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Studies in the pyrolysis and flash photolysis of azoethane Sandhu, Harbhajam Singh 1966

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STUDIES IN THE PYROLYSIS AND FLASH PHOTOLYSIS  OF AZOETHANE by HARBHAJAN SINGH SANDHU B.Sc. (Hons.) Panjab U n i v e r s i t y , 1959. M.Sc. Pa n j a b U n i v e r s i t y , 1961. A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n t h e Department o f CHEMISTRY We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d . THE UNIVERSITY OF BRITISH COLUMBIA May, 1966 In present ing th i s thes is in pa r t i a l fu l f i lment of the requirements for an advanced degree at the Un ivers i ty of B r i t i s h Columbia, I agree that the L ibrary sha l l make i t f r ee l y ava i l ab le for reference and study. I fu r ther agree that permission for ex-tensive copying of th i s thes is for scho la r l y purposes may be granted by the Head of my Department or by his representat ives . It is understood that copying or pub l i ca t ion of th is thes is for f i n an -c i a l gain sha l l not be allowed without my wr i t ten permiss ion. Department of The Un ivers i t y of B r i t i s h Columbia Vancouver 8, Canada The University of B r i t i s h Columbia FACULTY OF GRADUATE STUDIES PROGRAMME OF THE FINAL ORAL EXAMINATION FOR THE DEGREE OF DOCTOR OF PHILOSOPHY of HARBHAJAN SINGH SANDHU M.Sc, Panjab University, 1961 TUESDAY, JUNE 28, 1966, AT 3:30 P.M. IN ROOM 261, CHEMISTRY BUILDING COMMITTEE IN CHARGE Chairman: S. H. Zbarsky F. W. Dalby L. G„ Harrison B. A. Dunell J . G. Hooley L. Do Hayward G. B. Porter External Examiner: B. H. Mahan Department of Chemistry University of C a l i f o r n i a Berkeley, C a l i f o r n i a Research Supervisor: G, B. Porter STUDIES IN THE PYROLYSIS AND FLASH PHOTOLYSIS OF AZOETHANE ABSTRACT The p y r o l y s i s of azoethane has been studied i n a s t a t i c system i n the temperature range 245 to 308°C, and at i n i t i a l pressures between 15 and 110 m.ni. using gas chromatographic a n a l y s i s . In the i n i t i a l stages of the reaction, products i d e n t i f i e d are methane, ethylene, ethane, propylene, propane, n-butane, nitrogen and d i -and t r i - e t h y l amines. At large extents of the reaction, r a d i c a l - o l e f i n reactions occur extensively and produce a complex d i s t r i b u t i o n of products. Compounds containing carbon, hydrogen and nitrogen are also formed at higher percentage decompositions. The orders of formation of major products with respect, to azoethane, as well as a c t i v a t i o n energies, have been determined. The a c t i v a -t i o n energy for the i n i t i a t i o n reaction has been found to be 47.2 + 1 . 0 kcal per mole. The e f f e c t s of additives, such as. cis-butene-2, butene-1 and carbon dioxide, on the i n i t i a l rates of formation of various products have been investigated. A l l the additives have a common e f f e c t of lowering the i n i t i a l rate of ethane formation. Increase i n surface:volume ratio.has the e f f e c t of lowering the time rate of pressure change by 10 to 207o depending on temperature and pressure used and t h i s has been a t t r i b u t e d p a r t i a l l y to the increased surface re-combination of r a d i c a l s . The rate of formation of ethane i s also decreased i n the packed re a c t i o n v e s s e l . This study has pointed out that there i s a short chain i n the thermal decomposition of azoethane involving formation and subsequent decomposition, of CH.,j-CH-N= N-CH2-CH3 r a d i c a l . A mechanism has been proposed for the p y r o l y s i s of azoethane which accounts q u a l i t a t i v e l y f o r the nature and d i s t r i b u t i o n of products. The f l a s h photolysis of azoethane has also been investigated at room temperature i n pyrex and quartz reaction vessels. Thermally e q u i l i b r a t e d r a d i c a l s are produced i n the pyrex c e l l with outer pyrex jacket. Addition of large amounts of carbon dioxide i n the f l a s h photolysis of azoethane decreases the product y i e l d s i n d i c a t i n g the c o l l i s i o n a l deactivation of excited azoe-thane molecules. The r a t i o of ethyl r a d i c a l ' s dispro-portionation to combination has been found to be 0.12 /t 0.02. Reactions have been postulated to account for the products of f l a s h photolysis of azoethane. GRADUATE STUDIES F i e l d of Study. Physical Chemistry Modern Organic Chemistry Physical Inorganic Chemistry Topics i n Physical Chemistry Seminar i n Chemistry Topics i n Inorganic Chemistry Spectroscopy and Molecular Structure Chemical K i n e t i c s Other Studies: Applied Calculus and D i f f e r e n t i a l Equations Analysis Moder Physics Programming and Numerical Algorithms A. I. Scott H. Hochstrasser N. B a r t l e t t J . A. R. Coope A. Bree B„ A. Dunell N. B a r t l e t t H. C. Clark J . T. Kwon W. R. Cullen A. Bree C. Reid B. A. Dunell G. B. Porter D. G. L. James W. H. Gage E. Macskasy M. Bloom J. R„ H. Dempster PUBLICATION Comparative Study of the Thermodynamic Properties of Polyethylene Glycols, M.Sc. Thesis, Panjab University, 1961. ABSTRACT The pyrolysis of azoethane has been studied in a static system in the temperature range 245 to 308°C, and at initial pressures between 15 and 110 mm. using gas chromatographic analysis. In the initial stages of the reaction, products identified are methane, ethane, ethylene, propane, propylene, n-butane, nitrogen and di- and tr i - ethyl amines. At large extents of the reaction radical-olefin reactions occur extensively and produce a complex distribution of products. Compounds containing carbon, hydrogen and nitrogen are also formed at higher percentage decompositions. The orders of formation of major products with respect to azoethane, as well as activation energies, have been determined. The activation energy for the initiation reaction C2H5N2C2H5 • 2 C 2H 5 + N2 has been found to be 47.2 ± 1.0 kcal per mole. The effects of additives such as cis-butene-2, butene-1 and carbon dioxide on the initial rates of formation of various products have been investigated. All the additives have a common effect of lowering the initial rate of formation of ethane. Increase in surface: volume ratio has the effect of lowering the time rate of pressure change by 10 to 20% and this has been attributed to the increased surface recombination of radicals. The rate of formation of ethane is also decreased in the packed reaction vessel. This study has pointed out that there is a short chain in the thermal decomposition of azoethane due to the formation and subsequent decomposition of CH3CH-N=N-CH2CH3 radical. A mechanism has been p r o p o s e d f o r t h e p y r o l y s i s o f azoethane w h i c h a c c o u n t s q u a l i t a t i v e l y f o r t h e n a t u r e and d i s t r i b u t i o n o f p r o d u c t s . The f l a s h p h o t o l y s i s o f azoethane has a l s o been i n v e s t i g a t e d at room t e m p e r a t u r e i n p y r e x and q u a r t z r e a c t i o n v e s s e l s . T h e r m a l l y e q u i l i -b r a t e d r a d i c a l s a r e p r o d u c e d i n t h e p y r e x c e l l w i t h o u t e r p y r e x j a c k e t . A d d i t i o n o f l a r g e amounts o f ca r b o n d i o x i d e i n t h e f l a s h p h o t o l y s i s o f azoethane d e c r e a s e s t h e p r o d u c t y i e l d s i n d i c a t i n g t h e c o l l i s i o n a l d e a c t i ^ v a t i o n o f e x c i t e d azoethane m o l e c u l e s . The r a t i o o f e t h y l r a d i c a l s d i s p r o -p o r t i o n a t i o n t o c o m b i n a t i o n has been f o u n d t o be 0.12 -* 0.02. R e a c t i o n s have been p o s t u l a t e d t o a c c o u n t f o r t h e p r o d u c t s o f f l a s h p h o t o l y s i s o f a z o e t h a n e . iv TABLE OF CONTENTS Page Ti t le Page i Abstract i i Table of Contents iv List of Figures v i i Lis t of Tables ix Acknowledgements x i CHAPTER I INTRODUCTION 1 General . 1 Earl ier Studies on the Azoethane Decomposition 3 A Similar System Hg(C2H5) 2 7 The Object of the Present Investigation 9 CHAPTER II EXPERIMENTAL : 11 Materials: — 11 (i) Azoethane 11 ( i i ) Hydrocarbons 11 ( i i i ) Other Material 11 The Pyrolysis System 13 Description of a Typical Pyrolysis Run 15 Analysis of Products 16 Chromatographic Apparatus 16 (i) Thermal Conductivity Chromatograph 16 ( i i ) Hydrogen Flame Ionisation Chromatograph 16 V Page Chromatographic Columns 18 Sampling Loops 19 Infrared Analysis 20 Mass Spectrometric Analysis 20 Flash Photolysis Apparatus 20 CHAPTER III RESULTS 23 General: 23 Qualitative Identification of Products 23 Quantitative Analysis 28 Pyrolysis Results: 29 Disappearance of Azoethane and Formation of Nitrogen 29 Product Formation 39 The Overall Orders of Formation for Major Products . . . 39 Pressure Change Measurements 43 Activation Energies of Formation of Various Products 4 3 Surface Effects • 51 Effect of Added Gases on the Initial Rates of Formation 57 Carbon, Hydrogen and Nitrogen Material Balances 57 Flash Photolysis Results: 62 CHAPTER IV DISCUSSION 67 Pyrolysis: 67 Primary Step in the Thermal Decomposition of Azoethane 67 Hydrogen Abstraction by Ethyl Radicals 68 Addition of Ethyl Radicals to Azoethane 69 v i Page Reactions of Resonance S t a b i l i s e d Radical CH3CHN=NC2H5 71 Decomposition and Combination of CH3CHN=NC2H5 Radical .... 72 Addition of Ethyl Radicals to CH3CHN=NC2H5 Radical 73 Mechanism of the Thermal Decomposition of Azoethane 74 Flash Photolysis 94 CHAPTER V CONCLUSIONS AND SUGGESTIONS FOR FURTHER WORK ... 100 Conclusions 100 Suggestions f o r Further Work 101 BIBLIOGRAPHY 102 v i i LIST OF FIGURES Page 1. Mass s p e c t r u m o f pur e azoethane 12 2. Vacuum System ... 14 3. R e a c t i o n v e s s e l and a n a l y s i s u n i t 17 4. F l a s h p h o t o l y s i s a p p a r a t u s 21 5. Chromatogram showing h y d r o c a r b o n p r o d u c t s a f t e r 2 minutes r e a c t i o n time a t 295°C 24 6. Gas chromatogram o f p r o d u c t s o f p y r o l y s i s at h i g h e x t e n t o f r e a c t i o n a t 271.5°C 25 7. A p o r t i o n o f h i g h r e s o l u t i o n mass s p e c t r u m a t h i g h e x t e n t o f r e a c t i o n 27 8. Gas chromatogram o f t h e removal o f azoethane 31 9. D i s a p p e a r a n c e o f azoethane and f o r m a t i o n o f n i t r o g e n as a f u n c t i o n o f t i m e a t 271.5°C 35 10. F i r s t o r d e r p l o t f o r azoethane r e m o v a l a t 259°C. 36 11. O r d e r p l o t f o r t h e r e m o v a l 6 f azoethane a t 271.5°C 38 12. P r o d u c t f o r m a t i o n as a f u n c t i o n o f t i m e a t 261°C.: N i t r o g e n and ethane 40 13. P r o d u c t f o r m a t i o n as a f u n c t i o n o f t i m e a t 261°C.: E t h y l e n e , methane and n-butane 41 14. P r o d u c t f o r m a t i o n as a f u n c t i o n o f t i m e a t 2 6 1 ° C : P r o p y l e n e . 42 15. O r d e r p l o t f o r t h e f o r m a t i o n o f n i t r o g e n and ethane a t 286°C. 46 16. O r d e r p l o t f o r propane and p r o p y l e n e a t 286°C 47 17. O r d e r o f f o r m a t i o n p l o t f o r methane and e t h y l e n e a t 286°C. . 48 v i i i Page 18. O r d e r o f f o r m a t i o n p l o t f o r n-butane a t 286°C. 49 19. O r d e r p l o t f o r the o v e r a l l p r e s s u r e change 50 20. A r r h e n i u s p l o t s f o r e t h a n e , n i t r o g e n and n-butane 54 21. A r r h e n i u s p l o t s f o r e t h y l e n e and methane 55 22. P r e s s u r e change r e c o r d e d by t r a n s d u c e r b o t h i n p a c k e d and unpacked v e s s l e s 56 23. P r o d u c t f o r m a t i o n as a f u n c t i o n o f number o f f l a s h e s .... 65 24. I n i t i a l r a t e s o f f o r m a t i o n o f e t h a n e , n i t r o g e n and n-butane as a f u n c t i o n o f azoethane p r e s s u r e a t 295°C . . 77 25. I n i t i a l r a t e o f f o r m a t i o n o f e t h y l e n e and methane as a f u n c t i o n o f i n i t i a l a z oethane p r e s s u r e a t 295°C. 78 26. I n i t i a l r a t e s as a f u n c t i o n o f azoet h a n e p r e s s u r e at 295°C. propane and p r o p y l e n e 79 27. V a r i a t i o n o f r a t i o CaHtt/d+Hjo as a f u n c t i o n o f azoet h a n e p r e s s u r e 83 28. V a r i a t i o n o f r a t i o C^H^/CH^ as a f u n c t i o n o f azoet h a n e p r e s s u r e 84 29. P l o t o f r a t i o o f C2H4/N2 as a f u n c t i o n o f azoet h a n e p r e s s u r e .' 85 30. P l o t o f C3Hs/C3H 6 r a t i o as a f u n c t i o n o f azoet h a n e p r e s s u r e 86 31. V a r i a t i o n o f r a t i o C?H&/C2H^ as a f u n c t i o n o f azoet h a n e p r e s s u r e 87 32. V a r i a t i o n o f C2H6/N2 r a t i o as a f u n c t i o n o f added c i s - C ^ H g - 2 90 33. P r o d u c t y i e l d as a f u n c t i o n o f added c a r b o n d i o x i d e 98 i x LIST OF TABLES Page 1. A c t i v a t i o n e n e r g i e s and f r e q u e n c y f a c t o r s a t d i f f e r e n t 12 p r e s s u r e s c a l c u l a t e d by C l a r k e t a l , 6 2. P r o d u c t s from p y r o l y s i s o f 57.2 mm. azoethane a t 286°C .. 30 3. P r o d u c t d i s t r i b u t i o n from 54.8 mm. o f azoe t h a n e a t 271.5°C. 30 4. Removal o f azoethane and f o r m a t i o n o f n i t r o g e n as a f u n c t i o n o f t i m e a t 271.5°C. ,. . 32 5. Removal o f azoethane and f o r m a t i o n o f n i t r o g e n as a f u n c t i o n o f t i m e a t 259°C 33 6. D i s a p p e a r a n c e o f azo e t h a n e and appearance o f n i t r o g e n as a f u n c t i o n o f t i m e a t 246.5°C 34 7. I n i t i a l r a t e s f o r the re m o v a l o f azo e t h a n e a t 271.5°C. . 37 8. I n i t i a l r a t e s o f f o r m a t i o n o f v a r i o u s p r o d u c t s a t 295° and 286°C 44 9. I n i t i a l r a t e s o f f o r m a t i o n o f v a r i o u s p r o d u c t s i n t h e p a c k e d and unpacked v e s s e l a t 271.5°C 45 10. O r d e r s o f f o r m a t i o n o f v a r i o u s p r o d u c t s w . r . t . azoethane 52 11. A c t i v a t i o n e n e r g i e s f o r t h e f o r m a t i o n o f ma j o r p r o d u c t s w . r . t . azoethane 52 12. P r o d u c t f o r m a t i o n as a f u n c t i o n o f t i m e a t 246.5°C 53 13. P r o d u c t f o r m a t i o n w i t h added butene-1 53 14. I n i t i a l r a t e s o f f o r m a t i o n o f v a r i o u s p r o d u c t s i n t h e p r e s e n c e o f added gases 58 15. C a r b o n , n i t r o g e n and h y d r o g e n mass b a l a n c e s 59 16. I n i t i a l r a t e s o f f o r m a t i o n w i t h v a r i o u s added gases .... 60 X Page 17. F l a s h p h o t o l y s i s o f azoethane r e s u l t s a t room t e m p e r a t u r e 63 18. P r o d u c t y i e l d s i n t h e f l a s h p h o t o l y s i s o f azoethane i n t h e p y r e x r e a c t i o n v e s s e l w i t h o u t e r p y r e x j a c k e t a t room t e m p e r a t u r e 64 19. P r e v i o u s and p r e s e n t e s t i m a t e s f o r d i s p r o p o r t i o n a t i o n t o c o m b i n a t i o n r a t i o s o f e t h y l r a d i c a l s from v a r i o u s compounds 96 xi ACKNOWLEDGEMENT The work reported in this thesis has been carried out under the Supervision of Professor Gerald B. Porter, to whom the author is deeply indebted for his guidance and encouragement at all times. I am also thankful to the late Professor W. A. Bryce for supervising my research work from August 1962 to May 1964. I wish to express my thanks to all those, especially to Mr. John S. Mcintosh,who have helped me during this investigation in one way or another. Financial support of this research project from the Petroleum Research Fund, administered by the American Chemical Society, and a British Columbia Sugar Refinery Company scholarship to the author (1965-1966) is gratefully acknowledged. x i i To my friends GKS CHAPTER I INTRODUCTION Any e x p e r i m e n t a l i n v e s t i g a t i o n i n c h e m i c a l k i n e t i c s g e n e r a l l y demands t h e d e s i g n and e x e c u t i o n o f e x p e r i m e n t s t h a t w i l l p r o v i d e a p p r o p r i a t e d a t a f o r t h e d e t e r m i n a t i o n o f t h e r a t e s and mechanism o f t h e r e a c t i o n c o n c e r n e d . I n c h e m i c a l r e a c t i o n s p r o c e e d i n g by r a d i c a l c h a i n s , a s p e c i a l t a s k a r i s e s i n t h e d e t e r m i n a t i o n o f t h e e l e m e n t a r y r e a c t i o n s i n v o l v e d i n t h e mechanism. Many r e a c t i o n s , once b e l i e v e d t o p r o c e e d by a m o l e c u l a r mechanism, have been shown t o have c h a i n c h a r a c t e r . S u f f i c i e n t and a c c u r a t e d a t a were n o t a v a i l a b l e i n t h e p a s t t o d e c i d e about t h e n a t u r e o f the r e a c t i o n . E x p e r i m e n t a l d a t a on e l e m e n t a r y r e a c t i o n s a re o f g r e a t i m p o r t a n c e because o f t h e i r g e n e r a l i t y . Thus, t h e r a t e o f r e c o m b i n a t i o n o f e t h y l r a d i c a l s d e t e r m i n e d i n one s y s t e m can be a p p l i e d t o many systems i n wh i c h e t h y l r a d i c a l s a r e p r o d u c e d . However, such a " c a r r y o v e r " o f t h e d a t a f r o m one s y s t e m t o a n o t h e r i s d e f i n i t e l y n o t p e r m i s s i b l e i n a l l c a s e s . E n e r g e t i c a l l y e x c i t e d " h o t " r a d i c a l s may be p r o d u c e d i n some systems (as i n f l a s h p h o t o l y s i s ) and t h e s e e x c i t e d s p e c i e s do have a d i f f e r e n t b e h a v i o u r as compared t o n o r m a l ones. I f t h e e l e m e n t a r y r e a c t i o n s t u d i e d i s t h e s p l i t t i n g o f a m o l e c u l e o r r a d i c a l , t h e a c t i v a t i o n energy o f t h e r e a c t i o n can be i d e n t i f i e d w i t h t h e d i s s o c i a t i o n e nergy o f t h e bond b r o k e n , p r o v i d e d t h a t t h e a c t i v a t i o n energy o f t h e r e v e r s e r e a c t i o n i s z e r o . The a c t i v a t i o n e n e r g y f o r f r e e 1 2 r a d i c a l r e c o m b i n a t i o n r e a c t i o n s i s u s u a l l y s m a l l o r z e r o . ' In c h a i n r e a c t i o n , t h r e e s t a g e s can be d i s t i n g u i s h e d : i n i t i a t i o n , p r o p a g a t i o n and t e r m i n a t i o n . I n t h e t h e r m a l and p h o t o c h e m i c a l d e c o m p o s i t i o n o f a l i p h a t i c a z o a l k a n e s , i n i t i a t i o n w i l l t a k e p l a c e p r o b a b l y t h r o u g h t h e 2 c a r b o n - n i t r o g e n bond s p l i t because D(C-N) i s s i g n i f i c a n t l y l e s s t h a n D(C-C), D(C-H) and D(N=N). The i n i t i a t i o n i s p r o b a b l y u n i m o l e c u l a r . I t was once b e l i e v e d t h a t azomethane r e p r e s e n t e d a good example o f a 3 4 u n i m o l e c u l a r r e a c t i o n . ' E a r l i e r s t u d i e s were c a r r i e d out by t h e manometnc method whose l i m i t a t i o n s a r e w e l l known now f o r complex r e a c t i o n s . When t h e p r o d u c t s o f p y r o l y s i s o f azomethane were a n a l y s e d and f o u n d complex, doubts a r o s e about t h e u n i m o l e c u l a r i t y o f t h i s r e a c t i o n . Recent s t u d i e s 5 ' 6 have i n d i c a t e d t h e l i k e l i h o o d o f a r e a c t i o n c h a i n i n t h e azomethane p y r o l y s i s . E x c e l l e n t a c c o u n t s o f v a r i o u s t h e o r i e s o f c h e m i c a l k i n e t i c s can be 1 3 24 fo u n d i n s t a n d a r d t e x t b o o k s . ' ' 3 EARLIER STUDIES ON THE AZOETHANE DECOMPOSITION • 7 W e i n i n g e r and R i c e have i n v e s t i g a t e d t h e p h o t o l y s i s o f azoethane t o o b t a i n i n f o r m a t i o n on t h e d e a c t i v a t i o n o f e x c i t e d s p e c i e s . These a u t h o r s were o n l y i n t e r e s t e d i n t h e p r i m a r y p r o c e s s and t h u s measured t h e r a t e o f 8 n i t r o g e n f o r m a t i o n as a f u n c t i o n o f azoethane p r e s s u r e . A u s l o o s and S t e a c i e g and C e r f o n t a i n and K u t s d i k e have a l s o i n v e s t i g a t e d t h i s p h o t o l y s i s . They f o u n d t h a t t he r a t e o f p r o d u c t i o n o f n i t r o g e n , e t h y l e n e , e t h a n e , and n-butane c o u l d be e x p l a i n e d by t h e f o l l o w i n g sequence o f r e a c t i o n s : C2H5N=NC2H5 + hv C2H5N=NC 2H 5* + M C 2H 5N=NC 2H 5 N 2 C 2 H 5 2C 2 H 5 2C 2 H 5 C 2 H 5 + C 2 H 5 =NC 2 H 5 'C2Hi,N=NC2H5 C 2 H 5 + C2H1+N=NC2H5 2 C 2H l tN=NC 2H 5 2C 2 H 5 + C 2H 5N=NC 2H 5 C 2H 5N=NC 2H 5* ( I -1) C 2H 5N=NC 2H 5 (I- -2) C 2 H 5 + N 2 C 2 H 5 (I- -3) C 2 H 5 + N 2 (I -4) C 2 H g + C 2 H ^ ( I --5) Ci+Hio ( I --6) C 2 H 6 + C2H l 4N=NC2H5 (I- -7) C2Hit + N 2 + C 2 H 5 ( I -8) C1+H9N=NC2H5 (I -9) (CzH^'NCzBs) 2 (I -10) ( C 2 H 5 ) 2 N - N ( C 2 H 5 ) 2 ( I -11) Assuming e t h y l e n e , e t h a n e , and n-butane formed o n l y v i a r e a c t i o n s (5), (6) and 9 (7), t h e f o l l o w i n g r e l a t i o n s were d e r i v e d : C 2Hi| ^Ct^ Hio A denotes e x c i t e d s p e c i e s A. The e x c i t a t i o n may be v i b r o n i c , e l e c t r o n i c , o r b'oth. 4 R C 2 H 6 -RC2Hn k ? "CtfiO [AE] R + R 2k5 k7[AE] L 2" 6 «-• 2" 4 R = ~T*~ + kcK ^ 2 where R denotes the rate of formation of X and k's are the specific rate constants. [AE] represents azoethane concentration in appropriate concentration units. By plott ing logio (k7/k6V2) versus I/T, the value of E 7 - V2 E6 was found to be 7.5 * 0.1 Kcal per mole. The material balance B, defined as R- „ + R„ „ is less than unity, indicating addition — % ~ reactions. Ethyl radicals can also disappear by addition to azoethane giving tetraethyl hydrazine, according to reaction (1-11). The activation energy for this addition reaction was calculated to be 6.0 * 0.3 kcal per mole. The material balance B decreased as the temperature of the photolysis was increased. An increase in the (^He/C^H^ ratio was also observed at higher temperatures. Roquitte and Fu t re l l 1 ^ have investigated the flash photolysis of azoethane to obtain information concerning the .effect of indicent intensity and of azoethane pressure on the rate of i t s decomposition. The following reactions were postulated to account for the products formed. C2H5N=NC2H5 + hv, —*• 2 C 2 H 5 + N 2 2C 2H 5 n - C ^ o 2C2H5 — ' C2H4. + C2Hg The average value of the ratio ethylene to ethane was found to be 5 1.09 ± 0.02, s l i g h t l y h i g h e r t h a n t h e e x p e c t e d u n i t y . As a s m a l l amount o f h y d r o g e n was a l s o f o u n d i n t h e r e a c t i o n m i x t u r e , i t was assumed t h a t e x c e s s e t h y l e n e was formed by the d e c o m p o s i t i o n o f e t h y l r a d i c a l s as C 2 H 5 * • C2tti> + H The H atoms w o u l d then a b s t r a c t h y d r o g e n from a z o e t h a n e t o g i v e m o l e c u l a r h y d r o g e n . A s m a l l q u a n t i t y o f n i t r i c o x i d e was added t o azoethane and t h e m i x t u r e was s u b j e c t e d t o f l a s h p h o t o l y s i s . I t was o b s e r v e d t h a t NO c o m p l e t e l y p r e v e n t e d the f o r m a t i o n o f h y d r o c a r b o n s . However, no r u n i s r e p o r t e d w i t h added i n e r t gases. D i n g l e d y and C a l v e r t * * have a l s o s t u d i e d t h e f l a s h p h o t o l y s i s o f a z o e t h a n e w h i l e i n v e s t i g a t i n g e t h y l - o x y g e n r e a c t i o n s and f o u n d n i t r o g e n , e t h y l e n e , e t hane and n-butane as t h e p r o d u c t s o f d e c o m p o s i t i o n . T h e i r r e s u l t s i n t h e p y r e x f i l t e r e d - s y s t e m a r e q u i t e c o n s i s t e n t w i t h t h e p r e v i o u s 10 i n v e s t i g a t i o n . The o n l y s t u d y r e p o r t e d so f a r c o n c e r n i n g t h e t h e r m a l d e c o m p o s i t i o n 12 o f a z o e t h a n e i s o f C l a r k e and S w i n e h a r t . A mass s p e c t r o m e t e r was u s e d as an a n a l y t i c a l t o o l f o r f o l l o w i n g t h e change i n c o n c e n t r a t i o n w i t h r e s p e c t t o t i m e . T h i s i n v e s t i g a t i o n c o v e r e d t h e t e m p e r a t u r e and p r e s s u r e r a n g e s f r o m 250 t o 310°C and 1 0 5 t o 10 m i c r o n s , r e s p e c t i v e l y . They m a i n l y f o l l o w e d t h e p a r e n t peak o f a z o e t h a n e , mass t o charge r a t i o 86, as a f u n c t i o n o f t i m e . They f o l l o w e d t h i s peak f o r q u i t e l o n g p e r i o d s so t h a t compounds formed i n t h e r e a c t i o n might have s t a r t e d c o n t r i b u t i n g t o t h i s p a r e n t peak. O n l y f o u r runs were made i n o r d e r t o a n a l y s e t h e p r o d u c t s o f d e c o m p o s i t i o n . Toluene was u s e d t o e s t a b l i s h w h e t h e r o r n o t c h a i n s were i n v o l v e d i n t h e p y r o l y s i s . The i n c r e a s e o f s u r f a c e t o volume r a t i o d i d n o t a f f e c t t h e r a t e o f t h e r e a c t i o n and i t was C o n c l u d e d t h a t t h e d e c o m p s o i t i o n o f azoethane i s 6 u n i m o l e c u l a r , homogeneous, and f i r s t o r d e r o v e r t h e t e m p e r a t u r e and p r e s s u r e range s t u d i e d . H i g h p r e s s u r e r a t e c o n s t a n t s f o r s i x d i f f e r e n t t e m p e r a t u r e s were c a l c u l a t e d by e x t r a p o l a t i n g 1/k v e r s u s 1/P c u r v e s t o i n f i n i t e p r e s s u r e ( i . e . , 1/P = 0 ) . The v a l u e s r e p o r t e d f o r t h e a c t i v a t i o n energy and f r e q u e n c y f a c t o r s , c a l c u l a t e d from e x p e r i m e n t a l d a t a , as a f u n c t i o n o f p r e s s u r e are g i v e n i n T a b l e I . 12 TABLE I PRESSURE ACTIVATION ENERGY FREQUENCY_FACTOR ( M i c r o n s ) ( k c a l / m o l e ) ( s e c . ) I n f i n i t e 48.5 5.87 X 1 0 1 5 1.0 x 1 0 5 48.4 5.07 X 1 0 1 5 3.16 x 1 0 4 48.2 4.00 X 1 0 1 5 1.0 x 10k 48.0 2.92 X 1 0 1 5 3.16 x 1 0 3 47.7 1.97 X 1 0 1 5 1.0 x 1 0 3 47.4 1.22 X 1 0 1 5 3.16 x 1 0 2 46.9 6.03 X 1 0 l l t 1.0 x 1 0 2 47.1 4.76 X 1 0 1 4 3.16 x 10 48.2 1.43 X 1 0 1 5 1.0 x 10 49.4 1.43 X 1 0 1 5 3.16 51.2 4.09 X 1 0 1 5 I n t h e p r o d u c t s , no compound c o n t a i n i n g a s i n g l e n i t r o g e n atom was f o u n d even at h i g h e r e x t e n t o f t h e r e a c t i o n . The a u t h o r s were n o t , however, i n a position t o s a y much about t h e mechanism o f t h i s r e a c t i o n , w h i c h makes t h e i r c o n c l u s i o n s o f dub i o u s v a l u e . 7 A SIMILAR SYSTEM Hg(C ?HcQ ? Studies in the thermal decomppsition of mercury diethyl have been made both in the stat ic and flow systems.*^'*^'*^ The only hydrocarbons produced in any appreciable quantities were ethane, ethylene, and n-butane. The production of these hydrocarbons can be interpreted by the following sequence of reactions: (C 2H 5) 2Hg C2H5Hg 2C 2H 5 2C 2H 5 C2H5+ (C 2H 5) 2Hg C2H5+ C^H^HgC^Hs C 2 H 5 + HgC 2 H 5 C 2 H 5 + Hg C2Hij + C 2Hg C 2Hg + CjiH^HgC^Hs Ci tHgHgC 2H5 C 2 H 4 + Hg + C 2 H 5 The ratios of ethane to ethylene and C 2 to hydrocarbons were approximately unity and four, respectively. The photolysis and flash photolysis of diethyl mercury has been reported. 1 ^ '1^» 19,20 j ^ g ^ ^ s a g r o s s difference in the product d i s t r i -# 17 bution of low and high intensity photolysis. Ivin and Steacie found ethylene, ethane, n-butane and butene as the only products of low intensity photo-decomposition. Although the complete mechanism of the low intensity photolysis has not been established, the major products formation has been explained by the reactions (C 2H 5) 2Hg + hv 2C 2H 5 Hg + 2C 2H 5 Cit.Hio ' High intensity photolysis refers to flash photolysis. 8 2,C 2H 5 C 2 H 4 + C 2 H 6 These a r e t h e same r e a c t i o n s w h i c h were r e q u i r e d t o account f o r the p r o d u c t s o f p y r o l y s i s . The o b s e r v e d i n c r e a s e i n t h e r a t e o f p h o t o l y t i c d e c o m p o s i t i o n at h i g h e r p r e s s u r e s has been a s c r i b e d t o t h e f o l l o w i n g a d d i t i o n a l r e a c t i o n s : C 2 H 5 + ( C 2 H 5 ) 2 H g • C r tH 1 0-+ C 2H 5-+ Hg ^ C 2Hn + C 2 H 6 + C 2 H 5 + Hg 19 T h r u s h f o u n d methane, e t h y l e n e , e t h a n e , p r o p a n e , p r o p y l e n e , and n-butane as t h e f i n a l p r o d u c t s o f the f l a s h p h o t o l y s i s o f mercury d i e t h y l i n the f a r u l t r a v i o l e t u s i n g a q u a r t z r e a c t i o n v e s s e l . As t h e r a t i o o f ethane t o e t h y l e n e was l e s s t h a n u n i t y t h e f o l l o w i n g r e a c t i o n s were c o n s i d e r e d as a d d i t i o n a l s o u r c e s o f e t h y l e n e . C 2H5 C 2Hif + H C 2H5 + C 2 H 5 • C 2H i f + 2CH 3 C 2 H 5 + CH 3 • C 2H^ + CHit (a) R e a c t i o n (a) s h o u l d be c o n s i d e r e d a m i n o r s o u r c e o f e t h y l e n e i n any sy s t e m o f m e t h y l and e t h y l r a d i c a l s . The maj o r r e a c t i o n a r i s i n g f rom t h e i r i n t e r -a c t i o n i s : C 2 H 5 + C H 3 • C 3 H 8 Hydrogen can a r i s e by t h e i n t e r a c t i o n o f two hydr o g e n atoms as A + A + M — • • H 2 + M o r , more l i k e l y : ft + ( C 2 H 5 ) 2 H g — H 2 + CzH^HgCzHs 9 20 F i s c h e r and Mains have a l s o f o u n d t h e same p r o d u c t s , e x c e p t hydrogen r e p o r t e d by T h r u s h i n t h e h i g h i n t e n s i t y p h o t o l y s i s i n a q u a r t z v e s s e l . A d d i t i o n o f a few cm. o f p e r f l u o r o c a r b o n t o t h e s y s t e m had t h e e f f e c t o f l o w e r i n g b o t h t h e ethane and e t h y l e n e y i e l d s and r a i s i n g t h e n-butane y i e l d s . The e f f e c t o f i n e r t gas p r e s s u r e was q u i t e d i f f e r e n t from t h a t o b s e r v e d a t low i n t e n s i t y . The e f f e c t o f p a c k i n g t h e r e a c t i o n v e s s e l w i t h q u a r t z tubes on t h e p r o d u c t d i s t r i b u t i o n was a l s o i n v e s t i g a t e d . These a u t h o r s have a l s o r e p o r t e d the f l a s h p h o t o l y s i s o f mercury d i e t h y l and mercury d i e t h y l - d i Q m i x t u r e s . THE OBJECT OF THE PRESENT INVESTIGATION I t i s c l e a r f r o m the e x i s t i n g l i t e r a t u r e on t h e t h e r m a l d e c o m p o s i t i o n o f a zoethane t h a t t h e i n f o r m a t i o n a v a i l a b l e i s meager, u n r e l i a b l e and o f l i m i t e d v a l u e m e c h a n i s t i c a l l y , s i n c e t h e e a r l y s t a g e s o f t h e r e a c t i o n have n o t been i n v e s t i g a t e d . I n t h e p a s t , most o f t h e k i n e t i c s t u d i e s had been c a r r i e d out by o b s e r v a t i o n o f t o t a l p r e s s u r e change and i t s l i m i t a t i o n s are w e l l known now f o r complex r e a c t i o n s . The r a p i d development o f gas chromato-graphy has made i t p o s s i b l e t o s t u d y t h e k i n e t i c s o f d e c o m p o s i t i o n s more t h o r o u g h l y and a c c u r a t e l y . 21 Azoethane has been used as a t h e r m a l s o u r c e o f r a d i c a l s t o i n i t i a t e p o l y m e r i s a t i o n , hence a c c u r a t e k i n e t i c d a t a f o r i t s p y r o l y s i s a r e n e c e s s a r y . I t i s a l s o o f i n t e r e s t t o examine whether t h i s d e c o m p o s i t i o n i s u n i m o l e c u l a r o r n o t . I t was d e c i d e d t o b u i l d an a n a l y t i c a l s y s t e m w h i c h c o u l d be used t o f o l l o w b o t h d i s a p p e a r a n c e o f r e a c t a n t and appearance o f p r o d u c t m o l e c u l e s as a f u n c t i o n o f t i m e , t e m p e r a t u r e and p r e s s u r e . T h i s p a r t o f t h e i n v e s t i g a t i o n c o n s i s t s e n t i r e l y o f a n a l y t i c a l r e s u l t s o f the t h e r m a l d e c o m p o s i t i o n o f a z o e t h a n e . 10 A s u r v e y o f t h e s t u d i e s o f h i g h i n t e n s i t y p h o t o l y s i s o f d i e t h y l mercury have shown t h e complex n a t u r e o f the p r o d u c t s o f t h i s r e a c t i o n . There a r e g r o s s d i f f e r e n c e s i n t h e p r o d u c t d i s t r i b u t i o n i n h i g h and low i n t e n s i t y p h o t o l y s i s whereas p r o d u c t s o f p y r o l y s i s o f t h i s m o l e c u l e a r e t h e same as t h o s e o f low i n t e n s i t y . I n t h e case o f a z o e t h a n e , p r o d u c t s o f h i g h and low i n t e n s i t y p h o t o l y s i s a re c o n s i s t e n t w i t h each o t h e r b u t t h e p y r o l y s i s p r o d u c t s a r e d i f f e r e n t . The f l a s h p h o t o l y s i s s t u d y r e p o r t e d h e r e was u n d e r t a k e n t o g a i n a d d i t i o n a l i n s i g h t i n t o i t s mechanism as compared t o p y r o l y s i s and t o a s c e r t a i n t h e p r o d u c t d i s t r i b u t i o n s u s i n g v a r y i n g p r e s s u r e s o f i n e r t gas a t d i f f e r e n t f l a s h i n t e n s i t i e s b o t h i n p y r e x and q u a r t z r e a c t i o n v e s s e l s . 11 CHAPTER I I EXPERIMENTAL M a t e r i a l s : ( i ) A z o ethane: Azoethane was o b t a i n e d from Merck, Sharp and Dohme, M o n t r e a l , Canada, i n 5 gm. l o t s . I t was t h o r o u g h l y o u t g a s s e d and d i s t i l l e d a t low t e m p e r a t u r e i n t o a b l a c k e n e d s t o r a g e b u l b , w h i c h was always k e p t a t l i q u i d n i t r o g e n t e m p e r a t u r e (-196°C). S m a l l amounts, s u f f i c i e n t t o do runs f o r a week, were t r a n s f e r r e d t o a n o t h e r b l a c k e n e d b u l b ( k e p t a t d r y i c e t e m p e r a t u r e ) a d j a c e n t t o t h e main s t o r a g e . D u r i n g m a n i p u l a t i o n s , azoethane was s h i e l d e d from d a y l i g h t b y p a i n t i n g a l l c o n n e c t i n g t u b e s b l a c k and by w o r k i n g i n subdued l i g h t o r i n t h e e v e n i n g s . The gas c h r o m a t o g r a p h i c and mass s p e c t r o m e t r i c a n a l y s i s o f t h e f r a c t i o n a t e d sample showed t h e e s s e n t i a l absence o f i m p u r i t i e s . A mass s p e c t r u m o f pur e azoethane i s g i v e n i n F i g u r e 1. T a k i n g a l l t h e s e p r e c a u t i o n s , d i f f e r e n t b a t c h e s o f azoethane gave i d e n t i c a l r a t e s o f d e c o m p o s i t i o n w i t h i n e x p e r i m e n t a l e r r o r . ( i i ) H y d r o c a r b o n s : A l l t h e h y d r o c a r b o n s used i n gas c h r o m a t o g r a p h i c , mass s p e c t r o m e t r i c and i n f r a r e d a n a l y s i s were r e s e a r c h grade o b t a i n e d from P h i l l i p s P e t r o l e u m Company, B a r t l e s v i l l e , Oklahoma, (U.S.A.) T h e i r p u r i t y as s u p p l i e d v a r i e d f r o m 99.5 t o 99.9%. F u r t h e r p u r i f i c a t i o n o f some o f t h e s e h y d r o c a r b o n s was a c h i e v e d by d o i n g t r a p - t o - t r a p d i s t i l l a t i o n at t e m p e r a t u r e s o f -196°C. ( l i q u i d n i t r o g e n t e m p e r a t u r e ) and -78°C. ( s o l i d c a r b o n d i o x i d e and methanol s l u s h b a t h ) . The h y d r o c a r b o n s were s t o r e d i n p y r e x g l o b e s p r o v i d e d w i t h g r e a s e l e s s m e t a l l i c v a l v e s . ( i i i ) O t h e r Compounds: N i t r o g e n , h y d r o g e n , and car b o n d i o x i d e were t a k e n d i r e c t l y f r o m c y l i n d e r s s u p p l i e d by C a n a d i a n L i q u i d A i r Company and were us e d w i t h o u t f u r t h e r p u r i f i c a t i o n . H e l i u m , w h i c h was a l s o s u p p l i e d by x 10 10 x -§> • H <u X <a a. a> > • H +J nt i— i a> cc to x 10 x To" 30 15" 5IC^  6 0 Mass Number 70 Figure 1. Mass Spectrum of Pure Azoethane 13 L i q u i d A i r Company was used as a c a r r i e r gas i n chromatography. I t was p u r i f i e d by p a s s i n g t h r o u g h two t r a p s , one c o n t a i n i n g c h a r c o a l and t h e o t h e r c o n t a i n i n g anhydrous c a l c i u m c h l o r i d e . A l l o t h e r c h e m i c a l s used were o f r e s e a r c h grade. THE PYROLYSIS SYSTEM A c o n v e n t i o n a l s t a t i c vacuum s y s t e m was employed, as shown i n F i g u r e s 2 and 3. A l l s t o p c o c k s were l u b r i c a t e d w i t h a h i g h vacuum Dow C o r n i n g S i l i c o n e g r e a s e . A c y l i n d r i c a l p y r e x r e a c t i o n v e s s e l o f 694 ml. volume was p l a c e d i n s i d e a b r a s s c y l i n d e r wound w i t h h e a t i n g w i r e . The o u t s i d e o f the b r a s s b l o c k was c o v e r e d w i t h a s b e s t o s s h e e t , v e r m i c u l i t e and g l a s s w o o l t o p r o v i d e t h e r m a l i n s u l a t i o n . The r e a c t i o n v e s s e l was c o n n e c t e d t o t h e vacuum s y s t e m and t h e a n a l y s i s u n i t t h r o u g h t h e t h r e e way s t o p c o c k F. F o r f o l l o w i n g p r e s s u r e changes, i t c o u l d be c o n n e c t e d t o a p r e s s u r e t r a n s d u c e r by o p e n i n g s t o p c o c k G. A v a r i a b l e v o l t a g e was s u p p l i e d t o t h e h e a t i n g element o f t h e f u r n a c e by means o f a c o n t r o l l e d power s u p p l y . Three c a l i b r a t e d Chrome1-Alumel t h e r m o c o u p l e s , p l a c e d a t t h r e e d i f f e r e n t p o i n t s i n t h e b r a s s b l o c k (shown i n F i g u r e 2 ) , were u s e d t o measure t h e t e m p e r a t u r e o f t h e f u r n a c e . A H o n e y w e l l E l e c t r o n i c M i l l i v o l t C o n t r o l l e r Model No.Y156C18-V(S)H-61 c o n t r o l l e d t h e t e m p e r a t u r e . The d i f f e r e n c e i n t e m p e r a t u r e o v e r t h e l e n g t h o f t h e b l o c k was about ±0.2°C. The p y r e x r e a c t i o n v e s s e l was p a c k e d w i t h 170 p y r e x t y b e s , (18 cm. l o n g and 0.3 cm. i n d i a m e t e r ) w i t h f i r e p o l i s h e d ends, t o s t u d y t h e e f f e c t o f p a c k i n g on t h e p r o d u c t d i s t r i b u t i o n . The method o f a n a l y s i s was t h e same f o r b o t h p a c k e d and unpacked r e a c t i o n v e s s e l s . In a few r u n s , two 170.0 ml. q u a r t z c e l l s , one p a c k e d w i t h f i r e p o l i s h e d q u a r t z t u b e s and t h e o t h e r unpacked, were p l a c e d i n s i d e a d o u b l e Figure 2. Vacuum System. 15 f u r n a c e , whose t e m p e r a t u r e was c o n t r o l l e d u s i n g t h e t e c h n i q u e d e s c r i b e d above. The s u r f a c e r a t i o o f p a c k e d t o unpacked v e s s e l s was a p p r o x i m a t e l y e l e v e n . Both c e l l s were c o n n e c t e d t o t h e vacuum s y s t e m t h r o u g h q u a r t z c a p i l l a r i e s . P r o v i s i o n s were made f o r i n t r o d u c i n g the same p r e s s u r e o f azoethane t o b o t h v e s s e l s . The i n c r e a s e i n p r e s s u r e due t o d e c o m p o s i t i o n f o r each v e s s e l u n d e r i d e n t i c a l c o n d i t i o n s was r e c o r d e d w i t h a p r e s s u r e t r a n s d u c e r , whose o u t p u t was d i s p l a y e d on a 1.0 mV r e c o r d e r (Speedomax t y p e ) . I t must be n o t e d , however, t h a t no p r o d u c t a n a l y s e s were made i n t h i s s e r i e s o f e x p e r i m e n t s . The p y r e x and q u a r t z r e a c t i o n v e s s e l s were s e a s o n e d e i t h e r by p y r o l y s i n g i s o p e n t a n e o r by c a r r y i n g out s e v e r a l p r e l i m i n a r y d e c o m p o s i t i o n s o f a z o e t h a n e . T h i s p r o c e d u r e was r e p e a t e d whenever t h e v e s s e l s were exposed t o a i r . A f t e r d o i n g 25 t o 30 k i n e t i c runs a y e l l o w d e p o s i t was u s u a l l y f o u n d on t h e tube c o n n e c t i n g t h e r e a c t i o n v e s s e l t o t h e s t o p c o c k F. The volume o f t h i s c o n n e c t i n g tube o u t s i d e t h e f u r n a c e was a p p r o x i m a t e l y 1 ml. I t was h e a t e d e l e c t r i c a l l y t o a v o i d c o n d e n s a t i o n o f v a p o u r s . D e s c r i p t i o n o f a T y p i c a l P y r o l y s i s Run A f t e r pumping t h e vacuum s y s t e m , r e a c t i o n v e s s e l and t h e s a m p l i n g l o o p s t o a " b l a c k vacuum", t h e low t e m p e r a t u r e b a t h was removed from s t o r a g e b u l b AE2 and t h e r e a c t a n t v a p o u r was b r o u g h t t o room t e m p e r a t u r e by s u r -r o u n d i n g i t w i t h warm w a t e r . S t o p c o c k s D, E, and H were c l o s e d and s t o p c o c k C opened t o i n t r o d u c e azoethane i n t h e r e a c t i o n c e l l . S t o p c o c k F was i m m e d i a t e l y c l o s e d and t h e i n i t i a l p r e s s u r e was r e a d e i t h e r on a mercury manometer o r on t h e r e c o r d e r u s i n g a p r e s s u r e t r a n s d u c e r . The v a p o u r i n the vacuum l i n e was t h e n condensed back i n s t o r a g e b u l b AE2. A f t e r t h e d e s i r e d r e a c t i o n t i m e had e l a p s e d , s t o p c o c k F was opened t o t h e a n a l y s i s u n i t f o r about 10 seconds and t h e n i t was c l o s e d . The 16 h e l i u m f l o w was d i v e r t e d i n t h e s a m p l i n g l o o p s t o t a k e the r e a c t i o n m i x t u r e i n t h e c h r o m a t o g r a p h i c a p p a r a t u s . Then t h e s a m p l i n g l o o p s were e v a c u a t e d t o be r e a d y f o r t h e n e x t a n a l y s i s by c o n n e c t i n g them back t o t h e vacuum sy s t e m t h r o u g h s t o p c o c k F.: A n a l y s i s o f P r o d u c t s The i d e n t i t y o f each p r o d u c t was c o n f i r m e d by gas chromatography, mass s p e c t r o m e t r y and i n f r a r e d s p e c t r o s c o p y . Gas c h r o m a t o g r a p h i c i d e n t i -f i c a t i o n o f t h e p r o d u c t s f o l l o w e d a comprehensive c o m p a r i s o n o f t h e r e t e n -t i o n b e h a v i o u r w i t h t h o s e o f a wide range o f s t a n d a r d compounds u s i n g two d i f f e r e n t columns. C a l i b r a t i o n c u r v e s were c o n s t r u c t e d f o r v a r i o u s com-ponents o f t h e r e a c t i o n m i x t u r e and were ch e c k e d from t i m e t o t i m e u s i n g a s y n t h e t i c m i x t u r e . The a n a l y s e s o f h y d r o c a r b o n s were r e p r o d u c i b l e w i t h i n 5%. CHROMATOGRAPHIC APPARATUS ( i ) Thermal C o n d u c t i v i t y Chromatograph: The t h e r m a l c o n d u c t i v i t y c e l l 22 was d e s i g n e d by Ryce e t a l . The c e l l c u r r e n t and t e m p e r a t u r e o f t h e d e t e c t o r was always k e p t c o n s t a n t a t 120 mA and 5 5 ° C , r e s p e c t i v e l y , t h r o u g h -out t h e i n v e s t i g a t i o n . A.D.C. a m p l i f i e r (Leeds § N o r t h u p Type) was i n c o r -p o r a t e d t o i n c r e a s e t h e s e n s i t i v i t y o f t h e c e l l . The ouput from t h e a m p l i f i e r was d i s p l a y e d oh a lOmV Speedomax r e c o r d e r . The a r e a o f each chromatograph was measured u s i n g a p l a n i m e t e r ( A r i s t o t l e ) . The s e n s i t i v i t y o f t h e chromatograph was c h e c k e d t w i c e d u r i n g each week w i t h a s t a n d a r d sample o f n i t r o g e n and f o u n d always t o be w i t h i n ± 3 . 0 % e r r o r . ( i i ) Hydrogen Flame I o n i z a t i o n Chromatograph: A W i l k e n s A e r o g r a p h Model A 600 "HY-FI" w i t h a h y d r o g e n f l a m e i o n i s a t i o n d e t e c t o r was employed f o r (TC) Thermocouple (TC) P r e s s u r e t r a n s d u c e r R e c o r d e r E x t e r n a l Sample I n l e t H e l i u m To Hydrogen Flame Gas D e t e c t o r To Thermal C o n d u c t i v i t y D e t e c t o r F i g u r e 3. R e a c t i o n V e s s e l and A n a l y s i s U n i t . 18 c a r r y i n g out q u a n t i t a t i v e a n a l y s e s o f azoe t h a n e and h y d r o c a r b o n s . The p r e s s u r e o f hyd r o g e n was k e p t c o n s t a n t a t 10 p . s . i . by u s i n g a n e e d l e v a l v e . The f l a m e head was c l e a n e d from t i m e t o t i m e a c c o r d i n g t o t h e i n s t r u c t i o n s s u p p l i e d by t h e m a n u f a c t u r e r . C a l i b r a t i o n c u r v e s were r e c o n s t r u c t e d whenever t h e flame head was c l e a n e d . Copper t u b i n g 1/8" i n d i a m e t e r c o n n e c t e d t h e s a m p l i n g v a l v e t o the c h r o m a t o g r a p h i c columnc. The o u t p u t f r o m t h i s chromato-graph was d i s p l a y e d on a M i n n e a p o l i s - H o n e y w e l l 1.0 mV r e c o r d e r , e q u i p p e d w i t h a d i s c i n t e g r a t o r . Ch romat ograph i c Columns: A g l a s s column 150 cm. l o n g and w i t h an i n t e r n a l d i a m e t e r o f 5 mm., p a c k e d w i t h a l u m i n a and o p e r a t e d a t room t e m p e r a t u r e , was u s e d t o s e p a r a t e v a r i o u s components o f t h e r e a c t i o n m i x t u r e f o r q u a l i t a t i v e i d e n t i f i c a t i o n w i t h t h e t h e r m a l c o n d u c t i v i t y chromatograph. The s e p a r a t e d gases l e a v i n g t h e column c o u l d be t r a p p e d o u t o f t h e h e l i u m s t r e a m by means o f a s y s t e m o f c o l d t r a p s , and by o b s e r v i n g the t h e r m a l c o n d u c t i v i t y d e t e c t o r ' s r e s p o n s e on th e r e c o r d e r . The t r a p p e d components were t r a n s f e r r e d i n t o e v a c u a t e d b u l b s o r i n f r a r e d c e l l s f o r a n a l y s i s . A 160 cm. l o n g and 5 mm. d i a m e t e r g l a s s column, p a c k e d w i t h a c t i v a t e d c h a r c o a l , was u s e d f o r t h e q u a n t i t a t i v e a n a l y s i s o f n i t r o g e n . The column was h e a t e d e l e c t r i c a l l y t o 34°C. S i n c e a l l t h e h y d r o c a r b o n s and azoethane were r e t a i n e d on t h e column, a f t e r c a r r y i n g out s i x o r seven a n a l y s e s , t h e column was h e a t e d t o c a . 200°C. and f l u s h e d f o r two days. T h i s p r o c e d u r e was e s s e n t i a l i n o r d e r t o m a i n t a i n a good base l i n e on t h e r e c o r d e r . T h i s method, a l t h o u g h t i m e consuming, was i n d i s p e n s a b l e . S e p a r a t i o n o f azo e t h a n e from t h e r e a c t i o n m i x t u r e was e f f e c t e d u s i n g a 10* x 1/8" s t a i n l e s s s t e e l column, p a c k e d w i t h 20% Carbowax, 5% KOH on 19 60/80 DMCS Chromosorb W. The quantitative analyses for hydrocarbons were made with a 25' x 1/8" stainless steel column, packed with 20% hexadecane on 42/60 mesh firebrick. Azoethane was retained on the hexadecane column and, when it started coming out, the column was flushed for a day at ca. 50°C. Both columns were obtained from Wilkeris Instrument and Research Inc. The temperature of both columns was kept at 30°C. throughout this study. The following constant flow of helium was maintained through all the above columns. Column Flow (Ml./min.) Activated charcoal 60 1 1 Hexadecane 23 1 1 KOH-Carbowax 18 ± 1 Sampling Loops: A high-vacuum, 6 way linear gas sampling valve (Wilkens Instrument and Research Trie) was employed for injecting the reaction mixtures into the "HY-FI" gas chromatograph. The apparatus used for injecting the reaction mixtures into thermal conductivity chromatograph is shown in Figure 3. It was made of 1/4" copper tubing incorporating greaseless Hoke valves and worked as follows. To start, the valves 1 and 2 were closed and valve 3 opened. The volume enclosed by valves 1 and 2 was connected to the vacuum system for evacuation by opening stopcock F.. While this was in progress, the carrier gas was routed through a bypass by opening valve 4. After taking a sample of the products from the reaction vessel, valve 3 was closed. For injecting the reaction mixture in the gas chromatograph, valve 4 was closed and valves 1 and 2 opened, in that order. The total volume of the sampling loops and 20 of the line connecting these to the reaction vessel was approximately 7 ml. The analysis of the products from flash photolysis runs was made in the same manner by connebting the photolysis reaction vessel to the sampling unit through valve 5. Infrared Analysis: The qualitative identification of the components of the reaction mixture was achieved by analysis of the infrared spectra of all separated components using a Perkin-Elmer Model 21 instrument. The gaseous samples were handled in glass cells with sodium chloride windows, polished from time to time to maintain high transmittance. Mass Spectrpmetric Analysis: All mass spectra were taken either with an MS9 mass spectrometer, manufactured by Associated Electrical Industries Ltd. or with an Atlas CH4 mass spectrometer at 70 eV energies. The mass spectrum of each of the separated components was compared with the spectrum of the known compounds to confirm their identity. Exact mass measurements were made on the MS9 mass spectrometer using compounds of known mass numbers. FLASH PHOTOLYSIS APPARATUS The apparatus used in this study is essentially similar to that 23 described by Basco and Norrish and is shown in Figure 4. The quartz photolysis lamp (50 cm. long and 1 cm. in internal diameter), filled with krypton and a trace of nitrogen at a pressure of 60 mm. Hg, was placed inside a brass casing alongside the pyrex reaction vessel. The inside of the brass casing was covered with reflecting material. The flash lamp, with tungsten electrodes, was connected across a 33.3 microfarad capacitor. After c H i g h Power C h a r g i n g U n i t C a p a c i t o r To Vacuum System F l a s h lamp |j=| I 1 R e a c t i o n V e s s e l T r i g g e r "^ 1 Tungsten E l e c t r o d e s Brass C a s i n g F i g u r e 4. F l a s h P h o t o l y s i s A p p a r a t u s . B^Q cone 22 c h a r g i n g t h e condenser t o a h i g h v o l t a g e , t h e d i s c h a r g e t h r o u g h t h e f l a s h t u b e was i n i t i a t e d w i t h a T e s l a c o i l . The l i g h t e m i t t e d by t h i s lamp c o n s i s t s o f c o n t i n u o u s r a d i a t i o n i n t h e w a v e l e n g t h r e g i o n t r a n s m i t t e d by q u a r t z . The d u r a t i o n o f t h e f l a s h a t 8.0 KV was a p p r o x i m a t e l y 30 m i c r o -seconds . Two p y r e x r e a c t i o n v e s s e l s were used f o r c a r r y i n g out t h e f l a s h p h o t o l y s i s o f azo e t h a n e . One o f t h e v e s s e l s was a 48 cm. l o n g and 1 cm. i n d i a m e t e r p y r e x tube w i t h s e a l e d ends. P r o v i s i o n s were made f o r f i l l i n g ( c o n n e c t i n g ) the r e a c t i o n v e s s e l t h r o u g h a s t o p c o c k and BIO cone t o t h e vacuum l i n e . The volume o f t h i s c e l l was a p p r o x i m a t e l y 42 cm . The s e c o n d r e a c t i o n v e s s e l was a 53 cm. l o n g and 0.6 cm. i n i n t e r n a l d i a m e t e r p y r e x t u b e , w i t h one end s e a l e d . The o t h e r end was c o n n e c t e d t o a cone t h r o u g h a s t o p c o c k f o r f i l l i n g and e v a c u a t i n g t h i s c e l l . T h i s v e s s e l was s u r r o u n d e d w i t h a p y r e x tube j a c k e t w i t h open e n d s , h a v i n g d i m e n s i o n s o f the f i r s t r e a c t i o n c e l l . The volume o f t h i s c e l l was c a . 20 cm. 3. A f t e r t h e e v a c u a t i o n o f the r e a c t i o n v e s s e l t o ''black vacuum", a known p r e s s u r e o f azoethane was a d m i t t e d f i r s t , and t h e n t h e known p r e s s u r e o f t h e m o d e r a t i n g gas. A l l p r e s s u r e s were r e a d on a mercury manometer. The m i x t u r e was a l l o w e d t o s t a n d i n t h e dar k f o r an h o u r , f o r complete m i x i n g . A f t e r t h i s , t h e c o n t e n t s o f t h e v e s s e l were f l a s h p h o t o l y s e d u s i n g an a p p r o p r i a t e number o f f l a s h e s . The r e a c t i o n v e s s e l was t a k e n out o f t h e b r a s s c a s i n g and t h e p r o d u c t s were a n a l y s e d u s i n g gas chromatography as d e s c r i b e d e a r l i e r . The s e a r c h f o r h i g h e r p r o d u c t s w i t h mass number g r e a t e r t h a n 86 ( p a r e n t mass o f azoethane) was made u s i n g a mass s p e c t r o m e t e r . 23 CHAPTER I I I RESULTS T h i s c h a p t e r i s d i v i d e d i n t o t h r e e main s e c t i o n s under t h e h e a d i n g s : -i ) GENERAL i i ) PYROLYSIS RESULTS i i i ) FLASH PHOTOLYSIS RESULTS The d a t a , and t h e r e s u l t s c a l c u l a t e d from t h e d a t a o f t h e p y r o l y s i s and f l a s h p h o t o l y s i s o f a z o e t h a n e , w i t h m i n o r c o n c l u s i o n s a r e r e p o r t e d i n t h i s c h a p t e r . The i n t e r p r e t a t i o n o f e x p e r i m e n t a l r e s u l t s and major con-c l u s i o n s t o be drawn w i l l be t a k e n up i n C h a p t e r IV. GENERAL:-Q u a l i t a t i v e I d e n t i f i c a t i o n o f P r o d u c t s : -The c o n f i r m a t i o n o f the i d e n t i t y o f each p r o d u c t was made u s i n g gas chromatography, mass s p e c t r o m e t r y and i n f r a r e d s p e c t r o s c o p y . The columns used f o r t h e s e p a r a t i o n o f r e a c t i o n m i x t u r e have been l i s t e d i n C h a p t e r I I . A gas chromatograph o f h y d r o c a r b o n p r o d u c t s , formed and d e t e c t e d i n t h e i n i t i a l s t a g e s o f t h e r e a c t i o n i s shown i n F i g u r e 5. A l l c h r o m a t o g r a p h i c peaks were a t t e n u a t e d and t h e c o r r e s p o n d i n g a t t e n u a t i o n f a c t o r s a r e shown at t h e t o p o f each peak i n t h e F i g u r e . P o s i t i v e l y i d e n t i f i e d p r o d u c t s , formed i n t h e i n i t i a l s t a g e s o f t h e p y r o l y s i s a r e l i s t e d below. N 2 C H l + t r a n s - C i t H 8 - 2 C 2 H 4 n-Ci+Hjo CzHg d i e t h y l amine ( t r a c e q u a n t i t y ) x 16 C 2H 6 x 8 trans-Buten Figure 5. Gas Chromatogram Showing • Time Hydrocarbon Products a f t e r Integrator • 2 min. reaction Time at 295°C from 31.5mm Hg. AE. Figure 6 Gas Chromatogram of Products of Py r o l y s i s at High Extents of Reaction at 271.5°C. 2 6 C 3 H 6 t r i e t h y l amine ( t r a c e q u a n t i t y ) C3H.8: The p r o d u c t s o f the t h e r m a l d e c o m p o s i t i o n o f azoethane a r e v e r y complex when t h e r e a c t i o n i s c a r r i e d t o h i g h p e r c e n t a g e d e c o m p o s i t i o n . Hydrocarbons w i t h h i g h e r m o l e c u l a r w e i g h t t h a n t h a t o f a z o e t h a n e , a r e t h e n p r o d u c e d , most p r o b a b l y due t o i n d u c e d p o l y m e r i s a t i o n o f e t h y l e n e . A gas chromatogram o f C i t o Ci+ h y d r o c a r b o n r e a c t i o n m i x t u r e , p r o d u c e d at h i g h e x t e n t s o f r e a c t i o n i s shown i n F i g u r e 6 . The c h r o m a t o g r a p h i c peak f o r e t h y l e n e i s d i m i n i s h e d r e l a t i v e t o methane ( a t t h e same a t t e n u a t i o n ) , t h u s i n d i c a t i n g e s s e n t i a l l y t h e p o l y m e r i s a t i o n o f e t h y l e n e . The f o l l o w i n g a d d i t i o n a l h y d r o c a r b o n s were i d e n t i f i e d when t h e r e a c t i o n was c a r r i e d t o a h i g h p e r c e n t a g e d e c o m p o s i t i o n . b u t e h e - 1 c i s - b u t e n e - 2 pentenes and pentanes hexene, hexane, hepteine and heptane e t c . The p r o d u c t s o f p y r o l y s i s from a few r u n s , w h i c h had been c a r r i e d t o 40% d e c o m p o s i t i o n o r more, were condensed a t - 7 8 ° C . and t h e v o l a t i l e com-ponents o f t h e r e a c t i o n m i x t u r e were pumped away. The h i g h r e s o l u t i o n mass sp e c t r u m o f t h i s sample ( r u n on M S 9 mass s p e c t r o m e t e r ) was q u i t e complex. A p a r t o f i t from mass number 1 1 8 t o 1 4 0 i s shown i n F i g u r e 7 . By l o o k i n g at a p a r t i c u l a r mass number, one can e a s i l y see t h e c o n t r i b u t i o n r e s u l t i n g from m o l e c u l e s o r i o n s h a v i n g s l i g h t l y d i f f e r e n t masses f o r example, at mass 2 8 , e t h y l e n e ( 2 8 . 0 3 1 2 9 9 ) and n i t r o g e n ( 2 8 . 0 0 6 1 4 8 ) . T h i s shows t h a t compounds c o n t a i n i n g c a r b o n , hydrogen and n i t r o g e n are p r o d u c e d a l o n g w i t h m o l e c u l e s R e l a t i v e Peak H e i g h t OJ Ml W c H 2 P co co z c t-O" OJ OJ . .SC So-n-era -a 3^ O m r t X H -r t O CD 3 3 r t co O o. a: rt> T3 3' o a> r-" CO x o CO v_. H - C CO r t o 3 P co co CO (!) O r t H § OJ ^1" O J -LZ 28 c o n t a i n i n g c a r b o n and hydr o g e n o n l y , a t b o t h low and h i g h p e r c e n t a g e d e c o m p o s i t i o n . Q u a n t i t a t i v e A n a l y s i s : -The q u a n t i t a t i v e d e t e r m i n a t i o n o f t h e p r o d u c t s was made u s i n g gas chromatography as t h e a n a l y t i c a l t o o l t h r o u g h o u t t h i s i n v e s t i g a t i o n . A t s m a l l e x t e n t s o f r e a c t i o n , t h e p r o d u c t d i s t r i b u t i o n was made up s o l e l y o f CHi+, C 2 H 4 , C 2 H 6 , C3H 6, C3H8, trans-Ci+He, n-C^Hio a n d N 2 . K i n e t i c d a t a were o b t a i n e d o n l y f o r t h e above l i s t e d compounds. T a b l e 2 g i v e s t h e amount o f each p r o d u c t i n mm. Hg i n t h e r e a c t i o n v e s s e l a f t e r 60 seconds r e a c t i o n t i m e at 286°C. P r o d u c t d i s t r i b u t i o n fftrom 54.8 mm. Hg o f azoethane a t 271.5°C. f o r 60 seconds r e s i d e n c e t i m e i s l i s t e d i n T a b l e 3. E t h a n e , n-butane and n i t r o g e n c o n s t i t u t e t h e m a j o r p o r t i o n o f the r e a c t i o n m i x t u r e . I t i s q u i t e o b v i o u s from the d a t a i n t h e s e two t a b l e s t h a t C3H6, C3H8, and tr a n s - C i T H 8 - 2 a r e formed r e l a t i v e l y i n s m a l l amounts i n t h e e a r l y s t a g e s o f t h i s t h e r m a l d e c o m p o s i t i o n . 2 9 PYROLYSIS RESULTS D i s a p p e a r a n c e o f azoethane and F o r m a t i o n o f N i t r o g e n : -The d i s a p p e a r a n c e o f AE and f o r m a t i o n N 2 as a f u n c t i o n o f t i m e were s t u d i e d a t d i f f e r e n t t e m p e r a t u r e s and p r e s s u r e s . A t y p i c a l gas chromatogram showing t h e d i s a p p e a r a n c e o f azoethane a t 2 5 9 ° C . i s shown i n F i g u r e 8 . A l l C i t o C i t h y d r o c a r b o n s a r e e l u t e d from t h e column i n one peak on the chromato-gram. Two o r t h r e e a n a l y s e s were made from a s i n g l e r u n and t h r e e runs were u s u a l l y c a r r i e d out t o p l o t t h e c o n c e n t r a t i o n o f azoethane as a f u n c t i o n o f t i m e . The d a t a f o r t h e removal o f AE and appearance o f n i t r o g e n a t d i f f e r e n t t e m p e r a t u r e s and p r e s s u r e s a r e r e p o r t e d i n T a b l e s 4 , 5 , and 6 . T a b l e 5 was u t i l i s e d i n c o n s t r u c t i n g F i g u r e s 9 and 1 0 . I t i s w o r t h w h i l e t o n o t e from t h e s e t a b l e s t h a t t h e r a t e o f d i s a p p e a r a n c e o f AE i s always g r e a t e r t h a n t h e r a t e o f f o r m a t i o n o f n i t r o g e n a t t h e v a r i o u s t e m p e r a t u r e s and p r e s s u r e s s t u d i e d . The s i g n i f i c a n c e o f t h e f i r s t o r d e r r a t e c o n s t a n t at 2 5 9 ° C . i s i l l u s t r a t e d i n F i g u r e 1 0 . I t may be seen t h a t t h e app a r e n t f i r s t o r d e r r a t e c o n s t a n t changes a p p r e c i a b l y i n t h i s p r e s s u r e i n t e r v a l . However, i t can be s p e c u l a t e d t h a t u n i m o l e c u l a r i n i t a t i o n i n t h i s r e a c t i o n may t u r n out t o be p r e s s u r e dependent and i n t h a t c a s e , t h e f i r s t o r d e r r a t e c o n s t a n t s w i l l v a r y s i g n i f i c a n t l y a l s o . To c a l c u l a t e t h e o r d e r f o r t h e removal p f a z o e t h a n e , i n i t i a l r a t e s o f a z oethane d i s a p p e a r a n c e a t d i f f e r e n t i n i t i a l p r e s s u r e s Were measured a t 2 7 1 . 5 ° C . The r e s u l t s a r e g i v e n i n T a b l e 7 and p l o t t e d i n F i g u r e 1 1 . The method o f l e a s t s q u a r e s (a F p r t r a n prpgramme r u n pn IBM 7 0 4 0 ) was a p p l i e d t p t h e s e d a t a and t h e P r d e r was fpu n d t o be 1 . 2 0 * 0 . 0 5 . The r e s u l t s c o n c e r n i n g t h e r a t e o f f o r m a t i o n o f n i t r o g e n a t d i f f e r e n t i n i t i a l tempera-t u r e s and p r e s s u r e s w i l l be r e p o r t e d a l p n g w i t h o t h e r h y d r o c a r b o n p r o d u c t s . 30 TABLE 2 Products from p y r o l y s i s of 57.2 mm. Hg of azoethane  at 286°C. i n pyrex r e a c t i o n : v e s s e l r e a c t i o n , t i m e 60 sec. Product mm. Hg i n Reaction Vessel CHI+ 0.10 C 2H4 0.15 C 2H 6 1.85 C 3 H 6 0.013 C 3 H 8 0.025 CHH8-2 trace Ci+Hio 0.42 N 2 1.32 TABLE 3 Product d i s t r i b u t i o n from 54.8 mm. Hg of azoethane  at 271.5°C. a f t e r reaction time of 60 seconds Product Mole % of Total Products CH^ 2.3 C2Hi+ 2.7 C 2H 6 63.3 C 3H 6 0.01 C 3H 8 0.02 C^He-2 trace •Ci+HiQ 8.20 N 2 23.1 A z o e t h a n e Time • Integrator • Figure 8 Gas Chromatogram of the Removal of Azoethane at 259°C. 32 TABLE 4 Analytical results for the removal of azoethane and  formation of nitrogen as a function of time Time [Azoethane] [Nitrogen] (seconds) (mm. Hg in reaction (mm. Hg in reaction vessel) vessel)  TEWPERATURE 271.5°C. 0 60.8 0.0 120 57.4 1.5 240 54.2 2.7 360 51.6 4.0 600 47.0 6.5 780 44.0 8.0 960 41.6 9.70 1140 37.6 11.2 TEMPERATURE 271.S°C. 0 45.1 0.0 120 43.2 0.8 240 41.5 1.6 420 39.0 3.1 600 37.0 4.5 840 34.5 5.6 1140 32.0 7.5 1260 31.0 8.7 33 TABLE 5 Analytical results for the removal of azoethane  and formation of nitrogen as a function of time Time [Azoethane] [Nitrogen] (seconds) (mm. Hg of the product (mm. Hg of the pro-in vessel) duct in vessel) TEMPERATURE 2S9°C. 0 55.0 0.0 180 52.4 0.5 360 50.0 1.2 540 48.0 720 46.0 2.3 900 44.6 2.7 1080 43.0 3.6 1800 38.1 5.6 TEMPERATURE 259°C 0 33.2 0.0 120 32.8 0.2 240 32.1 0.45 600 31.0 0.9 840 30.4 1.2 1140 29.6 1.5 1560 28.5 2.0 1800 27.9 2.6 34 TABLE 6 A n a l y t i c a l r e s u l t s f o r t h e r e m o v a l o f az o e t h a n e and f o r m a t i o n o f n i t r o g e n as a f u n c t i o n o f t i m e Time [Azoethane] [ N i t r o g e n ] ( s e c o n d s ) (mm. Hg o f p r o d u c t i n (mm. Hg o f p r o d u c t i n .. t h e v e s s e l ) t h e v e s s e l )  TEMPERATURE 246.5°C. 0 49.1 0.0 240 46.7 0.3 480 44.8 0.7 600 44.0 0.87 840 42.6 1.15 1080 41.4 1.4 1440 40.1 1920 39.0 2.25 2400 38.2 2.6 TEMPERATURE 246.5°C. 0 45.5 240 43.2 480 41.5 720 40.5 960 39.5 1680 38.0 2400 37.9 37 TABLE 7 I n i t i a l r a t e s o f r e m o v a l o f azo e t h a n e as a f u n c t i o n  o f i n i t i a l a z o e t h a n e p r e s s u r e a t 271.5°C. p a z o e t h a n e I n i t i a l r a t e 1 (mm. Hg) (mm. Hg se c o n d " ) 19.9 7.41 1 0 " 3 24.0 8.22 1 0 " 3 32.3 1.32 1 0 " 2 39.8 1.50 1 0 " 2 45.1 1.78 1 0 ' 2 60.8 2.83 1 0 ~ 2 80.4 3.71 1 0 " 2 0 ^ Azoethane) 39 P r o d u c t F o r m a t i o n : -E x p e r i m e n t s , c o n s i s t i n g o f a d e t a i l e d a n a l y t i c a l s t u d y o f t h e f o r m a t i o n o f p r o d u c t s as f u n c t i o n o f time a t v a r i o u s i n i t i a l t e m p e r a t u r e s and p r e s s u r e s , were c a r r i e d o u t . F i g u r e s 12> 13 and 14 show t h e v a r i a t i o n o f p r o d u c t s , formed i n t h e e a r l y s t a g e s o f t h e r e a c t i o n as a f u n c t i o n o f time from 21.0 mm. o f azoe t h a n e a t 261.0°C. I t was o b s e r v e d t h a t t h e r a t e s o f f o r m a t i o n o f a l l p r o d u c t s o f p y r o l y s i s e x c e p t p r o p y l e n e a r e in d e p e n d e n t o f r e a c t i o n e x t e n t ( l e s s t h a n 4 0 % ) w i t h i n e x p e r i m e n t a l e r r o r , and t h a t t h e r a t e was a maximum r i g h t a t t h e b e g i n n i n g o f the r e a c t i o n . No i n d u c t i o n p e r i o d was o b s e r v e d f o r t h e f o r m a t i o n o f p r o d u c t s e x c e p t f o r p r o p y l e n e These f i n d i n g s c o n s i d e r a b l y s i m p l i f i e d t h e t a s k o f m e a s u r i n g i n i t i a l r a t e s o f f o r m a t i o n o f t h e v a r i o u s p r o d u c t s . I n a l l subsequent e x p e r i m e n t s , i n i t i a l r a t e s were measured by a s i n g l e a n a l y s i s o f t h e c o n t e n t s o f t h e r e a c t i o n v e s s e l a f t e r a s h o r t , b u t measured, r e s i d e n c e t i m e , always g r e a t e r t h a n 1 m i n u t e . The i n i t i a l r a t e s o f f o r m a t i o n o f v a r i o u s p r o d u c t s (mm. Hg se c . 1 ) at d i f f e r e n t i n i t i a l t e m p e r a t u r e s and p r e s s u r e s a r e g i v e n i n T a b l e s 8 and 9. T a b l e 9 a l s o l i s t s t h e i n i t i a l r a t e s o f f o r m a t i o n a t d i f f e r e n t i n i t i a l AE p r e s s u r e s a t 271.5°C. i n t h e p y r e x r e a c t i o n v e s s e l , p a c k e d w i t h 170 p y r e x t u b e s w i t h f i r e p o l i s h e d ends (18 cm. l o n g and 0.3 mm. d i a m e t e r ) . These r e s u l t s i n t h e pack e d v e s s e l w i l l be t a k e n up a g a i n i n t h e s e c t i o n " S u r f a c e E f f e c t s " . The O v e r a l l O r d e r s o f F o r m a t i o n f o r M a j o r P r o d u c t s The d a t a l i s t e d i n T a b l e s 8 and 9 were u t i l i s e d i n c a l c u l a t i n g t h e o r d e r s f o r t h e f o r m a t i o n o f major p r o d u c t s u s i n g t h e r e l a t i o n s h i p l°glo( Ri) = l o g 1 0 k + n i o B i o C A E ) * where R. and k denote t h e i n i t i a l r a t e o f f o r m a t i o n o f t h e p r o d u c t under o o 600 1200 1800 2400 Time (Seconds) • 600 1200 1800 2400 Time (Seconds) 0.Q4 Time (Seconds) • F i g u r e 14. F o r m a t i o n o f P r o p y l e n e as a F u n c t i o n o f Time a t 261°C. from 21 mm. o f Azoethane. 43 consideration and the specific rate constant, respectively. (AE)^  and n denote the initial AE concentration and order of product formation, respectively. The results of experiments at 286°C. are presented in the form of logarithmic plots in Figures 15, 16, 17 and 18, from which orders of formation of various products were calculated using the method of least squares. These various orders and the experimental uncertainty to which they are subject at three different temperatures are listed in Table 10. The significance of the orders of formation of the products will be discussed in Chapter IV. Pressure Change Measurements:-In a few experiments, the overall pressure change, AP, was measured as a function of time using a pressure transducer. Initial rates (mm. Hg sec. l) were measured at various initial pressures of azoethane at two temperatures. The experimental results are shown in the form of a logari-thmic plot in Figure 19 ajt 284 and 2 5 9 ° C , from which the overall apparent order of the reaction was calculated. Three runs were carried out at 2 4 6 . 5 ° C. at 40 mm. Hg for recording pressure change due to thermal decomposition with the transducer. In this case,no pressure change was observed for a reaction time of 50 minutes, although reaction did occur. The product formation as a function of time at 246 .5°C . is given in Table 12. This observation clearly points out that azoethane is disappearing to give compounds of low volatility, and that the lack of pressure change is fortui-tous . Activation Engergies of Formation of Various Products:-Twenty runs were carried out at different temperatures at azoethane concentrations of about 10~ 6 moles ml"1 to calculate activation energies TABLE 8 A n a l y t i c a l r e s u l t s f o r t h e i n i t i a l r a t e s o f f o r m a t i o n o f v a r i o u s p r o d u c t s a t d i f f e r e n t i n i t i a l t e m p e r a t u r e s and p r e s s u r e s o f a z o e t h a n e . ( A l l r a t e s e x p r e s s e d i n mm. Hg p e r s e c o n d ) P a z o e t h a n e ^ H ^ RC2^ RC2HS R C 3 H 6 R C 3 H 8 RCkHQ (mm. Hg) x 1 0 3 x 1 0 3 x 1 0 2 x 10h x 1 0 4 x 101* TEMPERATURE 295°C: 19.3 1.17 3.83 3.67 6.70 5.00 5.0 16.8 1.71 31.5 2.00 6.50 6.00 8.33 7.50 7.0 26.2 2.72 39.8 3.03 7.04 7.70 9.80 9.10 9.0 31.7 3.56 54.5 5.38 10.9 12.6 8.80 11.0 n.d. 38.8 5.32 72.5 7.67 15.2 16.8 11.7 15.0 10.0 51.7 7.08 TEMPERATURE 286°C.: 19.0 0.33 0.73 0.92 1.00 1.40 n.d 3.50 0.67 36.6 0.80 1.27 2.32 1.6 2.5 0.80 5.00 1.30 57.2 1.67 2.50 3.08 2.20 3.30 1.30 7.00 2.20 61.0 1.83 3.00 3.50 n.d. n.d n.d. 8.10 2.50 92.0 3.67 4.42 7.40 2.80 6.00 3.00 12.0 5.30 R C - H 1 0 R N 2 x 1 0 3 x 1 0 2 TABLE- 9 A n a l y t i c a l r e s u l t s f o r t h e i n i t i a l r a t e s o f f o r m a t i o n o f v a r i o u s p r o d u c t s at d i f f e r e n t i n i t i a l t e m p e r a t u r e s and p r e s s u r e s o f azoethane. ( A l l r a t e s a r e e x p r e s s e d i n mm. Hg p e r second,) ^ a z o e t h a n e R C 2 H 6 R C 3 H 6 R C 3 H 8 tCi+Hs ^ - C i t H i o N 2 (mm. Hg) x 1 0 3 x 1 0 3 x 1 0 2 x 10h x 1 0 4 x 10k x 1 0 3 x 10 : TEMPERATURE 271.5°C., UNPACKED PYREX REACTION VESSEL 18.2 0.125 0.20 0.433 n.d. n.d. n.d. 0.92 0.20 35.9 0.40 0.50 1.12 0.10 0.17 n.d. 1.50 0.45 54.8 0.80 0.933 2.18 0.33 0.66 0.20 2.83 0.80 87.5 1.27 1.30 2.77 0.65 1.70 0.84 3.17 1.27 108.5 1.58 1.50 3.45 1.00 3.33 1.0 3.67 1.50 TEMPERATURE 271.5°C, PACKED PYREX REACTION VESSEL: 20.7 0.170 0.28 0.40 0.41 0.83 n.d. 1.10 0.32 34.6 0.33 0.43 0.73 0:80 1.30 0.33 1.40 0.55 5 7,2 0.77 0.85 1.50 0.83 2.01 0.60 2.90 0.85 84.3 1.05 1.11 2.20 0.60 2.50 1.10 2.67 1.20 109.2 2.20 2.20 3.11 0.95 4.00 1.70 4.47 1.70 o 10 Azoethane) 51 associated with the formation of various products. The Arrhenius equation used to calculate the activation energies is: iQglo(RV) = logloA + nxlogio(AE). - E /2.303 RT where A is the pre-exponential factor of the rate constant and the other terms have the usufcl significance. The typical Arrhenius plots for the initial rates of formation of various products are presented in Figures 20 and 21. The activation energies for the formation of major products have been calculated as reported in Table 11. Surface Effects:-Two types of measurements were made in the packed vessels. Figure 22 illustrates the effect on the pressure increase due to thermal decomposition as a function of time at 271.5°C. and 286.0°C. due to packing the quartz cell with quartz tubes (11-fold increase in surface). The solid line is drawn by the pen of the recorder on the chart paper and the dotted lines correspond to alternate connection of the transducer to the packed and unpacked vessels. In all cases, the rate of the reaction as measured by pressure increase was lower in the packed than in the unpacked vessels. To study the effect of packing on the product distribution, the pyrex vessel (694 ml) was packed with 170 pyrex tubes (18 cm. long and 0.3 mm. in diameter) and seasoned for a week by doing various preliminary pyrolysis of azoethane at 271.5°C. The proper conditioning of the packed vessel is very important because of its bearing on the product distribution. The initial rates of formation of various products in the packed pyrex reaction vessel are given in Table 9. It can be seen that initial rates of formation of nitrogen are slightly higher in the packed than in the unpacked 52 TABLE 10 Orders of Formation of Various Products  w.r.t. Azoethane at three Different Temperatures Product 271.5°C 286.0°C 295.0°C CHi+ 1.42 * 0.10 1.60 * 0.30 1.47 ± 0.20 C 2 H 4 1.14 •* 0.10 1.19 ± 0.10 1.02 ± 0.10 C 2 H 6 1.16 * 0.12 1.16 ± 0.12 1.28 * 0.13 C 3 H 6 — - 0.66 * 0.10 0.40 ± 0.10 C 3 H 8 — - 0.83 * 0.10 0.81 ± 0.20 n-C^Hio °-80 * 0.10 0.88 * 0.15 0.83 * 0.15 N 2 1.14 ± 0.05 1.15 * 0.10 1.09 ± 0.05 TABLE 11 Activation Engergies of Formation of Major Products w.r.t. Azoethane. Concentration of AE = 1.08 x 10~e moles ml Product Activation Energy (kcal mole ) CHi+ 45.0 ± 4.0 <Z2WU 56.2 * 5.0 C^e 42.3 * 2.0 n - C ^ o 66.0 * 8.0 N 2 47.2 ± 1.0 TABLE 12 A n a l y t i c a l r e s u l t s f o r p r o d u c t f o r m a t i o n as a f u n c t i o n o f t i m e Time C H i * C 2 H 6 C 3 H 6 C 3 H 8 N 2 (Seconds) ( P r e s s u r e s o f p r o d u c t s i n mm. Hg i n t h e V e s s e l )  TEMPERATURE 246.5°C, I n i t i a l p r e s s u r e o f azo e t h a n e 44.0 mm. Hg: 600 0.03 0.025 0.50 0.01 0.02 0.08 0.35 1200 0.06 0.055 0.95 0.017 0.03 0.15 0.67 1800 0.09 0.08 1.46 0.03 0.055 0.20 0.98 2400 0.125 0.10 1.85 0.04 0.07 0.26 1.28 3000 0.150 0.12 2.30 0.05 0.09 0.30 1.50 t n W TABLE 13 A n a l y t i c a l r e s u l t s f o r p r o d u c t f o r m a t i o n as a f u n c t i o n o f t i m e  TEMPERATURE 2 7 1 . 5 ° C . , a z o e t h a n e p r e s s u r e 36.6 mm. Hg p l u s b u t e n e - 1 38.5 mm. Hg. 180 0.12 0.12 0.68 0.065 0.02 -- 0.97 300 0.20 0.20 1.14 0.12 0.03 -- 1.6 900 0.45 0.53 5.00 0.41 0.12 -- 4.6 1380 0.72 0.88 8.80 0.84 0.30 -- 7.0 2460 1.0 1.30 13.00 1.40 0.70 -- 11.5 Note: b u t e n e - 2 and n-butane were n o t measured i n t h i s s e t o f e x p e r i m e n t s . l / T x 1 0 3 (°K) 1.72 1.76 1.80 1.84 1.88 1.92 l / T x 1 0 3 (°K) 56 ^Unpacked 286.0°C 69.Omm Unpacked Packed 271.5°C Figure 22. Pressure Change Displayed on Recorded Chart Paper Using Pressure Transducer Both In Packed and 33.0mih Unpacked Quartz Vessels. ' Time 57 ve s s e l , whereas the i n i t i a l rate of formation of ethane i s sharply decreased in the packed vessel. However, the i n i t i a l rates of the other hydrocarbons are also somewhat affected by packing. E f f e c t o f Added Gases on the I n i t i a l Rates of Formation:-A ser i e s of 20 runs was c a r r i e d out at 261°, 271.5° and 286°C. with various added gases i n the unpacked reaction vessel i n which the pressure of AE was between 20.0 and 70.0 mm. Hg. The data obtained for the e f f e c t of added gases on the i n i t i a l rates of formation of various products at d i f f e r e n t i n i t i a l temperatures are presented in Table 14. The rate of ethane formation i s affected s i g n i f i c a n t l y as compared to other products. Carbon dioxide was used because of i t s inertness to ethyl r a d i c a l s , and also due to i t s non-interference with the product analysis. Butene-1 and cis-butene-2 were used because of t h e i r chain i n h i b i t i o n character i n p y r o l y t i c studies. Cis-butene-2 was e s p e c i a l l y useful because i t was possible to measure a l l hydrocarbon products q u a n t i t a t i v e l y i n i t s presence. In the p y r o l y s i s of AE and butene mixtures, higher compounds with 6,7 and 8 carbon atoms were detected i n the reaction mixture using gas chromatography and mass spectrometry. Carbon, Hydrogen and Nitrogen Material Balances:-The carbon, hydrogen and nitrogen balances attained i n the analysis of the products are i l l u s t r a t e d i n Table 15. This Table l i s t s the i n i t i a l rates of formation of major products, along with the i n i t i a l rates of C, H and N production for a t y p i c a l experiment at 271.5°C, with an i n i t i a l -6 -1 concentration of azoethane of 1.08 x 10 moles ml. . It can be seen that there are considerable discrepancies i n the mass balances even at low conversions. However, i t i s TABLE 14 ' A n a l y t i c a l r e s u l t s f o r i n i t i a l r a t e s o f f o r m a t i o n o f v a r i o u s p r o d u c t s a t d i f f e r e n t i n i t i a l t e m p e r a t u r e s  and p r e s s u r e s i n t h e p r e s e n c e o f v a r i o u s added gase s . ( A l l r a t e s e x p r e s s e d i n mm. Hg. p e r second.) Temperature °C. P azo ^added gas (mm. Hg) x 1 0 3 C 2 H t | x 1 0 3 R C 2 H 6 x 1 0 2 R C 3 H 6 x 10k R C 3 H 8 x 10" R C i+Hio x 1 0 3 x 1 0 2 271.5 37.0 29.2 ( c i s - b u t e n e ) 0.53 1.0 0.53 0.60 0.30 1.2 0.60 271.5 35.0 222.0 " 0.60 0.7 0.57 2.0 0.67 2.2. 0.70 271.5 35.5 200.5 " 0.62 0.65 0.60 1.8 0.70 2,20 0.72 ^261.0 36.6 129.0 " 0.23 0.27 0.25 0.83 0.21 0.83 0.26 261.0 38.0 130.0 " 0.24 0.30 0.28 0.80 0.25 0.81 0.26 286.0 25.5 20.0 " 1.0 1.50 1.13 2.90 2.10 3.75 1.20 261.0 63.5 0.0 0.27 0.30 0.64 n.d. 0.11 0.83 0.361 271.5 37.5 80.0 ( C 0 2 ) 0.33 0.35 0.50 0.50 1.30 n.d. 0.40 286.0 54.3 110.7 ( C 0 2 ) 1.3 2.0 2.20 0.22 0.50 n.d. 3.0 271.5 36.0 100.0 ( C 0 2 ) 0.30 0.34 0.45 0.40 1.20 1.1 0.40 286.0 50.3 120.0 ( C 0 2 ) 1.10 1.9 2.0 0.20 0V55 6.0 2.80 271.5 36.6 38.5 ( t - b u t - 2 ) 0.67 0.70 0.40 3.7 1.11 n.d. 0.60 n.d. Not d e t e r m i n e d . 59 TABLE 15 CARBON, HYDROGEN AND NITROGEN MASS BALANCES Products at 271.5°C. , from 35.9 mm. Hg of Azoethane I n i t i a l rate of formation C H 10 5 x mm. Hg per CH 4 40 40 160 C2H4. 50 100 200 C 2 H 6 1120 2240 6720 C 3H 6 1 3 6 C 3 H 8 2 6 16 trafis-CitHg -- -- --n-Ci+HiQ 150 600 1500 N 2 450 - — -- 900 Total 2989. 8602 900 60 TABLE 16 Initial rates of formation of various products in  the presence of cis-butene and carbon dioxide at 271.5°C. from 35.0 mm. Hg of azoethane along with  the rates in packed and unpacked vessels. P - J J j CH4 added gas (mm. Hg) With added Cis-Butene-2 C 2H 4 C 2H 6 C 3H 6 103x Initial Rate C 3H 8 (mm. C 4 H 1 0 N 2 Hg per second) 0 0.4 0.5 11.0 0.01 0.02 1.5 4.5 30 0.5 1.0 5.0 0.06 0.03 1.2 6.0 200 0.6 0,7 6.0 0.2 0.7 2.2 7.0 With added carbon dioxide 0 0.4 0.5 11.0 0.01 0.02 1.5 4.5 80 0.3 0.4 5.0 0.05 0.13 4.0 100 0.3 0.3 4.0 0.04 0.12 1.1 4.0 Unpacked 0 0.4 0.5 11.0 0.01 0.02 1.5 4.5 Packed 0 0.3 0.4 7.0 0.08 0.13 1.4 5.5 61 o b s e r v e d t h a t t h e d i s c r e p a n c y i n t h e m a t e r i a l b a l a n c e s i s l e s s pronounced a t h i g h e r t e m p e r a t u r e s t h a n a t l o w e r t e m p e r a t u r e s . The c a r b o n t o n i t r o g e n r a t i o f ound i n t h e p r o d u c t s i s h i g h e r t h a n t h e t h e o r e t i c a l v a l u e o f 2, whereas t h e h y d r o g e n t o c a r b o n r a t i o i s g r e a t e r t h a n 2.5, the t h e o r e t i c a l v a l u e , r e f l e c t i n g t h e h i g h p e r c e n t a g e o f hydrogen a c c o u n t e d f o r . T h i s r e s u l t was e x p e c t e d as t h e i n i t i a l r a t e o f f o r m a t i o n o f ethane (C/H=3) i s s i g n i f i c a n t l y h i g h e r t h a n t h e r a t e o f f o r m a t i o n o f o t h e r h y d r o c a r b o n s . The d i s c r e p a n c y i n the m a t e r i a l b a l a n c e must be t h e r e s u l t o f i n c o m p l e t e a n a l y s i s , p a r t i c u l a r l y o f h i g h m o l e c u l a r w e i g h t p r o d u c t s . F o r m a t i o n o f p o l y m e r s c o n t a i n i n g c a r b o n , h y d r o g e n and n i t r o g e n , w i t h v e r y low v a p o u r p r e s s u r e , a r e a s s o c i a t e d w i t h t h e y e l l o w compounds on t h e w a l l s o f t h e tube c o n n e c t i n g t h e r e a c t i o n v e s s e l t o t h e s t o p c o c k F, o b s e r v e d a f t e r t h e a p p a r a t u s had been i n use f o r s e v e r a l r u n s . However, i t i s b e l i e v e d t h a t t h i s d i s c r e p a n c y w i l l n o t have a marked b e a r i n g on w r i t i n g a mechanism f o r t h e r e a c t i o n . 62 FLASH PHOTOLYSIS RESULTS The results of flash photolysis of azoethane in the pyrex and quartz reaction vessels, with and without outer glass jackets are presented in Tables 17 and 18. The number of flashes, and the discharge voltage used in various experiments are reported in these tables. Quantitative analyses for the products were obtained by gas chromatography using columns discussed in Chapter II. All flash experiments were carried out at room temperature (25°C). In the runs without added gases, a much higher percentage decomposition was observed, probably because of both thermal and photo-chemical decomposition of AE. This uncontrolled decomposition was eliminated by the addition of moderating gases. It is quite reasonable to assume that the temperature increase due to flash niay not be more than 30°C. in the presence of added moderating gas. This conclusion that the experiments with added moderating gas and with outer glass jackets are essentially isothermal, is borne out by the expected ratio of CzHit/CttHio, i • e., disproportionation to recombination ratio of ethyl radicals. Carbon dioxide and helium were used as inert gases due to their chemical stability, high vapour pressure and non-interference with product analysis. It can be seen that the amounts of flash photolysis products were directly.proportional to the number of flashes, within experimental error, as shown in Figure 23. Therefore, secondary decomposition of the products is not taking place. It must be noted, however, from Table 17 that there is a marked change in the product distribution in the quartz reaction vessel as compared to pyrex cell. In order to establish that this unusual product distribution in the TABLE 17 R e s u l t s o f f l a s h p h o t o l y s i s o f azoethane a t room t e m p e r a t u r e , P y r e x r e a c t i o n v e s s e l w i t h o u t p y r e x j a c k e t . ( P r o d u c t y i e l d i n m i c r o moles) P a z o . Pmod.gas CH^ C2Hi+ C 2 H 6 C 3 H 6 C 3 H 8 n -CitH 1 0 N 2 2 / 1 1/3 (1+3) /4 Comments (mm. Hg) (1) (2) (3) (4) 71.2 0 0.441 0 .588 3.82 4.62 1..33 0. 115 0 .92 phot . f o r 1/2 0 - h r . w i t h 3660 A 70.3 0 -.420 0 .610 3.57 4.20 1.45 0. 117 0 .95 ti 119.5 380.0 (CO 2) 0.042 0.294 0 .210 n.d. n.d. 2.84 3.05 0.714 0. 104 1 .02 1 f l a s h 8.0 KV. 61.0 400.0(CO2) n.d. 0.400 0 .336 n.d. n.d. 2.94 3.47 0.84 0. 136 0 .96 5 - f l a s h e s 2.6 KV. 100.0 100.0(He) n.d. 0.137 0 .105 n.d. n.d. 0.735 0.84 ~0.77 0. 186 1 .04 2 i i 2.7 KV. 60.6 295.0(He) 0.031 0.567 0 .462 0.021 0.063 3.91 4.00 0.815 0. 145 1 .12 1 I I 8.0 KV. 90.2 360.0(He) 0.063 1.20 0 .'903 0.042 0.084 7.22 9.00 , .0.753 0. 166 0 .93 2 I I it KV. 41.0 624(C02) 0.042 0.567 0 .483 n.d. 0.042 3.15 3.80 0.850 0. 180 o .97 3 I I . it KV. 39.3 232.7(C02) 0.060 0.63 0 .588 0.021 0.063 3.57 4.62 0.933 0. 176 0 .91 it i i KV. 35.01 1 0 4.032 19.2 9 .41 5.04 8.40 25.2 n.d. 0.49 0. 760 2 it it KV. 36.0 300.0(CO2) 0.04 0.525 0 .441 n.d. 0.084 3.78 4.41 0.84 0. 139 0 .97 2 n I I KV. 6 7 . 5 b ' C 0 0.945 5.88 2 .69 1.43 2.31 10.3 19.8 0.457 0. 571 0 .81 1 it ti KV. 65 .5 b * C 0 0.84 5.67 2 .73 1.05 2.50 10.1 20.0 0.481 0. 481 0 .79 1 it it KV. a, h y d r o g e n n o t measured. b, h y d r o g e n , b u t e n e s , n-pentane, p e n t e n e were d e t e c t e d bu n o t measured q u a n t i t a t i v e l y . c, q u a r t z r e a c t i o n v e s s e l o f volume 19.0 ml. w i t h o u t e r q u a r t z j a c k e t (48 cm. l o n g and 1 cm i n d i a . ) w a s u s e d , n.d., n o t d e t e r m i n e d . TABLE 18 R e s u l t s o f f l a s h p h o t o l y s i s o f a z o e t h a n e a t room t e m p e r a t u r e . P y r e x r e a c t i o n v e s s e l w i t h p y r e x j a c k e t ( P r o d u c t c o n c e n t r a t i o n i n m i c r o moles) azoethane mod.gas C2H4 C 2 H 6 n-Ci+Hxo N 2 2/1 1/3 (l + 3 ) / 4 Comments (mm. Hg) (mm. Hg) CD (2) (3) (4) 22.9 0 0.076 0.078 0.67 0.69 1.03 0.113 1.08 5 f l a s h e s o f 33.5 0 0.126 0.136 1.01 1.20 1.08 0.125 0.95 » 50.8 0 0.168 0.178 1.26 1.47 1.06 0.133 0.97 8 1.0 0 0.220 0.200 1.80 2.00 0.91 0.122 1.06 » 97.2 0 0.294 0.294 2.52 2.73 1.00 0.117 1.03 120.0 0 0.360 0.370 3.40 n.d. 1.03 0.105 - » 85.0 0 0.168 0.168 1.58 1.70 1.00 0.106 1.02 4 " 8 5.0 0 0.210 0.210 1.90 2.21 1.0 0.111 0.95 5 " 83.1 0 0.294 0.305 2.73 3.15 1.04 0.108 0.96 8 " 85.0 0 0.735 0.735 5.67 6.30 1.00 0.129 1.02 14 " 8 2.8 60.0(CO 2) 0.200 0.210 1.70 1.90 1.05 0.118 1.00 5 82.2 300.0 (CO 2) 0.168 0.160 1.47 1.75 0.95 0.114 0.94 » 83.0 500.0(CO 2) 0.157 0.147 1.26 1.40 0.94 0.125 1.01 » 8 2.5 6 0 0 . 0 ( C 0 2 ) 0.128 0.136 1.28 1.50 1.06 0.100 0.84 » 8 2.2 275.5 (CO 2 ) 0.063 0.0170 0.525 0.630 1.10 0.120 0.93 a, a l l o w e d t o s t a n d o n l y f o r f i v e m i n u t e s ; p r o b a b l y i n c o m p l e t e m i x i n g . ,d. Not d e t e r m i n e d . Number of Flashes »• 66 q u a r t z and p y r e x p h o t o l y s i s c e l l s , and a l s o i n t h e p y r o l y s i s i n v e s t i g a t i o n was n o t a r i s i n g from an u n d e t e c t e d i m p u r i t y i n t h e a z o e t h a n e , t h r e e low i n t e n s i t y p h o t o l y s i s were c a r r i e d out i n t h e p y r e x r e a c t i o n v e s s e l o f volume 42.0 cm. 3 a t room t e m p e r a t u r e and around 70 mm. Hg AE p r e s s u r e w i t h o 3660 A r a d i a t i o n u s i n g a PEK 110 medium p r e s s u r e mercury lamp. N e i t h e r propane n o r methane c o u l d be d e t e c t e d among t h e p r o d u c t s and t h e CzH^/n - C i+Hio and C 2 H 4 / C 2 H 6 r a t i o s were q u i t e i n agreement w i t h t h e p r e v i o u s low i n t e n s i t y s t u d i e s . I t seems r e a s o n a b l e t o c o n c l u d e t h a t t h e f o r m a t i o n o f p r o d u c t s l i k e methane, propane and hydrogen i s due t o the r e a c t i o n s a r i s i n g e i t h e r as a r e s u l t o f h i g h l i g h t f l u x o r as a consequence o f t h e a b s o r p t i o n o f l i g h t o f s h o r t e r w a v e l e n g t h s i n t h e f l a s h p h o t o l y s i s o f a z o e t h a n e . T h i s r e s u l t , i . e . , absence o f CH^ and C 3 H 8 i n t h e low i n t e n s i t y p h o t o l y s i s , has a l s o s e r v e d as a check on t h e p y r o l y s i s p r o d u c t s , i n t h e sense t h a t m e t h y l r a d i c a l s must be formed i n the i n i t i a l s t a g e s o f t h e t h e r m a l d e c o m p o s i t i o n o f a z o e t h a n e . ( C 2 H 4 + C 4 H 1 0 ) The m a t e r i a l b a l a n c e d e f i n e d as B = n was always c l o s e N 2 t o u n i t y i n t h e h i g h i n t e n s i t y p h o t o l y s i s , i n d i c a t i n g t h a t m a j o r i t y o f t h e p r o d u c t s have been a c c o u n t e d f o r . The v a l u e o f B i n t h e p y r o l y t i c work i s much h i g h e r t h a n u n i t y . 67 Chapter IV  DISCUSSION Pyrolysis: PRIMARY STEP IN THE THERMAL DECOMPOSITION OF AZOETHANE It is quite reasonable to assume that the first step in the pyrolysis of azoethane is the reaction C 2H 5N 2C 2H 5 — * C 2H 5 + N 2C 2H 5 (IV-1) followed by splitting of highly unstable N 2C 2H 5 fragment, according to N 2C 2H 5 *• N2 + C 2H 5 (IV-2) The first and second bond dissociation energies, Di and D 2, in azoethane are defined as the endothermicities of the reactions (1) and (2), respectively. The heat of formation of AE has been calculated to be 33.0 kcal mole-1, and using the value generally accepted for the heat of formation of ethyl radical, the sum of the two bond dissociation energies, 41 - l Dj + D 2, was calculated to be 16.0 kcal mole (to within ±5.0 kcal mole ) by Gowenlock et al. These authors have concluded on the basis of thermo-chemical considerations that two and three fragment decomposition mechanisms for aliphatic azoalkanes seem equally plausible. If the reaction proceeds by one bond fission, then N 2C 2H 5 fragment will fly apart immediately and the total result will be the same as for the simultaneous breaking of two bonds in the azoethane decomposition. It appears quite impossible that the process like, C 2H 5N 2C 2H 5 • CH3 + CH2N2C2H5 (IV-3) 68 c o u l d be the p r i m a r y s t e p i n t h e d e c o m p o s i t i o n due t o t h e c o m p a r a t i v e l y h i g h c a r b o n - c a r b o n bond energy. There i s e v i d e n c e i n t h e l i t e r a t u r e , i n c l u d i n g t h e p r e s e n t work, t h a t t h e t h e r m a l d e c o m p o s i t i o n o f a z o a l k a n e s i n v o l v e s f r e e r a d i c a l s . I f t h e n i t r o g e n were o n l y formed i n t h e p r i m a r y p r o c e s s , t h e n t h e r a t e o f t h e i n i t i a t i o n r e a c t i o n c o u l d be a s s o c i a t e d w i t h t h e f o r m a t i o n o f n i t r o g e n . However, t h i s i s n o t t h e case i n azoethane d e c o m p o s i t i o n . N i t r o g e n can be pr o d u c e d by o t h e r s e c o n d a r y r e a c t i o n s w h i c h w i l l be c o n s i d e r e d i n su b s e q u e n t s e c t i o n s . The s e c o n d a r y r e a c t i o n s o c c u r r i n g i n AE p y r o l y s i s must be i n i t i a t e d by t h e e t h y l r a d i c a l s p r o d u c e d i n t h e p r i m a r y d e c o m p o s i t i o n . E t h y l r a d i c a l s can r e a c t w i t h a zoethane i n d i f f e r e n t ways l e a d i n g t o t h e p r o d u c t i o n o f o t h e r r a d i c a l s w h i c h f u r t h e r r e a c t , making t h e o v e r a l l r e a c t i o n q u i t e complex. Hydrogen A b s t r a c t i o n by E t h y l R a d i c a l s E t h y l r a d i c a l s , i n p r i n c i p l e , can a b s t r a c t h ydrogen atoms from a o r 3 carbon atoms o f a z o e t h a n e , a c c o r d i n g t o t h e r e a c t i o n s SECONDARY REACTIONS IN THE THERMAL DECOMPOSITION OF AZOETHANE C 2 H 5 + H-CH2CH2N2C2H5 C 2 H 6 + CH2CH2N2C2H5 (IV-4) C 2 H 5 + CH3CHN2C2H5 C 2 H 6 + CH3CHN2C2H5 (IV-5) H The r a d i c a l CH 3CHN=NC2H 5 can be w r i t t e n i n v a l e n c e bond terms as t h e f o l l o w i n g two r e s o n a n c e s t r u c t u r e s . CH 3CH-N=N-C 2H5 f CH 3CH=N-N-C 2H 5 69 The resonance stabilisation of the resulting radical suggests the preferen-tial abstraction of a hydrogen atoms from azoethane. There are few thermochemical data available for azo compounds to provide a quantitative estimate of the resonance energy involved, but a reasonable resonance stabilisation energy can be expected. This resonance stabilised radical can react in different ways which will be discussed in later sections. The abstraction of hydrogen atoms by ethyl radicals from unreacted azoethane must be considered to be the main process responsible for the formation of ethane. As the rate of formation of ethane is frequently higher than twice the rate of nitrogen formation, it is evident that the ethyl radicals generated in the primary step are not alone responsible for the total amount of ethane formed. Ethyl radicals must originate to some extent from chain carrying reactions. The activation energy for the abstraction reaction of hydrogen atom from azoethane by ethyl radicals, in the low intensity photolysis, has been found to be 8.0 * 0.2 kcal mole 9 by Cerfontain and Kutschke. Addition of Ethyl Radicals to Azoethane Ethyl radicalsj besides abstracting hydrogen from AE molecule, can add to its double bond. C 2H 5 + C2H5-N=Nr-C2H5 • C2H5-N-N (C2H5) 2 C 2H 5 + C2H5-N-N(C2H5)2 • (C2H5)2N-N(C2H5)2 The activation energy for this addition reaction has been found to be 6.0 * - l 9 0.3 kcal mole from the low intensity photolysis. The reverse decomposition of C2H5-N-N(C2H5)2 radicals will depend on the possibilities for the distribution of the excess energy into different degrees of freedom and on the deactivation by collisions. The life time of 70 t h e s e r a d i c a l s w o u l d be q u i t e s h o r t and t h e y c o u l d decompose, a c c o r d i n g t o C 2 H 5 - N - N ( C 2 H 5 ) 2 • C 2H 5-N + N ( C 2 H 5 ) 2 The d i e t h y l amino r a d i c a l s o r monoethyl amino d i r a d i c a l s c o u l d f u r t h e r combine w i t h e t h y l r a d i c a l s o r a b s t r a c t h y d r o g e n from azoethane t o g i v e t r i e t h y l amine o r d i e t h y l amine. These p r o d u c t s were, i n f a c t , d e t e c t e d i n t h e r e a c t i o n m i x t u r e u s i n g mass s p e c t r o m e t r y . Two o f t h e r a d i c a l s , C 2 H 5 ^ N - N ( C 2 H 5 ) 2 , can a l s o combine t o g i v e h e x a e t h y l t e t r a z a n e as g i v e n below. 2 C 2 H 5 - N = N ( C 2 H 5 ) 2 • C 2 H 5 - N - N ( G 2 H 5 ) 2 ( C 2 H 5 ) 2 N - N C 2 H 5 A t t h e t e m p e r a t u r e s used i n t h e p r e s e n t i n v e s t i g a t i o n , h e x a e t h y l t e t r a z a n e w i l l be q u i t e u n s t a b l e and must decompose g i v i n g o t h e r p r o d u c t s . S i m i l a r compounds have been d e t e c t e d i n t h e p h o t o l y s i s o f azomethane a t h i g h e x t e n t s o f r e a c t i o n by o t h e r w o r k e r s . ^ T e t r a e t h y l h y d r a z i n e formed by t h e a d d i t i o n o f two e t h y l r a d i c a l s t o a z o e t h a n e w i l l a l s o be q u i t e u n s t a b l e a t t h e t e m p e r a t u r e s o f t h e p y r o l y s i s due t o a weak n i t r o g e n - n i t r o g e n bond and w i l l decompose, a c c o r d i n g t o ( C 2 H 5 ) 2 N - N ( C 2 H 5 ) 2 • 2 ( C 2 H 5 ) 2 N The d i e t h y l amino r a d i c a l s can e i t h e r add t o e t h y l r a d i c a l s t o g i v e t r i e t h y l amine o r a b s t r a c t hydrogen from azoethane t o g i v e d i e t h y l amine. As m e n t i o n e d e a r l i e r , ; t h e s e two compounds were d e t e c t e d i n t h e p r o d u c t s o f p y r o l y s i s o f t h i s r e a c t i o n . 43 • I t has been s u g g e s t e d t h a t ( C 2 H 5 ) 2 N r a d i c a l s might decompose, a c c o r d i n g t o 71 (C2H5)2N • CH3 + C2H5-N=CH2 The methyl radicals formed in this way will either abstract hydrogen or combine with ethyl radicals, which are present in excess, to give methane and propane, respectively. Methane and propane have been found in this study and the significane of this and other reactions resulting in the formation of methane and propane will be discussed in later sections. There is a likelihood that C2H5-N=CH2 will polymerise giving higher molecular weight products. Reactions of Resonance Stabilised Radical CH3-CHN=N-C2H5 It was discussed earlier that the hydrogen abstraction from AE would give rise to the following two radicals. CH2-CH2-N=N-C2H5 CH3-CH-N=N-C2H5 (I) (H) It is reasonable to assume that the abstraction reaction will predominantly lead to the formation of radical II due to its resonance stabilisation. 14 15 In the pyrolysis of diethyl mercury ' a similar compound to azoethane, in the temperature interval of 250 to 300°C. in a static system, equal amounts of ethane and ethylene were observed in the products within experimental error. Abstraction of hydrogen from diethyl mercury by ethyl radicals will give rise to the radicals listed below. CH2CH2-Hg-C2H5, CH3CH-Hg-C2H5 Neither of the resulting radicals are resonance stabilised due to the absence of a double bond in diethyl mercury molecule, and hence these two radicals can readily decompose, according to 72 C H 2CH 2HgC 2H 5 C2Hk + Hg + C 2 H 5 CH 3CHHgC 2H 5 C 2 H 4 + Hg + C 2 H 5 These two p r o c e s s e s w i l l l e a d t o t h e f o r m a t i o n o f e q u a l amounts o f C 2 H 6 and C 2H ( +. The r a t i o C 2Hi i/C ( +Hio was g r e a t e r t h a n 0.12, t h e u s u a l l y a c c e p t e d v a l u e ' f o r t h i s r a t i o , hence a b s t r a c t i o n by e t h y l r a d i c a l s f r o m d i e t h y l m e r c u r y , f o l l o w e d by t h e d e c o m p o s i t i o n o f t h e r e s u l t i n g r a d i c a l , was o c c u r r i n g . The absence o f methane and propane from t h i s r e a c t i o n m i x t u r e i n d i c a t e s t h a t m e t h y l r a d i c a l s a r e n o t p r o d u c e d t o any a p p r e c i a b l e e x t e n t . As t h e r e a r e c o n s i d e r a b l e d i f f e r e n c e s i n t h e y i e l d s o f e t h a n e , e t h y l e n e and n-butane f r o m azoethane as compared w i t h H g ( C 2 H s ) 2 i t i s e v i d e n t t h a t t he hydrogen a b s t r a c t i o n f r o m A E i s much e a s i e r t h a n i n d i e t h y l mercury. T h i s may be due t o two r e a s o n s . One o f t h e s e c o u l d be t h a t t h e c i s -. t r a n s c o n f o r m a t i o n o f azoethane m o l e c u l e makes i t e a s i e r f o r t h e a hydrogen t o be p u l l e d away, t h u s i n c r e a s i n g t h e s t e r i c f a c t o r . The o t h e r f a c t o r , i . e . r e s o n a n c e s t a b i l i s a t i o n o f t h e r e s u l t i n g r a d i c a l CH3CH-N 2-C 2H 5, would l o w e r t h e a c t i v a t i o n e n e r g y . Both e f f e c t s w o u l d i n c r e a s e t h e r a t e o f a b s t r a c t i o n . • • D e c o m p o s i t i o n and C o m b i n a t i o n o f CH3CH-N=N-C9Hc; R a d i c a l T h i s r a d i c a l can decompose i n v a r i o u s ways. F o l l o w i n g two r e a c t i o n s a p p e a r most p r o b a b l e . CH 3CH-N=N-C 2H 5 C2HL» + N 2 + C 2 H 5 CH 3CH-N=N-C 2H 5 C 2 H 4 N 2 + C 2 H 5 D i a z o e t h a n e fbrmed i n t h e se c o n d r e a c t i o n can f u r t h e r decompose o r p o l y m e r i s e t o g i v e o t h e r p r o d u c t s . The d e c o m p s o s i t i o n o f 73 CHgCH-N=N-C2Hs radical to give ethylene will be greatly increased at higher temperatures. Such a behaviour has been observed in the present investi-gation. The ethyl radicals formed in this way propagate the chains making the reaction quite complex. Since the CH3CHN=NCH2CH3.can be expected to be relatively unreactive because of a reasonable degree of resonance stabilisation, the steady state concentration of these radicals will be high. Therefore, chain termination should occur partly by the combination of these radicals either with each other 2 C^Hi+N^Hs • CH 3 -CH-N»N-C 2 H 5 C2H5-N=N-HC-CH3 or with other radicals in the system. The cyclisation of dimer could be responsible for the polymers found in the present study. These high molecular weight compounds will contain excess nitrogen. A shortage of nitrogen in the mass balance has been pointed out in Chapter III. Trans-butene-2 has been detected in the early stages of the reaction at higher pressures of azoethane. As carbon-nitrogen bonds are quite weak in the dimer of the radical C2H1TN2C2H5, it appears that dimer will decompose to give nitrogen, ethyl radicals arid trans-butene-2. Moreover, it is reasonable to assume that trans-butene-2 formed by disporportionation of n-butyl radicals will be quite negligible; The dimer can also decompose to give other products at the temperatures of the pyrolysis. Addition of Ethyl Radical to CH3CH-N=N-C2Hs Radical As the relative concentration of the later radical is expected to be high, its reactions with ethyl radicals, which are present in large proportion, are bound to take place. Two plausible reactions are written below. 74 i i C 2 H 5 + CH3CH-N=N-C2H5 • CH3CH-N=N-C2H5 C 2 H 5 • C2H5CH-N=N-C2H5 + CH3 Ethyl radicals can add on' to CH3CH-N=N-C2H5 radicals with the elimination of CH3 radical, which can further end up as methane molecules. The ethyl-butyl diimide formed in the first reaction will be relatively unstable as compared to azoethane and will decompose CitHgN^Hs • Ci+Hc, + N2 + C 2 H 5 The C ^ H g radical formed in the above reaction can further decompose as Ci+Hg >• CH3 + C3Hg ^ C2HII+ C 2H5 As no hydrogen has been detected in the products, the radicals produced can not decompose to give hydrogen atoms and the corresponding olefin. The detailed variation of products as a function of pressure and temperature will be taken up later. MECHANISM OF THE THERMAL DECOMPOSITION OF AZOETHANE The reactions discussed earlier have been incorporated in the following sequence of reaction steps to account for the nature and distri-bution of products in the thermal decomposition of azoethane. Initiation Reaction: C 2 H 5 N 2 C 2 H 5 • C 2 H 5 N 2 + G2H5 C2H5N2 -2- • C 2 H 5 + N2 Radical-Molecule Reactions: C 2 H 5 + AE CH 3 + AE C 4 H 9 + AE C 3 H 7 + AE (C 2 H 5 ) 2 N + AE C 2 H 5 + AE Radical Decomposition Reactions: (C 2 H 5 ) 2 N Ci|Hg Radical-Radical Reactions: 2 C 2 H 5 CH 3 + C 2 H 5 CH 3 + CH 3 C2H5 + C ^ N ^ H s CH 3 + C 2 H 4 N 2 C 2 H 5 2 C 2 H l t N 2 C 2 H 5 C2H5 + ( C 2 H 5 ) 2 N 2 C 2 H 5 C2H5 + (C 2 H 5 ) 2 N C2H5 + C2HLJN2C2H5 CHi* + C2H1+N2C2H5 CitM 1 0 + C 2 H t t N 2 C 2 H 5 C 3 H e + C2Ui^202H5 CC 2H 5) 2NH + C 2H4N 2 C 2 H 5 8 —»• • (C 2 H5)2N 2C2H5 9 —>- C2H1+ + N 2 + C 2 H 5 10 —+ C2H4N2 + C2H5 11 — CH 3 + C2H5N=CH2 12 • C3H5 + CH3 12' • .C2H5 + C2Hi + 13 C2HL> + C 2 H 6 14 15 C 3 H 8 15 ' C 2 H 6 16 Ci t H 9 N2C 2 H5 17 —+• C 3 H6N 2 C 2 H 5 + CH 3 118 —> C 3 H 7 N 2 C 2 H 5 19, —> (C 2 HuN 2 C2H 5 )2 20 —> ( C 2 H 5 ) 2 N 2 ( C 2 H 5 ) 2 21 —• (C 2H 5) 3N 76 Decomposition Reactions: ( C 2 H 5 ) 2 N 2 ( C 2H5) 2 2 2 > 2 (C 2 H 5 ) 2 N C 4 H 9 N 2 C 2 H 5 — — — „ ^ H g + N z + c 2 H 5 C 3 H 7 N 2 C 2 H 5 — — — • C 3 H 7 + N 2 + C 2 H 5 Additional reactions will add more complexity to the already complex sequence of reactions. The proposed mechanism has the important characteristics mentioned below. The thermal decomposition of azoethane is assumed here to proceed by a complex free radical mechanism. The chains are not very long, but secondary reactions are numerous. Ethyl radicals are produced in the pyrolysis- by different reactions. A large number of reactions have been included in the mechanism to account for the various products of this reaction. The major features of the reaction scheme presented above will be justified on the basis of various experimental findings. The rate constants for the disappearance of azoethane and formation of products were not obtained because the orders calculated,from the experimental results were non-integral. The orders of formation of various products calculated from the data are reported in Table 10. The proposed mechanism reasonably accounts for the observed orders of formation of various products. The ratio of ethyl radical disporportionation to combination is accepted to be 0.12, independent of temperature. The values obtained for this ratio by various workers using different compounds containing ethyl group are reported in Table 19. Since the ratio C^Hit/CitHio was always observed to be greater than 0.12 at all temperatures in the pyrolysis of azoethane, the.excess — O O O— n-butane © @ Methane 80 e t h y l e n e must a r i s e by r e a c t i o n (9) o f t h e r e a c t i o n scheme. The g r e a t e r p a r t o f ethane comes from a b s t r a c t i o n r e a c t i o n ( 3 ) . As a b s t r a c t i o n r e a c t i o n i s dominant i n azoethane p y r o l y s i s , t h e h i g h e r o r d e r f o r t h e f o r m a t i o n o f ethane compared w i t h t h a t o f n-butane i s q u i t e e x p e c t e d . The f o r m a t i o n o f butane by a r e a c t i o n s u c h as C 2 H 5 .+ AE • CifH.io + N 2 C 2 H 5 would p a r t i a l l y a c c o u n t f o r t h e o b s e r v e d o r d e r o f 0.8 (See T a b l e 1 0 ) . The o r d e r o f butane f o r m a t i o n s h o u l d be s m a l l e r i f e t h y l r a d i c a l c o m b i n a t i o n i s 49 the o n l y s o u r c e o f i t s p r o d u c t i o n . However, P r a t t and P u r n e l l have d i s c o u n t e d t h i s r e a c t i o n i n t h e p y r o l y s i s o f t e t r a e t h y l l e a d on t h e b a s i s o f a s i m i l a r o r d e r o f bu t a n e f o r m a t i o n found by them. R e a c t i o n (11) and (17) have been p r o p o s e d f o r t h e f o r m a t i o n o f me t h y l r a d i c a l s . The m a j o r i t y o f t h e m e t h y l r a d i c a l s e i i d up as methane i n an a l o g y t o e t h y l r a d i c a l s and etha n e . The o r d e r o f f o r m a t i o n o f methane w i t h r e s p e c t t o azoethane i s h i g h e r t h a n o t h e r p r o d u c t s i n d i c a t i n g p r e s s u r e dependent f o r m a t i o n o f m e t h y l r a d i c a l s . The o r d e r o f d i s a p p e a r a n c e o f azoethane i s f o u n d t o be 1.2 a t 271.5°C. T h i s i s q u i t e e x p e c t e d as t h e o r d e r s o f f o r m a t i o n o f v a r i o u s p r o d u c t s , p r o d u c e d from azoethane a r e g r e a t e r t h a n u n i t y t o o . The o v e r a l l o r d e r s f o u n d by p r e s s u r e measurements a l s o s u p p o r t t h e p r o p o s e d mechanism. As t h e p r o p o s e d mechanism i n c l u d e s a g r e a t many r e a c t i o n s , t h e p o s s i b i l i t y o f a p p l y i n g a complete s t e a d y s t a t e t r e a t m e n t t o t h e mechanism i s p r e c l u d e d . I t does not seem r e a s o n a b l e t o make many d u b i o u s a s s umptions about complex r e a c t i o n s i n o r d e r t o do so m e t h i n g q u a n t i t a t i v e . We f e e l t h a t any q u a n t i t a t i v e t r e a t m e n t i n v o l v i n g a s s u m p t i o n s i n t h e t h e r m a l d e c o m p o s i t i o n o f azoethane w i l l n o t y i e l d any m e a n i n g f u l r e s u l t s e x c e p t 81 h a v i n g a good e x e r c i s e i n " a p p r o x i m a t i o n s " . F i g u r e s 24, 25, and 26 show t h e p l o t s o f t h e i n i t i a l r a t e o f f o r m a t i o n o f t h e major p r o d u c t s as a f u n c t i o n o f i n i t i a l a zoethane p r e s s u r e a t 295°C. A l l p r o d u c t s show dependence o f t h e i r i n i t i a l r a t e s o f f o r m a t i o n on azoethane p r e s s u r e s , b u t each t o a d i f f e r e n t e x t e n t . However, when t h e i n i t i a l r a t e s o f p r o p y l e n e and propane ( r e p o r t e d i n T a b l e 9) at 271.5°C. i n the unpacked v e s s e l a re p l o t t e d , t h e dependence i s more pronounced. The shape o f t h e c u r v e s a t 271.5°C. l e a d s . t o t h e c o n c l u s i o n t h a t t h e o r d e r s f o r t h e f o r m a t i o n o f propane and p r o p y l e n e s h o u l d be g r e a t e r t h a n u n i t y . The o r d e r s c a l c u l a t e d from the d a t a a r e r e p o r t e d i n T a b l e 10. The h i g h o r d e r s o b s e r v e d f o r p r o p y l e n e and propane may be a t t r i b u t e d t o t h e i r i n a c c u r a t e a n a l y s i s because t h e s e p r o d u c t s a r e r e l a t i v e l y formed i n s m a l l amounts i n t h e i n i t i a l s t a g e s o f t h e r e a c t i o n . As e t h y l e n e a r i s e s from r e a c t i o n s (9) and (13) p o s t u l a t e d i n t h e mechanism, an i n c r e a s e i n tempera-t u r e w i l l i n c r e a s e t h e r a t e o f r e a c t i o n (9) as compared t o t h a t o f the e l i m i n a t i o n r e a c t i o n ( 1 7 ) . Thus, t h e r a t i o C^H^/CH^, would be h i g h e r at h i g h e r t e m p e r a t u r e s and would t e n d t o i n c r e a s e as t h e p r e s s u r e o f AE i s i n c r e a s e d due t o an i n c r e a s e d r a t e o f a b s t r a c t i o n by m e t h y l r a d i c a l s . The p l o t o f t h i s r a t i o i s shown i n F i g u r e 28 a t two d i f f e r e n t t e m p e r a t u r e s . The c u r v e has the e x p e c t e d t r e n d . The r a t i o C 2 H i t / N 2 > shown i n F i g u r e 29, does n o t have a marked dependence on i n i t i a l a z o e t h a n e p r e s s u r e . N i t r o g e n i s p r o d u c e d m a i n l y by r e a c t i o n s (2) and (9) whereas e t h y l e n e a r i s e s by r e a c t i o n s (9) and (13) o f t h e p o s t u l a t e d r e a c t i o n scheme. An i n c r e a s e i n t e m p e r a t u r e w i l l enhance th e r a t e o f a l l t h e s e above r e a c t i o n s , r e l a t i v e l y , and th u s t h e r a t i o C 2 H i + / N 2 as a f u n c t i o n o f AE p r e s s u r e w i l l have t h e same s m a l l s l o p e a t two d i f f e r e n t t e m p e r a t u r e s w i t h i n e x p e r i m e n t a l e r r o r . 82 The CzHg/CzHit r a t i o , p r e s e n t e d i n F i g u r e 31 a l s o does not depend ma r k e d l y on azoethane p r e s s u r e s . The v a l u e o f t h i s r a t i o i s h i g h e r a t 2 7 1 . 5 ° C . t h a n at 295°C. due t o t h e i n c r e a s e d r a t e o f f o r m a t i o n o f C 2 H 4 by r e a c t i o n (9) a t h i g h e r t e m p e r a t u r e s . I t can be seen from F i g u r e 27 t h a t t h e r a t i o CzHi+ZCi+Hio i s c o n s i d e r a b l y h i g h e r t h a n 0.12, t h e v a l u e u s u a l l y a c c e p t e d f o r t h i s r a t i o (see T a b l e 19) when bu t a n e and e t h y l e n e a r e pr o d u c e d o n l y by c o m b i n a t i o n and d i s p r o p o r t i o n a t i o n o f the e t h y l r a d i c a l s . T h i s r a t i o tends t o i n c r e a s e a t h i g h azoethane p r e s s u r e s , p r o b a b l y , b e c a u s e t h e r e l a t i v e r a t e s o f f o r m a t i o n and d e c o m p o s i t i o n o f t h e CzHt^CzHs r a d i c a l w i l l t h e n be enhanced. The l o w e r v a l u e o f t h i s r a t i o o b s e r v e d a t h i g h e r t e m p e r a t u r e s may be due t o an i n c r e a s e d r a t e o f r e a c t i o n (5) o f t h e mechanism scheme. The r a t i o C3H8/C3H6, as d i s p l a y e d i n F i g u r e 30, i n c r e a s e s as a f u n c t i o n o f azoethane p r e s s u r e , and d e c r e a s e s w i t h i n c r e a s i n g t e m p e r a t u r e . The low v a l u e o f t h i s r a t i o a t h i g h t e m p e r a t u r e s may be a t t r i b u t e d t o t h e i n c r e a s e d r a t e o f r e a c t i o n (12). I t i s e v i d e n t from F i g u r e 22 t h a t t h e i n c r e a s e i n s u r f a c e : volume r a t i o d e c r e a s e s t h e t i m e r a t e o f p r e s s u r e change. T h i s a p p a r e n t r a t e o f p y r o l y s i s was l o w e r i n t h e p a c k e d r e a c t i o n v e s s e l by about 10 t o 20% o f the unpacked r a t e d e p e n d i n g upon the p a r t i c u l a r t e m p e r a t u r e and p r e s s u r e used. W a l l s can a f f e c t r a t e s o f b o t h i n i t i a t i o n and t e r m i n a t i o n i n a r e a c t i o n p r o c e e d i n g by r a d i c a l c h a i n s . I f t h e r a t e o f i n i t i a t i o n i s enhanced s i g n i f i c a n t l y as compared t o t e r m i n a t i o n r a t e , t h e n e t e f f e c t w i l l be an i n c r e a s e i n t h e o v e r a l l r a t e due t o p a c k i n g . I f , on t h e o t h e r hand, the r a t e s o f t e r m i n a t i o n p r o c e s s e s a r e i n c r e a s e d , t h e t o t a l e f f e c t w i l l be a de c r e a s e i n t h e o v e r a l l r a t e as measured by p r e s s u r e change. However, i n s e n s i t i v i t y o f t h e o v e r a l l r a t e t o t h e s u r f a c e / v o l u m e r a t i o does n o t e x c l u d e t h e p o s s i b i l i t y o f a he t e r o g e n e o u s c o n t r i b u t i o n t o t h e o b s e r v e d r a t e 0.6 * 9 Temperature 295°C. 30.0 50.0 70.0 90.0 110.0 p Azoethane (mm.) *• F i g u r e 27. V a r i a t i o n o f r a t i o CzHu/CttHio as a F u n c t i o n o f Azoethane P r e s s u r e . F i g u r e 28. V a r i a t i o n o f C2Hit/CHi+ r a t i o as a F u n c t i o n o f Azoethane P r e s s u r e . 0.25 Azoethane (mm.) F i g u r e 29. V a r i a t i o n o f C2HL>/N2 R a t i o as a F u n c t i o n o f I n i t i a l Azoethane P r e s s u r e . 30.0 50^0 7u7o ' 90^0 110.0 Azoethane (nun.) —• F i g u r e 30. V a r i a t i o n o f C 3 H 8 / C 3 H 6 R a t i o as a F u n c t i o n o f Azoethane P r e s s u r e . i g " r e 3 1 • V a r i a t i o n o f ^H^^H^ R a t i o as a F u n c t i o n o f I n i t i a l Azoethane P r e s s 25. 20.0__ 15.0--10.0 — ~0~ -—4-o G -A-30.0 4-Unpacked V e s s e l O Temperature 2 71.5°C Packed V e s s e l <B Temperature 271.5°C. Temperature 295°CV 50.0 70.0 90.0 p Azoethane (mm.) 1 110.0 88 T h i s w i l l happen i n c h a i n r e a c t i o n s i f c h a i n s a r e b o t h i n i t i a t e d and t e r m i n a t e d a t t h e w a l l s a t t h e same r e l a t i v e r a t e s as i n t h e gas phase. The d e c r e a s e i n t h e apparent r a t e i n t h e t h e r m a l d e c o m p o s i t i o n o f azoethane can be a t t r i b u t e d p a r t i a l l y t o an i n c r e a s e d s u r f a c e r e c o m b i n a t i o n o f r a d i c a l s . The i n i t i a l r a t e s o f v a r i o u s p r o d u c t s a r e a f f e c t e d by p a c k i n g b u t t o d i f f e r e n t degrees as can be seen from T a b l e 9. I t seems r e a s o n a b l e t o assume t h a t a r a d i c a l - r a d i c a l r e a c t i o n such as 2 C 2H4N 2C2H5 • ( C 2 H i r N 2 C 2 H 5 ) 2 ( a) wo u l d have a h i g h e r r a t e o f c o m b i n a t i o n i n t h e pa c k e d c e l l . The r a t e s o f t h e r e a c t i o n s C2H4N2C2H5 • C2Hk + N 2 + C 2 H 5 *• C 2Hi tN2 + C 2Hg r e l a t i v e t o r e a c t i o n (a) w o u l d p r o b a b l y be d e c r e a s e d , t h u s l e a d i n g t o t h e o b s e r v e d l o w e r r a t e o f f o r m a t i o n o f ethane i n t h e pa c k e d r e a c t i o n v e s s e l . I t w ould be dangerous t o p r o l o n g t h e d i s c u s s i o n because o f a l i m i t e d s t u d y on t h i s a s p e c t o f the r e a c t i o n . However, t h i s i n v e s t i g a t i o n does p o i n t out t h a t t h e t h e r m a l d e c o m p o s i t i o n o f azoe t h a n e p r o c e e d s by a complex f r e e r a d i c a l mechanism i n v o l v i n g a t l e a s t a s h o r t c h a i n . The r e s u l t s o b t a i n e d on t h e i n i t i a l r a t e s o f f o r m a t i o n o f v a r i o u s p r o d u c t s i n t h e p r e s e n c e o f d i f f e r e n t added gases such as bu t e n e s and carbon d i o x i d e a t v a r i o u s t e m p e r a t u r e s a r e r e p o r t e d i n T a b l e s 14 and 16. I t s h o u l d be n o t e d from t h e s e T a b l e s t h a t t h e a d d i t i o n o f c i s - b u t e n e - 2 , t - b u t e n e - 2 , b u t e n e - 1 (see T a b l e 13) o r c a r b o n d i o x i d e has a common e f f e c t o f l o w e r i n g t h e i n i t i a l r a t e o f f o r m a t i o n o f ethane a t a l l t h e t e m p e r a t u r e s u s e d i n t h i s s t u d y . The i n i t i a l r a t e s o f f o r m a t i o n o f n i t o r g e n a r e s l i g h t l y i n c r e a s e d . I n t h e case o f added b u t e n e , t h e r a t e o f f o r m a t i o n off p r o p y l e n e 89 was always f o u n d t o be g r e a t e r t h a n t h e r a t e o f f o r m a t i o n o f propane The i n c r e a s e d r a t e o f f o r m a t i o n o f p r o p y l e n e compared t o propane c o u l d be due t o t h e d e c o m p o s i t i o n o f butenes as • CH 3-CH=CH-CH 3 • CH 3 + CH=CH-CH3 or CH 3CH 2CH=CH 2 • CH 3 + CH 2-CH=CH 2 and f o l l o w e d by subsequent a b s t r a c t i o n o f hydrogen by CH= CH-CH 3 o r CH 2-CH=CH 2 r a d i c a l s from azoethane o r bute n e s t o g i v e p r o p y l e n e . The , r a t e s o f r e a c t i o n w i l l n o t be v e r y h i g h a t t h e t e m p e r a t u r e s o f t h e p y r o l y s i s . The i n c r e a s e d r a t e o f f o r m a t i o n o f CH^ i n t h e p r e s e n c e o f buten e s can a l s o be a t t r i b u t e d p a r t i a l l y t o t h e o c c u r r e n c e o f t h e r e a c t i o n . The m e t h y l r a d i c a l s formed i n t h i s r e a c t i o n can p a r t i c i p a t e i n v a r i o u s p r o c e s s e s and one o f t h e r e a c t i o n s w i l l be t h e a b s t r a c t i o n o f hydrogen atom from azoethane o r fr o m butene i t s e l f . However, t h e i n c r e a s e d r a t e o f f o r m a t i o n o f p r o p y l e n e can a l s o be a t t r i b u t e d t o t h e a d d i t i o n o f e t h y l r a d i c a l t o t h e doub l e bond i n b u t e n e s , and t h e n t h e subsequent e l i m i n a t i o n o f p r o p y l e n e from t h e r e s u l t e d r a d i c a l . The r a t e s o f f o r m a t i o n o f o t h e r p r o d u c t s a r e a l s o a f f e c t e d b u t each t o a v a r y i n g degree. The v a r i a t i o n o f r a t i o C 2 H 6 / N 2 as a f u n c t i o n o f added butene a t 271.5°C, a t an i n i t i a l p r e s s u r e o f 35.5 mm. Hg o f azoe t h a n e i s shown i n F i g u r e 33. The c u r v e has a u s u a l shape o f i n h i b i t i o n p l o t s w i t h added o l e f i n s . 6 As t h e v a l u e o f r a t i o C 2 H i + / C i t H i o i s always f o u n d t o be g r e a t e r t h a n 0.12 (see F i g u r e 2 7 ) , i t i s e v i d e n t t h a t t h e r e a c t i o n (9) o f t h e mechanism scheme does o c c u r a t an a p p r e c i a b l e r a t e . From the p r e v i o u s arguments i t may be c o n c l u d e d t h a t t h e r e i s a c h a i n i n azoe t h a n e p y r o l y s i s , p r o b a b l y due t o t h e f o r m a t i o n and subsequent d e c o m p o s i t i o n o f t h e 2.0" o 1.0 - -o.o 75!.0 150.0 225.0 added c i s - B u t e n e - 2 (mm. Hg) F i g u r e 32. V a r i a t i o n o f C2H6/N2 R a t i o as a F u n c t i o n o f Added C i s - B u t e n e - 2 at 271.5°C; I n i t i a l P r e s s u r e o f A zoethane = 35. 5 mm.. Hg. 91 C 2H 4N 2C2H5 r a d i c a l . The o b s e r v e d l o w e r i n g i n t h e r a t e o f f o r m a t i o n o f ethane ( F i g u r e 33) i s c o n s i s t e n t w i t h such an i n t e r p r e t a t i o n . R e d u c t i o n i n t h e r a t e o f f o r m a t i o n o f ethane i m p l i e s the s c a v e n g i n g o f C2H4N2C2H5 r a d i c a l o r i t s p r e c u r s o r C2H5 r a d i c a l . As most o f ethane comes by a b s t r a c t i o n o f hydrogen from azoethane by e t h y l r a d i c a l s , i t i s q u i t e d i f f i c u l t t o d e c i d e unambiguously as t o wh i c h o f t h e r a d i c a l s from C 2 H i +N 2C 2H5 and C 2H 5 i s removed s e l e c t i v e l y by t h e b u t e n e s . The i n t e r f e r e n c e o f t h e o l e f i n i n t h e r a t e o f f o r m a t i o n o f ethane depends upon t h e mode o f a c t i o n o f but e n e s as i n h i b i t o r s . F o r s t and R i c e ^ have d i s c u s s e d the a c t i o n o f e t h y l e n e , p r o p y l e n e and n i t r i c o x i d e on t h e 45 t h e r m a l d e c o m p o s i t i o n o f azomethane around 300 C. L a i d l e r e t a l have s u g g e s t e d a t h e o r y f o r p r o p y l e n e i n h i b i t i o n , a c c o r d i n g t o wh i c h t h e i n h i b i t i o n o f p r o p y l e n e i s ca u s e d by hydrogen a b s t r a c t i o n by t h e a l l y l r a d i c a l . A s i m i l a r s o r t o f argument can be advanced f o r butene i n h i b i t i o n . 45 T h i s t h e o r y w i l l n o t be a p p l i c a b l e i n t h e case o f azoethane d e c o m p o s i t i o n s i n c e t h e t e m p e r a t u r e s u s e d i n t h i s s t u d y a r e much l o w e r t h a n t h e tempera-t u r e s f o r w h i c h t h i s t h e o r y has been s u g g e s t e d . 12 C l a r k e t a l have c o n c l u d e d from a s t u d y o f t h e t h e r m a l decom-p o s i t i o n o f azoethane t h a t t h e r a t e c o n s t a n t f o r the removal o f azoethane does n o t change i n t h e p r e s e n c e o f a p p r e c i a b l e amounts o f t o l u e n e , and th u s t h i s r e a c t i o n does n o t i n v o l v e any c h a i n s . However, F o r s t and R i c e ^ o b s e r v e d a s i m i l a r e f f e c t o f t o l u e n e on t h e r a t e c o n s t a n t f o r t h e f o r m a t i o n o f n i t r o g e n . However, t h e r a t e c o n s t a n t was l o w e r e d i n t h e p r e s e n c e o f added o l e f i n s and n i t r i c o x i d e . The c r i t e r i o n o f no i n h i b i t i o n by t o l u e n e does n o t mean n e c e s s a r i l y t h e absence o f a s h o r t c h a i n i n azoethane p y r o l y s i s as p u t f o r w a r d by C l a r k e t a l . I t w o u l d t h u s appear t h a t at t h e t e m p e r a t u r e s used i n t h i s i n v e s t i -92 g a t i o n , b u t e n e s m a i n l y engage i n r emoving r a d i c a l s . The p r o c e s s o f i n h i b i t i o n by b u t e n e s i s c e r t a i n l y a complex phenomenon and we f e e l t h a t i t w i l l be more a p p r o p r i a t e t o c a l l t h e s e o l e f i n s " c o m p l i c a t o r s " i n s t e a d o f i n h i b i t o r s . N i t r i c o x i d e has been u s e d as an i n h i b i t o r f o r a l o n g time and 46 47 v a r i o u s t h e o r i e s have been advanced t o a c c o u n t f o r i t s mode o f a c t i o n . ' As the t h e r m a l d e c o m p o s i t i o n o f azoethane w i t h and w i t h o u t added b u t e n e s i s complex, i t was d e c i d e d n o t t o put n i t r i c o x i d e i n t h i s s y s tem. I t would p r o b a b l y add more c o m p l e x i t y t o an a l r e a d y complex r e a c t i o n . Carbon d i o x i d e , w h i c h was u s e d as an i n e r t g a s , a l s o had t h e e f f e c t o f l o w e r i n g t h e r a t e o f f o r m a t i o n o f e t h a n e . The y i e l d s o f o t h e r p r o d u c t s were a l s o a f f e c t e d b u t n o t t o any s i g n i f i c a n t e x t e n t . I t i s q u i t e an u n u s u a l r e s u l t i n t h e sense t h a t a s u p p o s e d l y " i n e r t " gas has a s i m i l a r e f f e c t t o b u t e n e on t h e r a t e o f f o r m a t i o n o f e t h a n e . One i s tempted t o i n v o k e " h o t " e t h y l r a d i c a l p a r t i c i p a t i o n i n t h i s t h e r m a l r e a c t i o n t o e x p l a i n t h i s phenomenon b u t t h e o t h e r e x p e r i m e n t a l d a t a are n o t c o n s i s t e n t w i t h t h i s i n t e r p r e t a t i o n . However, i t i s p o s s i b l e t h a t c a r b o n d i o x i d e i s n o t b e h a v i n g l i k e an i n e r t gas i n t h i s t h e r m a l d e c o m p o s i t i o n . Thus, no q u a n t i t a t i v e c o n c l u s i o n s about t h e c h a i n l e n g t h can be drawn from t h e d a t a . However, o u r r e s u l t s do i n d i c a t e t h e p r o b a b l e e x i s t e n c e o f a s h o r t c h a i n i n azoethane p y r o l y s i s . I t i s o b v i o u s from t h e p r o p o s e d mechanism t h a t t h e n i t r o g e n i s formed by v a r i o u s r e a c t i o n s . However, n i t r o g e n f o r m a t i o n o t h e r t h a n by r e a c t i o n s (2) and (9) w i l l be q u i t e s m a l l i n t h e i n i t i a l s t a g e s o f t h e r e a c t i o n . I t i s p o s s i b l e t o c a l c u l a t e t h e r a t e o f f o r m a t i o n o f n i t r o g e n by r e a c t i o n (9) i f we assume t h a t b u t a n e a r i s e s t o t a l l y f r o m e t h y l r a d i c a l r e c o m b i n a t i o n . The r a t e o f f o r m a t i o n o f e t h y l e n e a r i s i n g f rom 93 e t h y l r a d i c a l d i s p r o p o r t i o n a t i o n w i l l t h e n be e q u a l t o 0.12 x r a t e o f f o r m a t i o n o f b u t a n e . By s u b s t r a c t i n g t h i s c a l c u l a t e d r a t e from t h e t o t a l r a t e o f f o r m a t i o n o f e t h y l e n e one can a r r i v e a t t h e r a t e o f f o r m a t i o n o f e t h y l e n e , and hence t h e r a t e o f f o r m a t i o n o f n i t r o g e n , by r e a c t i o n (9). A l o g a r i t h m i c p l o t o f t h e r e s u l t i n g e x p r e s s i o n K * i x, " (K * i n u - 0.12 x R r u ) = k x (AE) n t o t a l N 2 t o t a l C2Hi+ C I ^ Q .• . y i e l d s t h e v a l u e o f n e q u a l t o 1.1. The a c t i v a t i o n e nergy f o r n i t r o g e n f o r m a t i o n has been c a l c u l a t e d t o be 47.2 ± 1.0 k c a l p e r mole. T h i s a c t i v a t i o n e n e r g y can be r e g a r d e d as t h a t o f t h e i n i t i a t i o n r e a c t i o n , w i t h i n e x p e r i m e n t a l e r r o r . The f o l l o w i n g e x p r e s s i o n can be w r i t t e n f o r t h e r a t e o f f o r m a t i o n o f n i t r o g e n by t h e i n i t i a t i o n r e a c t i o n a t 271.5 °C. at a l l p r e s s u r e s . D n r .n 1 1* -47.200/544.5 R , A r j . l . l -1 RXI = 7.5 x 10 x e (AE) mm. s e c . N 2 The a c t i v a t i o n energy f o r b u t a n e f o r m a t i o n i s h i g h e r t h a n w o u l d be e x p e c t e d i f e t h y l r a d i c a l s are a l o n e r e s p o n s i b l e f o r i t s f o r m a t i o n . T h i s r e s u l t p o i n t s o u t t h a t b u t a n e may a l s o be formed by o t h e r r e a c t i o n s . The low a c t i v a t i o n e nergy f o r ethane f o r m a t i o n as compared t o n i t r o g e n i n d i c a t e s t h e e x i s t e n c e o f a s h o r t c h a i n i n t h e a zoethane p y r o l y s i s . 94 F l a s h P h o t o l y s i s : There i s a s t r i k i n g d i f f e r e n c e i n t h e p r e s e n t and p r e v i o u s i n v e s t i -g a t i o n s on t h e f l a s h d e c o m p o s i t i o n o f azo e t h a n e . P r o d u c t s l i k e CHi+, C 3 H 6 , and C 3 H 8 were n o t d e t e c t e d by D i n g l e d y and C a l v e r t * * i n a q u a r t z v e s s e l i n t h e p r e s e n c e o f a m o d e r a t i n g gas. However, t h e r a t i o s C2Hi+/C2H6 and CzHtj/Ci+Hio were much h i g h e r t h a n t h e u s u a l l y a c c e p t e d v a l u e s o f t h e s e q u a n t i t i e s (see T a b l e 19). These a u t h o r s have i n v o k e d t h e i d e a o f t h e p a r t i c i p a t i o n o f h i g h l y e x c i t e d e t h y l r a d i c a l s i n t h e p h o t o l y s i s o f azQ-ethane a t s h o r t w a v e l e n g t h s t o e x p l a i n t h e s e r e s u l t s , as azoethane has a o s t r o n g a b s o r p t i o n band below 2300 A. The above mentioned p r o d u c t s were n o t o b s e r v e d by R o q u i t t e and F u t r e l l * ^ a l s o , whereas a s m a l l amount o f hydrogen was d e t e c t e d i n t h e f l a s h p h o t o l y s i s p r o d u c t s i n a p y r e x r e a c t i o n v e s s e l . In t h e f l a s h p h o t o l y s i s o f a z o e t h a n e , t h e p r i m a r y s t e p s w i l l be C 2 H 5 N 2 C 2 H 5 • C 2 H 5 N 2 C z H s (1) C 2H 5N 2C 2H5 + M • C 2 H 5 N 2 C 2 H 5 + M (2) C 2H 5N 2C 2H5 • C 2 H 5 + N 2 C 2 H 5 (3) f o l l o w e d by t h e d e c o m p o s i t i o n o f t h e h i g h l y u n s t a b l e fragment N 2 C 2 H 5 • N 2 + C 2 H 5 (3a) The e t h y l r a d i c a l s g e n e r a t e d i n t h e p r i m a r y p r o c e s s can r e a c t f u r t h e r as 2 C 2 H 5 • C^HJQ (4) • CzHk + C 2 H 6 (5) As t h e d u r a t i o n o f t h e f l a s h i s q u i t e s h o r t , r a d i c a l - m o l e c u l e r e a c t i o n s h a v i n g a p p r e c i a b l e a c t i v a t i o n e n e r g i e s can be n e g l e c t e d . The o n l y i m p o r t a n t r e a c t i o n s under t h e c o n d i t i o n s o f t h e f l a s h e x p e r i m e n t s w i l l be t h e r a d i c a l -r a d i c a l o r the r a d i c a l - m o l e c u l e r e a c t i o n s w i t h low a c t i v a t i o n e n e r g y . I n 95 the pyrex reaction vessel with a pyrex outer jacket, where thermally equili-brated radicals are produced even in the absence of a moderating gas, the value of the quantity of CzHe/Ci+Hio was always close to 0.12, the usually accepted value of this ratio. The abstraction reaction C 2 H 5 + C 2 H 5 N 2 C 2 H 5 -*• C2Hg + C 2 H 1 + N 2 C 2 H 5 followed by the subsequent decomposition of . C 2HilN2C2H5 >• C 2 H 5 + N 2 + C 2 H 4 does not occur at all . Highly vibrationally excited ethyl radicals would enhance the rate of reaction (5) and lower the rate of reaction (4 ) leading to a high value for the ratio C2Hi t /Ci +Hio-The addition of large amounts of the inert gas decreases the product yields significantly which can be attributed to collisional deactivation of 7 9 26 AE ' ' . Figure 33 shows the variation of product yields with added carbon dioxide from 82.5 mm. Hg of azoethane. Table 19 lists the value calculated from the experimental results for the disporportionation to combination ratio of the ethyl radicals. The obtained value is consistent with the previous estimates reported in the same Table. In the quartlz reaction vessel, other processes have to be postulated which would account for the products like CH^, C 3 H 6 and C 3 H 8 , observed in the reaction mixture. As hydrogen is one of the products", and that the € 2 ^ yield is much higher than C 2 H 6 , a "hot" excited ethyl radical is formed in the primary process due to the absorption of light of low wave lengths. This "hot" radical can decompose in various ways. Following reactions seem quite plausible. C 2 H 5 • C2H1 + + H (6) 96 TABLE 19 D i s p r o p o r t i o n a t i o n t o c o m b i n a t i o n r a t i o s o f t h e e t h y l r a d i c a l s i n t h e gas ph ase System Temp.Range °C. k d i s / k com Ref. Phot: ( C 2 H 5 ) 2 H g 75-200 0.38-0.43 26 P y r o : ( C 2 H 5 ) 2 H g 320-370 0.25 27 P h o t : EtCHO 35 0.10 28 P h o t : EtCHO 50-215 0.15 29 Phot: ( C 2 H 5 ) 2 C O 25-101 0.11 30 100-250 0.12 31 50-150 0.13 32 25-225 0.14 33 110 0.13 34 ( C H 3 C D 2 ) 2 C 0 24-138 0.10 35 50-197 0.10 36 Phot: ( C 2 H 5 ) 2 N 2 26-178 0.13 8 27-175 0.12 9 -65 - +40 0.16-0.12 37 F l a s h - P h o t : ( C 2 H 5 ) 2 N 2 22 0.11 11 25 0.117 10 Hg. ( 3 P ) - s e n s i t i z e d h y d r o g e n a t i o n o f C 2 H i | 25 0.15 38 25 0.11 39 25 0.14 40 P r e s e n t Work ll F l a s h - P h o t : •) 25 0.12*0.02 P y r o : 246.5-308.5 0.25*0.5 C 2 H 5 + C 2 H 5 • C2aH + 2 C H 3 (7) C 2 H s + C 2 H 5 • C H 3 + C 3 H 7 (8) • C u H 1 0 (9) The unimolecular decomposition of v i b r a t i o n a l l y excited ethyl r a d i c a l s i . e . , reaction (6) has been proposed by others i n the f l a s h photolysis of d i e t h y l 5 11 19 20 mercury and d i e t h y l ketone. > ' ' As n-pentane was found i n the products of t h i s reaction i n a quartz reaction v e s s e l , formation of propyl r a d i c a l s i s established. Propyl r a d i c a l s , i n the presence of excess ethyl r a d i c a l s 19 w i l l give r i s e to n-pentane. Thrush proposed reaction (8) i n the f l a s h photolysis of d i e t h y l mercury to account f o r the observed products, even without f i n d i n g n-pentane i n the reaction mixture. 20 Fischer and Mains could not detect n-pentane i n the products of diethyl mercury f l a s h photolysis and they concluded that n-propyl r a d i c a l s are not formed i n any s i g n i f i c a n t quantity under the conditions of t h e i r f l a s h experiments. Butane formed by reaction (9) w i l l further decompose to give other products, thus increasing the C 2 H i + / C i t H i o r a t i o . Reaction (10) can not be a major source of ethylene and methane i n a system of methyl and ethyl r a d i c a l s . The important reaction a r i s i n g from t h e i r i n t e r a c t i o n w i l l be reaction (11). C H 3 + C 2 H 5 • C 2 H 4 + C H 4 (10) C H 3 + C 2 H 5 • C 3 H 8 (11) The hydrogen observed i n the products can ar i s e by the reaction H + H + M • H 2 + M Such a reaction has been postulated i n the f l a s h photolysis of d i e t h y l 2.4. 9? mercury and d i e t h y l ketone. ' It should be noted that no reactions have been suggested so f a r f o r the trace amounts of products formed i n the pyrex reaction vessel without the outer pyrex jacket. Surface reactions have been proposed to account f o r butene and propylene formation i n the f l a s h photo decomposition 20 of d i e t h y l mercury. It may be. however, that the high l i g h t f l u x i n t h i s case causes some concurrent thermal decomposition. 100 CHAPTER V CONCLUSIONS AND SUGGESTIONS FOR FURTHER WORK Conclusions: It may be concluded from the present study that the thermal decomposition of azoethane proceeds by a complex free r a d i c a l mechanism inv o l v i n g a short chain, because of the formation and subsequent decomposition of CH3CH-N=N-C2H5 r a d i c a l . The p y r o l y s i s of azoethane i s not f i r s t order with respect to azoethane. However, as the nitrogen i s mainly formed by the unimolecular i n i t i a t i o n reaction, i t s order of formation with respect to azoethane i s r e l a t i v e l y close to unity. The addition reaction of ethyl r a d i c a l s to azoethane and the induced polymerisation of ethylene complicate the reaction products. The thermal decomposition of azoethane i s p a r t l y heterogeneous. The rates of formation of a l l products are affected due to packing, but each to a d i f f e r e n t extent. The addition of butenes or carbon dioxide decreases the rate of formation of ethane. In the fl a s h photolysis of azoethane thermally e q u i l i b r a t e d r a d i c a l s are produced i n the pyrex f i l t e r e d system whereas "hot" e t h y l r a d i c a l s are formed i n the quartz reaction vessel. The c o l l i s i o n a l deactivation of excited azoethane molecules i n the presence of carbon dioxide has been observed. The disproportionation to combination r a t i o of the ethyl r a d i c a l s has been found to be 0.12 ± 0.02. Hence, the f l a s h decomposition of azoethane i s a very convenient and clean source of producing and studying the reactions of ethyl r a d i c a l with other species compared to d i e t h y l mercury and d i e t h y l ketone. 101 Suggestions f o r Further Work: It i s a well known saying i n chemical k i n e t i c s that "A mechanism may be disproven but never proven". Mechanisms have been proposed f o r the p y r o l y s i s and f l a s h photolysis of azoethane, based on the experimental data, c o l l e c t e d within a reasonable period of time. To test and put the mechanisms on firm footing, a d d i t i o n a l experiments would have to be c a r r i e d out. As the hydrogen abstraction reaction i s very dominant i n azoethane p y r o l y s i s , i t would be of i n t e r e s t to investigate further the thermal decomposition of p a r t i a l l y deuterated azoethanes, such as CH3CD2N2CD2CH3 and CD3CH 2N 2CH2CD3. The analysis of the decomposition products of these could confirm or disprove many reactions suggested i n the mechanism. It i s not possible to decide unambigously from the present study about the reaction by which methyl r a d i c a l s are formed i n the p y r o l y s i s of azoethane. The study of the suggested deutero compounds would probably resolve t h i s s i t u a t i o n . By doing f l a s h photolysis of p a r t i a l l y deuterated azoethanes, i t would be possible to confirm further the fact that the head to head c o l l i s i o n of ethyl r a d i c a l s leads to the formation of butane, and that the head to t a i l c o l l i s i o n r e s u l t s i n the disproportionation of ethyl . 35, 37, 50 r a d i c a l s . ' . ' . We could not perform the suggested experiments because of the non-a v a i l a b i l i t y of the deutero compounds. 102 BIBLIOGRAPHY 1. Benson, S.W., The Foundations of Chemical K i n e t i c s , McGraw-Hill Book Company, Inc., 1960. 2. 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