e n t to operate the rela y , but due to l a g e f f e c t s , the furnace temperature would r i s e or f a l l about 0.2° further before the e f f e c t of the current was apparent. The platinum resistance thermometer used for this investigation had been very c a r e f u l l y c a l i b r a t e d by Love11 , and as the writer's values at 0° and 100° C. checked very well with h i s , further c a l i b r a t i o n was not made. Temperatures were therefore read from Lovell's large scale resistance-temperature graph. Pressure Measurement : Since a c l i c k e r guage was not available f o r t h i s investigation, the following procedure was used for measuring pressures. With the cathetometer properly alli g n e d , and with the cross hairs i n l i n e with the p o s i t i o n of the two menisci of the "U" for equal pressure i n both arms, (usually determined before the run started, when both reaction chamber and guage were evacuated) a i r was admitted to the pressure measuring system u n t i l the pressure there was about 0.5 mm greater than the pressure i n the reaction chamber. Then t h i s pressure could be meastired with the compensated MacLeod guage at any convenient time before or after the two pressures were observed to be equal. To aid i n observation, the "U" was illuminated from behind with a small lamp shielded from d i r e c t view with white paper. One arm of the "U" was always automatically tapped throughout a run with a motor tapper conveniently mounted. A l l scale readings were made with the a i d of a hand lens. Pressures were found reproducible to about 0.05 mm, after the exercise of usual precautions. The guage and compensating manometer were of course r e - c a l i b r a t e d f or each series of runs, but no v a r i a t i o n i n the apparatus constants was observed. Values obtained agreed well with, those of Love11. The formula used for pressure c a l c u l a t i o n was p . ( G i ~ + M(l + a) + L cm. E i where i s guage reading, g^ i s guage zero pressure, M is manometer reading (Ml), a i s f a l l of lower M 1 meniscus per cm r i s e for the upper meniscus, L is the zero correction for M 1, and i s the guage r a t i o corresponding to the g^ used. The values obtained for these four constants are as follows : g-^  12.22 cm (Ratios 2 and 3 were r a r e l y used) a 6.02 cm/cm 1 25.38 cm logR 1 0.69609 De t a i l s of these c a l i b r a t i o n s are to be found i n Log Books I and II. Time Measurement : Times were measured on a large faced e l e c t r i c clock equipped with a second hand ( Fisher Interval Timer). As an a u x i l i a r y to this a stopwatch was used, the watch being started as the second hand of the clock passed zero and being stopped when the top of the mercury meniscus i n the ,rU" was observed to just touch the h a i r l i n e as seen through, the cathetometer. Seconds measurements were thus obtained with quite f a i r accuracy. The Preparation of Mixtures : The usual procedure was as follows : N i t r i c oxide from the storage bulb was admitted to the dead-space connecting tothe reaction chamber and i t s pressure measured by cathetometer on the azomethane manometer, for which purpose the azomethane was frozen out , p r e f e r --ably with l i q u i d a i r , but when th i s was not available, with a dry ice-ether mixture. In the l a t t e r case, the vapour pressure of azomethane at -78° C. had to be corrected f o r . This was done by cathetometer measurement before the mixture was prepared, and sometimes after the residue had been pumped from the dead-space as well. Such measurements were always i n good agreement, and the correction d i d not vary more than from 0.590 to 0.720 throughout the investigation. Azomethane was next introduced by bubbling through the cutoff M 3 u n t i l the desired pressure was reached, after which a cathet-ometer reading was taken of the t o t a l mixture pressure. It was then of course necessary to correct for the pressure lowering of the n i t r i c oxide due to the increase i n the dead space volume. This was done by means of a formula of obvious derivation, P = P l ( _ l L _ _ J ( V i +1»'r2h) is a conveniently chosen volume of defined extent (See Log Book II, p 84) which was measured by p a r t i t i o n with the reaction chamber, the volume of which had been previously determined. Temperature fluctuations were never large enough during the prep-aration of a mixture to necessitate correction f o r that factor. When a large mixture f o r use i n several runs was prepared, the 500 ml mixture storage tube was used, and for such cases, no volume correction of this type need toe considered. When l i q u i d a i r was at hand, i t was of course preferable to prepare a mixture by f r e e z i n g the azomethane out i n a trap attached -17-to the deadspace a f t e r i t had heen measured and then add the n i t r i c oxide. Run Procedure : Before each run the reaction chamber was thoroiighly evacuated while hot, u s u a l l y for a period of four or f i v e hours. The residual pressure was always less than 3x10™ and usually about 2x10 cm, before the pumps were shut o f f . With the f i r s t temperature control system - the mercury thermo-regulator - the furnace was operated on the regulator for at least four hours before an experiment was commenced. With the photo-e l e c t r i c control, however, once the furnace had reached the desired temperature, the control mechanism could be switched i n and within an hour i t was possible to s t a r t the run. Usually more time than t h i s was allowed to ensure puoper thermal equilibrium. Then, after the mixture had been prepared, the cathetometer l i n e d up and t r a i n e d on the proper meniscus po s i t i o n , with the l i g h t and tapper operating, of course, a i r was admitted to the manometer system to the expected i n i t i a l pressure, and the second and minute hands of the clock were synchronized. Next, one minute before zero time, the stop-watch was started, the resistance of the resistance thermometer measured and the temperature of the reaction mixture observed. Five seconds before zero time, the r e a c t i o n chamber stopcock was opened and the mixture admitted to the furnace; then at zero time the stopcock was closed. As soon as possible thereafter, a pressure measurement was rA made ; i f circumstances were favourable, this was often possible within 60 seconds. Following the f i r s t reading, observations were made as frequently as was deemed necessary to follow the course of the reaction. When the decomposition had slowed down somewhat, the i n i t i a l pressure was found by measuring the residual gas pressure i n the deadspace. For t h i s purpsee, the cathetometer had to be turned from i t s p o s i t i o n f a c i n g the "U" and focussed on M 3, the azomethane manometer. The temperature was also observed and any necessary corrections made to convert t h i s value to that under i n i t i a l conditions. The cathetometer was then swung back to focuss on the "U". Movement of the instrument i n t h i s way d i d not appear to a f f e c t subsequent readings as they invariably f e l l on the same smooth curve as the previous measurements. This method of determining i n i t i a l pressures was checked by means of three blank runs, the results of which are recorded i n Table I. It i s to be noted that there i j _ a difference between the TABLE I Run 15-A Run 15-B Run 15-0 T - 290°. T -312° . T - 312°. P mm, Time p Time p Time 5 0 » ° 0 62.0 0 65.2 0 50.1 76 62*5 100 65.4 164 50.2 600 63.2 390 65.6 505 P 50.9 1440 63.2 620 65.6 900 50.9 2820 manometer and guage measured pressures, but i n every case,the curve i s l i n e a r and extrapolation passes through the manometric value,* which, i s good evidence f o r the r e l i a b i l i t y of the method i n th i s p a r t i c u l a r set-up. This small pressure change i s , however, of interest. A possible explanation may be proposed as a combination of three factors at lea s t , oxidation of tarry residues on the reaction chamber wailft, ( N i t r i c oxide and a i r were used for these calibrations.) slow heating of the gas in the reaction chamber dead-space, and perhaps an i n i t i a l Joule Thomson cooling. F i n a l pressures were measured very c a r e f u l l y , and for most of the completed runs, two i d e n t i c a l values at least a half-hour apart were obtained. GAS ANALYSIS : To supplement the rate measurements, gas analyses were c a r r i e d 38 out i n the semi-micro gas analysis described by Morrison and by . 59' Shipton , with the reagents described by them. The order of analysis was u s u a l l y as follows : NO with acid FeS04 C 3H 6,etc 87 % H 2S0 4 C I active HgSO^ CD Cu 2S0 4 H p Pd - sodium picr a t e . References to a l l these reagents except the f i r s t w i l l be found i n the above mentioned theses; i t was prepared according to Mellor ' , and contained the equivalent of 15 g anhydrous FeS0 4, 15 g of .64 % H 2SC 4, and 70 g HgO . Rather extensive experiments were also made on the use of s i l i c a gel f o r the se l e c t i v e , adsorption, at low temperatures of - 2 0 -various gases, according to the method of Delaplace 6, hut though the method indicated good separation of a l l gases l i k e l y to he present from hydrogen, at l i q u i d a i r temperatures, and of azomethane from gases of the "boiling point range of methane at dry ice-ether temperatures, the method was perforce discarded after several t r i a l s because of the high tenacity with which many of these gases are held by such a surface, even at f a i r l y high temperatures. For example, a methane sample with a pressure of 42.8 cm. at 22° o i n the 5 ml bulb was pumped over the gel at -78 . After 1 0 minutes, 55.4 cm. could be pumped o f f . Upon warming to room temperature, a further recovery of only 0 . 9 cm was made and even after heating to 65° for 15 minutes, of the 6.5 cm absorbed by the gel, a t o t a l of only 1.5 cm was recoverable, 5.2 cm being l e f t on the g e l . These re s u l t s indicate that without d i f f u s i o n pumps to remove the desired products from the adsorbent, the method i s not a p p l i c -able to ordinary gas analysis. The use of a d i f f u s i o n pump i n such work as the present would not be fe a s i b l e due to the i n s t a b i l i t y of any azomethane remaining i n the gas. THE PRESENT AT ION OF DATA : In such an i n v e s t i g a t i o n as t h i s , which must be of an explor-atory nature, i t i s d i f f i c u l t to assemble the data i n a systematic form, hut i t w i l l be seen that Table II contains a summary of a l l experiments, grouped according to n i t r i c oxide concentration, while the various subdivisions of Table III, i n which the complete data on each experiment i s presented, are grouped according to -21-temperature ( to "be inserted as appendix). This table also includes data on gas analysis where analyses were made, ("further analysis and data on hydrocarbons present w i l l be presented at a l a t e r date). Sev e r a l c h a r a c t e r i s t i c runs have been pl o t t e d on the accompanying graphs DISCUSSION The f i r s t problem which must concern us i s the disappearance of n i t r i c oxide. If we assume that n i t r i c oxide reacts only with rad-i c a l s , then we have two p o s s i b i l i t i e s before us, either the reaction i s reversible or i r r e v e r s i b l e . A reversible reaction is i n accord with the conclusions of Echolis and Pease who, i n a study of the butane pyrolysis-'- 3 came to the conclusion that the function of the n i t r i c oxidewas to set up the equilibrium B • •+ NO ENO and therefore did not influence the nature of the products, but only served to deactivate r a d i c a l s . Such conclusions are i n sensible ag-reement with the gas analyses of Buns 26 and 27, but not with the an-alyses of others at more extreme temperatures and n i t r i c oxide pres-sures, such as 28, 10, 12, and 13, where considerable n i t r i c oxide was used presumably in an oxidation process such as that mentioned previously i n a discussion of the work of Thompson and Meissner where, they concluded that the o v e r a l l reaction, using the methyl r a d i c a l as an example was CH 3 -+ NO = NH3 •*• CO Banford on the other hand i n the photolysis of trimethyl amine i n the presence of n i t r i c oxide showed the presence of cyanide and wrote the r e a c t i o n as CH3 •+ NO = HON +- H 20 It i s possible that both these reactions occur, as well as the formation of nitrogen and possibly carbon monoxide or water. With p o s s i b i l i t i e s of such undoubted complexity i t would be almost hopeless to determine a mechanism, but from a consideration of the p l o t -ted r e s u l t s we may make plau s i b l e assumptions as to the temperature r e -gions i n which such reactions, w i l l mask any i n h i b i t y e f f e c t . Before proceeding to such considerations'.however i t is necessary to consider another important complication, which has been observed i n several runs. It was noted i n several runs (Cf. Buns IS, 14, 18, 19, 23, 26, 30, 31) that during the f i r s t 150 seconds there was a small decrease i n pressure before the normal expanding reaction occured. This has been investigated a s c a r e f u l l y as possible but i t is r e a l l y unfeasible to i n -vestigate such a reaction i n a s t a t i c system. However considerable c a l -culations have been made on t h i s phenomenon. It i s d i f f i c u l t to see how t h i s could be a reaction with r a d i c a l s as such reactions unless more than two r a d i c a l s react with n i t r i c oxide cannot give a pressure decrease, ca l c u l a t i o n s however on the p o s s i b i l i t y of an addition with azomethane i t s e l f followed by a gradual brea&down of the complex are i n agreement with experiment for certain runs. (Reference may be had to the d e t a i l e d calculations of t h i s research f o r such matters). The fact that i n some cases (Cf Run 26) the f i n a l pressure was cl o s e l y i n accord with that of a simple azomethan decomposition and the analysis indicated no destruct-ion of n i t r i c oxide i n the process, i s also strong support for such a mechanism. A further complication was the negative slope accompanied by the formation found i n high temperatures. -23-We may therefore s&j that rims there are at least three possible r e -actions to "be taken into account before we can say that the azomethane de-composition is subject to i n h i b i t i o n by n i t r i c oxide: 1. association of azomethane with n i t r i c oxide at temperatures above 285. (see curves) 2. oxidation at high temperatures and n i t r i c oxide pressures. 3. the re a c t i o n of n i t r i c oxide with azomethane decompositoin matter. A glance at the curve for l / t 5 0 NQ/Azo at 2980 would indicate that a true i n h i b i t i o n is a decided p o s s i b i l i t y but u n t i l further evidence is at hand, detai l e d calculations i n chain lengths are meaningless and dangerous,. CONCLUSION The decomposition of azomehtane i n the presence of n i t r i c oxide has been investigated over a range of temperatures and pressures. The reaction has bee:n found to have at least three complicated side reactions which tend to mask any possible i n h i b i t i o n . However on the basis of some experimental evidence presented i t may be t e n t a t i v e l y stated that i n the region of 300°C, with a n i t r i c oxide concentration not more than one tenth the azomethane concentration some i n h i b i t i o n i s observed. , , BIBLIOGRAPHY (1) A l l e n , J. Am.Chem.Soc 56, 2053, '34 (2) A l l e n and Rice J . Am. Chem. Soc. 57, 310, '34 (2a) A l l e n and Sickman J . Am. Chem. Soc. 56, 2031, <34 (3) Bamford, J.Chern. Soc. pl7, 1939 (4) Burton Davis and Taylor, J . Am. Chem. Soc. 59, 1038, 1989, (5) Davis, Jahn and Burton, J . Am. Chem. Soc. 60, 10, '38 (6) Delaplace, Comptes Rend. 204, 1940, '37 (7) Fletcher and Roll|fson, J . Am. Chem. Soc. 58, 2135, '36 (8) Forbes Heidt and Sickman, J . Am. Chem. Soc. 5£, 1935, '35 (9) Gay and Travers, Nature, 138, 546, '36 (10) Goldfinger, 1 Compt. Rend. 202, 1502, '36 (•11) Hatt, " Q r g a n i c Synthesis" XVI, pl8-21, 1936 Wiley (12) Heidt and Forbes, J . Am. Chem. Soc. 57, 2331, '35 (13) E c h o l l s and Pease, J. Am. Chem. Soc. 60, 1701, '38 (14) Leermakers, J . Am. Chem. Soc. 55, 3499, '33 (15) " J. Am. Chem. Soc. 55, 4508, '33 (16) L o v e l l , Master's Thesis, This Un i v e r s i t y (17) L o v e l l , Unpublished data, (18) Mellor, "Comprehensive Treatise on Theoretical and Inorganic Chemistry" VIII, p418,p423 (19,);.) Pat at (a) Naturwissen, 23, 801, '35 quoted p. Chem* Abst. 30, 940, '36 (19B) (b) Nach*. Gess. Y/iss. Gottingen, Math.-physik. Klasse, Fachgruppe II (N.F.) 2, 77, '36 quoted Chem. Abst. 31, 6112, '37 (20) Ramsperger, J. Am. Chem. Soc, 49, 912, 1495, '27 (21) . Ramsperger, J. Phys. Chem. 34, 669, '30 (22) R i b l e t t and Rubin, J. Am. Chem. Soc. 59, 1537, '37 (23) Rice, F.O. & K.K. "The A l i p h a t i c Free Radicals',' Baltimore, John Hopkins Press. 1935 (24) Rice and Evering, J . Am. Chem. Soc. 55, 3898, '33 .(25) Rice & Herzfeld, J . Am. Chem. Soc. 56, 284, '34 (26) Sickman & Rice, J. Chem. Phys.(a) 4, 239, '36 (b) 4, 608, '36 (29) Staveley & Hinshelwood, Nature, 137, 29, '36 (30a) " Proc. Roy. Soc. 154A, 335, »36 (30b) " i " r 0 C s Roy. Soc. 159A, 32, '37 (31) " J . Chem. Soc. pl569, '37 (32) idem. Proc. Roy. Soc. 162A, 557, '37 (33) Thiele, Ber. 42, 2575, '09 (54) Ure A Aovell, unpublished data (35) Thompson & Meissner, Nature, 139, 1018, '37 (37) Winkler Fletcher & Hinshelwood, Proc. Roy. Soc. 146A, 345, '34 (38) Morrison, Batchelor's Thesis, This University. (39) Shipton, Batchelor's Thesis, This University. Run # Temp. °c P j_ 9 mm PN0 11111 NOfAzo, P/P A PAoT 1 TABLE II (Summary of a l l Experiments) 28 287.0 73.1 __* inc. .45x1c - 4 10 , sec -1 25 297.7 50.9 1.09 1.7x10 .180 ,-4 17 297.8 68.6 1.06 1 308.5 74.4 1.10 ,175 Run # Temp. P i PN0 NO/Azo. P/P A P A d t , 22 298.0 67.6 0.3** .004 .87' Exp l . 18 297.7 73.4 0.88 .012 1.07 19 298.5 69.5 1.34 .020 1.00 #1.7xl0~ 4 #1.6x10*4 29 287.0 61.7 .039 .85 .3x10" Run # Temp. P i PN0 NO/Azo. P/P, I-dPi P A d t 30 297.0 54.3 2.04 .639 1.04 #1.8xl0~ 4 31 304.9 60.6 g. 28 .039 1.07 2.7x10 ,-4 4 309.0 58.1 2.50 .045 1.09 6.8x10 -4 2 312.5 29.9 (meas 35.6 (extr 1.2 or 1.5 .045 1.18 9.8xl0 - 4. "TABLE II (cont'd) Run.# 3 27 26 23 Temp* 339 287 279.6 298.0 P i 53.5 80.7 72.8 52.3 PH0 2*30 3 © 90 5.8C l 4.75 NO/Azo ,045 .051 .086 .100 P/P A 1.12 1.007 1.07 1.06 P Adt 18.6xl0" 4 .25X10"4 #1.2xl0 - 4 #1.8xl0~ 4 Run # 24 21 20 12 Temp. 297.8 , 197.6 279.4 281.0 300. 5 P i 66.9 -37-6.6 94.6(7) 71.5 % 0 7.5 8.73 15.0 . 32.9 10/Azo .126 .129 .189 .852 P/P A .57 058 " (.48) .71 l.dPj 1.54X10"4 ™,— 1.83xl0~ 4 PAdt Run # 14 13 7 6 Temp. 286.0 287.0 311.0 312.5 P i 38.9 40.8 32 o 3 60.8 PK0 17.9 18 98 10.3 27.9 NO/Azo o852 .825 .852 .852 P/P A i n c . .65 .84 .93 IdPj 0 0 4.8xl0" 4 5.5x10' P Adt ,-4 Run # 6 16 5 11 Temp. 312.5 314.5 341.0 348.0 p i 76,4 24,7 91.3 62.5 p NO 35J2 11.4 42.0 28.8 NO/Azov .852 .852 .852 .852 P/P A 1.90 inc 1.83 1.69 P A d t 6.9xl0- 4 1.4x10" " 4 17.6x10-^ Run # 9 10 Temp 349.5 550 (circa) P i 56.9 Explosion PN0 2 6 9 2 NO/Azo .8523 P/P A 1.97 l.dP.i 22.8xl0~ 4 P A d t Legend; P/P A i s f i n a l pressure change divided hy the i n i t i a l pressure. When marked ( l ) i s P m a x PA * NO added l a t e r -**N6 pressure very approximate. Error i n manipulation. # In these cases i n i t i a l slope was negative. Recorded value is f i r s t p o s i t i v e slope. ( l l ) For th i s run P^na was value indicated i n bracket. P4 4- o »> o O 9 — "0 1 •V o 9^ '•0 o 0 * i f \ -\ 0 r\ 6 o Q O 0 0 


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