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Photochemical and themal reaction of crotonaldehyde and 3-butenal Sifniades, Stylianos 1965

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The U n i v e r s i t y 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 STYLIANOS SIFNIADES Diploma i n Chemistry, Uni v e r s i t y of Athens 1957 M.Sc, The University of B r i t i s h Columbia 1962 THURSDAY. OCTOBER 7th, 1965 AT 3;00 P.M. IN ROOM 261., CHEMISTRY BUILDING COMMITTEE IN CHARGE Chairman; I, McT. Cowan S D„ Cavers C. A. McDowell W. R, Cullen D. E. McGreer L, D. Hayward D. C. Walker External Examiners R. G. W. Norrish Department of Physical Chemistry U n i v e r s i t y of Cambridge, , Cambridge, England. PHOTOCHEMICAL AND THERMAL REACTIONS OF CROTONALDEHYDE AND 3-BUTENAL ABSTRACT In the f i r s t part of t h i s work the thermal reaction crotonaldehyde ^ *: 3-butenal ^ 1 was studied thermodynamically and k i n e t i c a l l y i n the gas phase and at the temperature range 150 to 210°C, The equilibrium composition was found to be very unfavourable to 3-butenal which constitutes 0.35% of the mixture at 150°C and 1.04% at 210°C. The enthalpy of the reaction was estimated to be 7.20 + 0.09 k c a l / mole. In the k i n e t i c study both the forward and reverse reactions were found to be heterogeneous and of the f i r s t order with respect to surface area and pressure of reactant. The rate constants obeyed the r e l a t i o n s k x = 1 0 ° * 4 0 t 0 , 9 3 „exp [- (14,220 + 1 3920)/RTJ s e c " 1 k - x = 1 0 " ° ' 5 2 + 1 = 1 7 .expQ -(7,,050 + 2.400)/RT] sec" 1 k i was determined i n a vessel with surface to volume r a t i o equal to 1.2 and i n a vessel with r a t i o equal to 4.1. The s i g n i f i c a n c e of the experimental Arrhenius parameters was discussed in terms of the theory of absolute rates as applied to surface reactions and of a mechanism based on a fast adsorption-desorption of the Langmuir type. In the second part of the. work the photochemical isomerization of crotonaldehyde to 3-butenal was studied in the gas phase and at the temperature range 25 to 140°C E x c i t i n g r a d i a t i o n of the wavelengths 3130„ 3340 and 3660 A was used. It was found that the quantum y i e l d of the isomerization obeys the Stern-Volmer equation 1/$ = a + bP ein/mole at the temperature and wavelength range studied, The pressure,,, P 3 of crotonaldehyde was varied from 0.4 to 34 mm Hg. The value of parameter a varies from 1.1. to 37 ein/mole with the low values observed at 3130 A* and the high values at 3660 A, The parameter b varies from 0.19 to 7.5 ein.mole (mm Hg) with the low values also observed at the shorter wavelength. At constant wavelength b. decreases with increasing temperature. A mechanism was discussed according to which the excited s i n g l e t '(n,"^ ) i s the reacting species and i t was shown that a "strong c o l l i s i o n " deactivation and a c l a s s i c a l energy d i s t r i b u t i o n function predict q u a l i t a t i v e l y the dependence of b on temperature and wavelength. An attempt to predict t h i s dependence i n a quantitative manner f a i l e d . A refinement to the "strong c o l l i s i o n " mechanism using the same energy d i s t r i b u t i o n function could not be tested numerically because of computational d i f f i c u l t i e s . An a l t e r n a t i v e mechanism was discussed involving a c i s - t r a n s e quilibrium of crotonaldehyde i n the ground state and i t was shown that the parameters a and b.may be interpreted i n more than one way. In the t h i r d part of the work the photolysis and photochemical oxidation of 3-butenal were studied i n the gas phase and the temperature range 25 to 140°C. o In the photolysis e x c i t i n g r a d i a t i o n of 3130 and 3340 A was used. The products were carbon.monoxi.de, propylene and b i a l l y l . .They were found to obey the r e l a t i o n CO < propylene + 2 . b i a l l y l The o v e r a l l quantum y i e l d s taken as equal to $ (propylene) + 2 $ ( b i a l l y l ) obeys the Stern-Volmer equation (1). The Value of the parameter a varies from 0.994 to 1.226 ein/mole and that of b from 1.15 x 10" 2 to 3.75 x 10 ein, mole (mm Hg) . The lowest values for both parameters are observed at 3130 A and 140°C and the highest at 3340 A and 25°C. The, s i g n i f i c a n c e of parameter a was discussed and i t was shown that i t s dependence on temperature and wavelength can be predicted within the l i m i t s of the experimental error by using the c l a s s i c a l energy d i s t r i b u t i o n function. In the photochemical oxidation r a d i a t i o n of 3130 A was used. The major products with the quantum y i e l d s at 25°C shown i n paranetheses were: carbon monoxide (3 . 0 - 3 . 4 ) , a l l y l alcohol (1„ 1-2.4) ..acrolein (1.2-1.9), carbon dioxide (0,4-1.0).propylene (0.4-0.5), peroxide (0,20-0.27) and ethylene (0.22-0.27). V a r i a t i o n of the experimental conditions at constant temperature had l i t t l e e f f e c t on the quantum y i e l d s . Increase of temperature to 140°C resulted i n decrease of the y i e l d of a l l y l alcohol and a c r o l e i n and increase of the y i e l d of a l l the other products. A mechanism was discussed which explains the r e s u l t s i n a q u a l i t a t i v e way. GRADUATE STUDIES F i e l d s of Study: Chemistry Molecular Structure A, Bree K. B. Harvey S t a t i s t i c a l Mechanics R. F. Snider Seminar i n Chemistry C. Reid J. P. Kutney Related Studies: Modern Physics M. Bloom D i g i t a l Computer J. R. H. Dempster Programming PUBLICATIONS C. A. McDowell and S. Sifniades; Oxygen-1.8 Tracer Evidence for the Termination Mechanism i n the Photochemical Oxidation of Acetaldehyde, Can. J. Chem. _41, 300 (1.963),, C. A. McDowell and S. Sifniades; Isomerization as a Primary Process i n the Photolysis of Crotonaldehyde. J. American Chem. Soc. 84, 4606 (1962). PHOTOCHEMICAL AND THERMAL REACTIONS OF ' arc, CROTONALDEHYDE AND J-BUTENAL by STYLIANOS SIFNIADES D i p l . Chem., University of Athens, 1 9 5 7 M.Sc., University of B r i t i s h Columbia, 1 9 6 2 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of Chemistry We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA May, 1 9 6 5 In presenting t h i s thesis in p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library shall.make i t f r e e l y a v a i l a b l e for reference and study. I further agree that per-mission for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by his representatives,; It is understood that copying or p u b l i -cation of t h i s thesis for f i n a n c i a l gain shall not be allowed without my written permission. Department of Chemistry The University of B r i t i s h Columbia Vancouver 8, Canada Date October 8, 1965 i i Supervisor: Professor Charles A. McDowell Abstract In the f i r s t part of this work the thermal reaction k] ^ crotonaldehyde ^ * J-butenal was studied therraodynamically and kinetically in the gas phase and at the temperature range ly0 to 210°C. The equilibrium composition was found to be very unfavorable to J-butenal which constitutes 0 . 3 5 % of the mixture at 1 5 0°C and 1.04% at 210°C. The enthalpy, of the reaction was estimated to be 7 .20±0.09 kcal/mole. In the kinetic study both the forward and reverse reactions were found to be heterogeneous and of the f i r s t order with respect to surface area and pressure of reactant. The rate constants obeyed the relations k x = 10°- A 0 ± 0- 95. e x p[_(i4,220+ 1,920 )/RT] s e c - 1 k_x = 1 0 ~ 0 , 5 2 1 1 , 1 7 - e x p [ - ( 7 , 0 5 0 + 2 , 4 0 0 )/RT] s e c - 1 k-^  was determined i n a vessel with surface to volume ratio equal to 1.2 and k_^ in a vessel with ratio equal to 4.1 . The significance of the exper-imental Arrhenius parameters was discussed i n terms of the theory of ab-solute rates as applied to surface reactions and of a mechanism based on a fast adsorption-desorption of the Langmuir type. In the second part of the work the photochemical isomerization of crotonaldehyde to ^""butenal was studied in the gas phase and at the temper-ature range 2 5 to 140° C. Exciting radiation of the wavelengths JIJO, yyk0 and 5 6 6 O A was used. It was found that the quantum yield of the isomerization obeys the Stern-Volmer equation i i i l A p -z. a +• bP ein/mole (1) a t t h e t e m p e r a t u r e and w a v e l e n g t h range s t u d i e d . The p r e s s u r e , P, o f c r o t o n a l d e h y d e was v a r i e d f r o m 0.4 t o j 4 mm Hg. The v a l u e of parameter a v a r i e s f r o m 11 t o 37 e i n / m o l e w i t h t h e low v a l u e s o b s e r v e d a t $IJ0 A and t h e h i g h v a l u e s a t 566O A. The parameter b v a r i e s f r o m 0.19 t o 7»5 ein*mole~-'-(nim Hg) ^ w i t h t h e low v a l u e s a l s o o b s e r v e d a t t h e s h o r t e r wave-l e n g t h . At c o n s t a n t w a v e l e n g t h b 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 . A mechanism was d i s c u s s e d a c c o r d i n g t o w h i c h t h e e x c i t e d s i n g l e t ^ ( n , " ^ * ) i s t h e r e a c t i n g s p e c i e s and i t was shown t h a t a " s t r o n g c o l l i s i o n " d e a c t i -v a t i o n and a c l a s s i c a l energy d i s t r i b u t i o n f u n c t i o n p r e d i c t q u a l i t a t i v e l y t h e dependence o f b on t e m p e r a t u r e and w a v e l e n g t h . An a t t e m p t t o p r e d i c t t h i s dependence i n a q u a n t i t a t i v e manner f a i l e d . A r e f i n e m e n t t o t h e " s t r o n g c o l l i s i o n " machanism u s i n g t h e same energy d i s t r i b u t i o n f u n c t i o n c o u l d n o t be t e s t e d n u m e r i c a l l y because o f c o m p u t a t i o n a l d i f f i c u l t i e s . An a l t e r n a t i v e mechanism was d i s c u s s e d i n v o l v i n g a c i s - t r a n s e q u i l i b r i u m o f c r o t o n a l d e h y d e i n t h e ground s t a t e and i t was shown t h a t t h e parameters a and b may be i n t e r p r e t e d i n more t h a n one way. I n t h e t h i r d p a r t o f t h e work t h e p h o t o l y s i s and p h o t o c h e m i c a l o x i d a t i o n o f 3 ~ b u t e n a l were s t u d i e d i n t h e gas phase and t h e t e m p e r a t u r e range 25 t o l40°C. I n t h e p h o t o l y s i s e x c i t i n g r a d i a t i o n o f 515° a " d 35^0 A was u s e d . The p r o d u c t s were ca r b o n monoxide, p r o p y l e n e and b i a l l y l . They were found t o obey t h e r e l a t i o n The o v e r a l l quantum y i e l d , t a k e n as e q u a l t o ^ ( p r o p y l e n e ) + 2 l ( b i a l l y l ) obeys t h e S t e r n - V o l m e r e q u a t i o n ( l ) . The v a l u e o f t h e parameter a v a r i e s CO ^, p r o p y l e n e + 2 ' b i a l l y l (2) i v from 0.994 to 1.226 ein/mole and that of b from 1.15 x 10" 2 to 5.75 x 10~ 2 ein-mole~-'-(mm Hg) The lowest values for both parameters are observed at JIJO A and l40°C and the highest at 3540 A and 25°C. The s i g n i f i c a n c e of parameter a was discussed and i t was shown that i t s dependence on temper-ature and wavelength can be predicted within the l i m i t s of the experimental error by using the c l a s s i c a l energy d i s t r i b u t i o n function. In the photochemical oxidation r a d i a t i o n of 31J0 A. was used. The major products with the quantum y i e l d s at 25°C shown i n parentheses were: carbon monoxid^(3« u -5*.4), a l l y l alcohol (1.1-2 . 4 ) , a c r o l e i n ( l . 2 - 1 . 9 ) , carbon dioxide (0 .4-1.0), propylene (0 .4-0.5), peroxide (0.20-0.27) and ethylene (0.22-0 .27). V a r i a t i o n of the experimental conditions at constant temperature had l i t t l e e f f e c t on the quantum y i e l d s . Increase of temperature to l40°C resulted i n decrease of the y i e l d of a l l y l alcohol and a c r o l e i n and increase of the y i e l d of a l l the other products. A mechanism was discussed which explains the r e s u l t s i n a q u a l i t a t i v e way. V CONTENTS Page. CHAPTER I . INTRODUCTION 1 CHAPTER I I . PREPARATION OF MATERIALS 1. CROTONALDEHYDE 9 2. 2-BUTENAL 9 J . OXYGEN 13 4. MATERIALS FOR CALIBRATION OF GAS CHROMATOGRAPH 14 5. SOLUTION OF AMMONIUM THIOCYANATE IN'METHANOL 14 6. SOLUTION OF FERROUS THIOCYANATE I N METHANOL 14 7. ACTINOMETRIC SOLUTIONS 14 CHAPTER I I I . THERMODYNAMICS AND MECHANISM OF THE THERMAL EQUILIBRIUM CROTONALDEHYDE 3-BUTENAL 1. APPARATUS The Vacuum System 16 The Gas Chromatography A p p a r a t u s 18 2. EXPERIMENTAL D e t e r m i n a t i o n o f R e a c t i o n P r o d u c t s 20 E x p e r i m e n t a l P r o c e d u r e 20 3. RESULTS E q u i l i b r i u m S t u d i e s 21 K i n e t i c S t u d i e s 25 4. DISCUSSION Thermodynamic S t u d i e s 32 K i n e t i c S t u d i e s 35 CHAPTER IV. DETAILED STUDY OF THE MECHANISM OF PHOTOCHEMICAL ISOMERIZATION OF CROTONALDEHYDE 1. APPARATUS The Optical System 40 The Photometer Unit 42 2. -EXPERIMENTAL Determination of the Absorption Curves of Crotonaldehyde 46 Actinometry 51 Determination of Reaction Products 54 3. RESULTS Effect of the Condition of the Wall and of Wavelength on the Photolysis 55 Effect of Pressure and Temperature on the Quantum Yield of  Isomerization 59 4..DISCUSSION Surface Effect 63 The Excited State 65 Vibronic Distribution of the Ground State,P(£) 66 Energy of the Exciting Radiation, hy 67 Selection Rules 68 Processes Open to the Molecules in the Excited State 68 Estimation of k-^ n and k^ n for a "Strong Collision Mechanism" 70 Theoretical, Estimation of k l n for a "Strong Collision  Mechanism" 73 Refinement of the "Strong Collision Mechanism" 76 Alternative Mechanism ' 78 CHAPTER V PHOTOLYSIS AND PHOTOCHEMICAL OXIDATION OF 3-BUTENAL v i i Page. 1. APPARATUS 82 2. EXPERIMENTAL Determination of the Absorption Curves f o r ^ -Butenal at 5130 .and 3340 A 82 Determination of Reaction Products 84 Experimental Procedure 88 3. RESULTS Photolysis 90 Photooxidation 95 Photooxidation of 3~Butenal with Oxygen-18 101 4. DISCUSSION Photolysis 106 Photooxidation 115 BIBLIOGRAPHY 123 1 v i i i LIST OF TABLES Table. Page. I Equilibrium Composition of Crotonaldehyde and J-Butenal at the Temperature Region 150° - 210°C 22 II Thermal Isomerization of Crotonaldehyde 25 III Thermal Isomerization of 3-Butenal 28 IV Photolysis of Crotonaldehyde in a "Seasoned" Vessel 57 V Photolysis of Crotonaldehyde in a "Clean" Vessel 59 VI Photolysis of Crotonaldehyde at 3130 A 60 VII Photolysis of Crotonaldehyde at 366O A 6 l VIII Least Square Estimation of Slopes and Intercepts for the Stern-Volmer Plots of Crotonaldehyde 63 IX Photolysis of 3-Butenal at 313O A 90 X Photolysis of 3-Butenal at 3340 A 91 XI Least Square Estimation of Slopes and Intercepts for the Stern-Volmer Plots of 3-Butenal 95 XII Formation of Peroxide in Thermal Oxidation of 3-Butenal 96 XIII Photooxidation of 3-Butenal; Analysis by Method "A" 98 XIV Photooxidation of 3-Butenal; Analysis by Method "B" 100 XV Values of E a +E 0_ 0 Estimated from the Intercepts of the Stern Volmer Plots 112 i x LIST OF FIGURES F i g u r e . Page. 1. Vapor P r e s s u r e o f C r o t o n a l d e h y d e and J - B u t e n a l 11 2. A p p a r a t u s f o r P r e p a r a t i o n o f 3 - B u t e n a l 12 J . H i g h Vacuum A p p a r a t u s 17 4. Chromatography A p p a r a t u s 19 5. v a n ' t H o f f P l o t o f Data f r o m T a b l e I 24 6. A r r h e n i u s P l o t o f Data f r o m T a b l e I I 27 7. A r r h e n i u s P l o t f o r I s o m e r i z a t i o n o f J - B u t e n a l 29 8. Dependence o f Rate o f I s o m e r i z a t i o n o f 3 - B u t e n a l on S u r f a c e J l 9. O p t i c a l System f o r C r o t o n a l d e h y d e P h o t o l y s i s 4l 10. A b s o r p t i o n S p e c t r a o f F i l t e r C o m b i n a t i o n s 4j 11. P h o t o m e t r i c C i r c u i t 44 12. A b s o r p t i o n Curve o f C r o t o n a l d e h y d e a t J I J O A 47 15- A b s o r p t i o n Curves o f C r o t o n a l d e h y d e a t 366O A 48 14. U.V. S p e c t r a o f C r o t o n a l d e h y d e and 3 - B u t e n a l a t 25°C 50 15. P h o t o l y s i s o f C r o t o n a l d e h y d e 5& 16. P h o t o l y s i s o f C r o t o n a l d e h y d e ; S t e r n - V o l m e r P l o t s 64 17. C a l c u l a t e d S t e r n - V o l m e r P l o t s f o r C e r t a i n V a l u e s o f ab and k 2 n / k l n 72 18. C a l c u l a t e d R a t i o s k 1(l40°C)/k 1(25°C) and ^ (3130 A)/k 1(3660 A) 75 19* A b s o r p t i o n Curves o f 3 - B u t e n a l 83 20. S u c c e s s i v e A b s o r p t i o n S p e c t r a o f 12.6 mm Hg 3 " B u t e n a l and 6.8 mm Hg 02 87 21. P h o t o l y s i s o f 3-Butenal a t 313O A, 25°C 92 22. P h o t o l y s i s o f 3-Butenal; S t e r n - V o l m e r P l o t s 93 23. P h o t o l y s i s o f 3-Butenal: ( p r o p y l e n e ) / ( b i a l l y l ) r a t i o s 94 X Figure. Page. 24. Thermal Oxidation of J-Butenal at l40°C 97 25. Mass Spectrum of Gases from Photooxidation with 0 l 8 0 l 8 10J 26. Mass Spectrum of A c r o l e i n from Photooxidation with 0 l 8 0 1 8 104 27. Mass Spectrum of A l l y l Alcohol from Photooxidation with 0 l 8 0 1 8 105 28. C l a s s i c a l Energy D i s t r i b u t i o n of 3-Butenal. L 'o 111 x i ACKNOWLEDGEMENT The present work was conducted under the supervision of Professor C.A. McDowell to whom I wish to express my gratitude f o r h i s l a s t i n g i n t e r e s t and guidance, throughout the course of the work. I am g r a t e f u l to the National Research Council of Canada for a Student-ship and to the University of B r i t i s h Columbia for Teaching Assistantships both covering the period 1962 - 1965-F i n a l l y , I wish to thank the Computing Center s t a f f f o r t h e i r assistance i n the computational part of t h i s work and the glassblowing, e l e c t r o n i c and workshop s t a f f f o r t h e i r assistance i n the construction of parts of the apparatus. 1 I. INTRODUCTION The i n t e r e s t of research workers i n the thermal and photochemical reactions of aldehydes, mainly i n the gas phase, i s well explained by the f a c t that they are among the most reactive compounds known. They undergo p y r o l y s i s and thermal oxidation at much lower temperatures than the cor-responding hydrocarbons; and t h e i r property of absorbing l i g h t at the e a s i l y accessible part of the u l t r a v i o l e t spectrum makes them suitable f o r photo-chemical studies. It i s important to understand the reactions of aldehydes i n the gas phase, because i t i s known that they constitute intermediate products during the oxidation of hydrocarbons. Knowledge of the way aldehydes behave when heated or subjected to r a d i a t i o n alone or i n mixture with oxygen w i l l help to elucidate the much more complex reaction occurring during combus-t i o n of hydrocarbons. As early as 1930, Bone et a l (1,2) and Naylor and Wheeler (3) noticed that addition of aldehydes to a- hydrocarbon-oxygen mixture resulted i n a reduction of the induction period of the combustion. Norrish ( 4 ) presented evidence that aldehydes are the most generally important branching agents i n the oxidation of hydrocarbons, without excluding the p a r t i c i p a t i o n of intermediate peroxides at low temperatures. It i s understandable that the work on aldehydes has been almost con-f i n e d to the lower members of the series considering that the vapour pres-sure of the higher members i s quite low at room temperature. Thus, while formaldehyde, acetaldehyde and to some extent propionaldehyde have been f a i r l y thoroughly investigated i n both t h e i r thermal and photochemical reactions with oxygen or alone, only fragmentary work has been undertaken 2 f o r aldehydes with four or more carbon atoms. However, such work could be of value i n understanding c e r t a i n aspects of hydrocarbon oxidation, because there i s evidence of the existence of higher aldehydes i n the cool flames i n the i g n i t i o n of higher hydrocarbons. Crotonaldehyde i n p a r t i c u l a r has been detected i n the cool flames of butane (5), pentane (6) and four i s o -meric hexanes (7). This, and the f a c t that crotonaldehyde possesses a double bond conjugated with the carbonyl group, make the compound an i n t e r -esting subject f o r study. The work done on gaseous crotonaldehyde oxidation and decomposition, both photochemical and thermal, can be summarized as follows. In 19J6, Blacet and Roof (8) studied the photolysis of gaseous croton-aldehyde at 30°C and seven wavelengths, ranging from 366O to 2J99 A and found no detectable decomposition. The only evidence of chemical re a c t i o n which they were able to observe was polymerization at y660 A with a quantum y i e l d of 0.02 based on the number of molecules disappearing from the gaseous phase per quantum of l i g h t absorbed. They proposed a predominating reverse reaction of the type C H J C H J G H C H O CH^ CH-.CH' + CHO* to account f o r the f a c t that no products were detected even i n the region where l i g h t absorption by crotonaldehyde i s apparently continuous. In the same work they noted b r i e f l y that oxidation of crotonaldehyde by oxygen does not proceed at a measurable rate i n the dark, but i t i s accelerated by l i g h t . At the same time DeLisle et a l (9) studied the thermal decomposition at 430-462°C and found CO and a mixture of hydrocarbons, mostly propylene, to be the main products. The reaction was heterogeneous-and i n general of 5 the second order. No mechanism was proposed. Three years later Blacet and his co-workers studied the photolysis at elevated temperatures (10) and the photochemical oxidation at *>0°C In the photolysis at 150-400°C the main products were found to be 60% C O , 27% unsaturated hydrocarbons, 6% hydrogen and 7% methane. The rate of decom-position increased with increase in temperature and decrease i n wavelength. No mechanism was proposed. In the same work they studied briefly the thermal decomposition and found that i t is not appreciable below 150°C and very slow below 275°C» At higher temperatures they confirmed the results found by DeLisle et al (9)» In the photochemical oxidation at 5 0 ° C they followed the course of the reaction by recording pressure decrease and postulated that the solid which accumulated in the c e l l during reaction was crotonic acid. They suggested that the overall reaction i s 2CHjCH:CHCH0 + Q>2 » 2GH3CH: CHC00H but also noted the possibility of a chain mechanism, because the quantum yield could be as high as 2*2 under certain circumstances. The introduction of nitrogen caused a marked decrease of quantum yield, therefore they con-cluded that reaction takes place mainly through activated molecules. Later Volman et al (12) used the mirror technique and detected free radicals during the photolysis of crotonaldehyde. They suggested that a bond rupture of the type ( C H ^ C H - . C H C H O ) > C H J C H J C H 2 4 + O H O * must also occur. Tolberg and Pitts (13) in a brief study of the photolysis at high 'temperature and short wavelength found 2-butene among the products and 4 postulated the displacement reaction CHj° +• CH5CH:CHCHO — C H ^ C H : CHCHj + CHO to explain i t s formation. Harrison and Leasing (l4) studied the mercury-photosensitized low temperature decomposition, i n an e f f o r t to define the primary process taking place i n crotonaldehyde photolysis. They used a mass spectrometer to follow the reaction products which were mainly CO and propylene. Added methyl r a d i c a l s , CH^ , showed that a r a d i c a l s p l i t t i n g was not s u f f i c i e n t to account f o r the amount of products formed. They concluded that the main process taking place was molecular re-arrangement to CO and propylene, accompanied by some r a d i c a l s p l i t t i n g , as follows: CHjCH:CH2+- CO+Hg :main process CH*CH:CHCHO +• Hg* * CH^CH:CH + CHO + Hg j . . j minor processes CHINCH: CHCO + H + Hg J Recently Osborne and Skirrow (15) reinvestigated the thermal oxidation using modern techniques of analysis to i d e n t i f y and determine the products. They found peroxycrotonic acid to be the main product at 166°C and adopted the general mechanism proposed some years e a r l i e r by McDowell and Farmer (16) f o r both the thermal and photosensitized oxidation of aldehydes. This mechanism postulates that r a d i c a l s , R, or excited molecules, R'CHO, pro-duced thermally or photochemically react very f a s t with oxygen molecules. The r e s u l t i n g r a d i c a l s , R", abstract then the aldehydic hydrogen from the excess aldehyde producing the r a d i c a l R'CO which becomes the chain c a r r i e r by reacting f a s t with oxygen to produce the peroxy-radical R'COOO* which i n turn abstracts an aldehydic hydrogen to produce the peracid, R'COOOH, disproportionation i n v o l v i n g the peroxyradicals, R'COOO*. The v a l i d i t y of t h i s mechanism has been confirmed on several occasions, (17-22). Recently the photolysis of crotonaldehyde was investigated i n t h i s laboratory (23,24) at room temperature using a medium pressure mercury lamp and a Corex 986 f i l t e r . Given the absorption spectrum of crotonaldehyde t h i s combination r e s u l t s i n the aldehyde absorbing r a d i a t i o n mostly at the wavelengths of 51$0, 3?40 and 366O A. Under these experimental conditions previous workers had found no reaction taking place except f o r some poly-merization (8) . In t h i s work, however, i t was found that crotonaldehyde upon absorption of r a d i a t i o n isomerizes to 3-butenal with a quantum y i e l d of 0.1 . The f a c t that the reaction, at le a s t i n i t s early stages, i s not accompanied by any appearance of fragmentation compounds explains the ap-parent s t a b i l i t y of crotonaldehyde to r a d i a t i o n , observed i n e a r l i e r work. When the reaction was prolonged i n time, however, fragmentation compounds did appear i n the form of propylene, carbon monoxide and b i a l l y l . The presence of the l a t t e r hydrocarbon combined with the f a c t that the quantum y i e l d s of a l l three compounds increase l i n e a r l y with time showed that these products a r i s e from photolysis of 3-butenal according to the scheme B i a l l y l then i s probably produced by recombination of two a l l y l r a d i c a l s . and regenerate the chain c a r r i e r , R'CO. Termination i s by r a d i c a l - r a d i c a l CH2:CHCH2CH0 +-Kv 2CH 2:CHCH 2 > CH2:CHCH2CH2 CH:CH2 6 These conclusions were further reinforced by studying the photolysis of J-butenal: propylene, carbon monoxide and b i a l l y l were found to be the products and t h e i r quantum y i e l d was constant. The t o t a l quantum y i e l d , based on the number of a l l y l groups appearing i n the products either as propylene or b i a l l y l , was 1. Although the primary process of the photolysis of crotonaldehyde was thus convincingly shown to be an isomerization, c e r t a i n points remained s t i l l unexplained. These were the following: (a) The quantum y i e l d of isomerization, 0 . 1 , showed that nine out of ten crotonaldehyde molecules absorbing r a d i a t i o n f a i l to react. (b) There i s no information about the fa t e of these non-reacting molecules. No phosphorescence or fluorescence could be detected i n crotonaldehyde vapour (8) . T r a n s i t i o n to a t r i p l e t state with an energy i n excess of 65 Kcal/mole has been postulated i n order to explain the crotonaldehyde photosensitized isomerization of butene-2 (25)» (c) The quantum y i e l d of isomerization was not affected by change i n the pressure of crotonaldehyde, a f a c t which implies that the e l e c t r o n i c a l l y excited molecules react i n a time much shorter than the time between two c o l l i s i o n s . But t h i s conclusion cannot be reconciled with the low quantum y i e l d . It was the object of the present work to provide answers to these questions. One obvious reason f o r the low value of quantum y i e l d could be that the majority of the crotonaldehyde molecules in.the e l e c t r o n i c a l l y excited state are not produced with enough energy to surmount the energy b a r r i e r to isomerization. This could be e a s i l y the case, because the l i g h t absorbed consisted of a spectrum encompassing the JIJO, yyk0 and 366O A bands, with the longest wavelength p r e v a i l i n g . A way of checking t h i s pos-7 s i b i l i t y is by determining the quantum yield at selected wavelengths. It was also thought interesting to investigate the influence of temperature on quantum yield, given the controversy existing'on this matter in the l i t e r a -ture (26-28). Before proceeding, however, to a study of photolysis at higher tem-peratures i t was necessary to ascertain that no dark reaction takes place to an appreciable extent under these conditions. It was already known that 3-butenal, the product of photolysis, is unstable at high temperatures (24, 29), isomerizing to crotonaldehyde, but no study of this reaction had been undertaken. Crotonaldehyde i t s e l f was known to be completely stable below 150°C and to decompose to only- a very small extent below 275°C (l°)« Absence of decomposition, however, did not exclude the possibility of iso-merization and a few preliminary runs showed that crotonaldehyde vapour did isomerize thermally to J-butenal. This result i s hardly surprising, because tautomeric equilibria of the form H H -C-C:C-X -> -C:C-CrX l I I ^ i 1 i K p * & P <* where X is -COOH, -COOR or -CN, are well known ($0), although no studies have been made^  in the case of aldehydes and ketones. This latter fact made a study of the thermal isomerization a l l the more important. Another objective of the present work was to study in some more detail the photolysis of J-butenal. The fact that the quantum yield for this re-action is unitj under a variety of pressures and intensities of illumination (24) implies that J-butenal molecules in the electronically excited state decompose in a time much shorter than the time required for collisional deactivation. There i s , of course, some uncertainty about this conclusion, 8 because the radiation used was not entirely mono-chromatic. Here again the way to eliminate the uncertainty is to determine the quantum yield using radiation of a narrow wavelength. Study of the photolysis at higher temper-ature was also desirable,, because under these conditions the a l l y l radicals produced might be able to abstract the aldehydic hydrogen and the activation energy for the process could be established. Finally, a study of the in-fluence of molecular oxygen on the course of photolysis was undertaken, mostly in order to establish the fate of the a l l y l radicals in the presence of this molecule. 9 II. PREPARATION OF MATERIALS 1. CROTONALDEHYDE CP. grade crotonaldehyde was purified by gas chromatography using a 5 foot long column packed with dinonyl phthalate on firebrick. The carrier gas was helium. The operating conditions were: pressure at entrance of column 8.5 psi, pressure at exit slightly above atmospheric, temperature of column 60°C. The middle section of the peak corresponding to crotonaldehyde was trapped at dry ice temperature, the head and t a i l being diverted to the vent. The product was further degassed by bulb to bulb d i s t i l l a t i o n . After this treatment i t showed less than 0.01% impurity when analyzed gas-chromatographicaliy using a flame ionization detector. 2. 3-BUTENAL This isomer of crotonaldehyde has been prepared by chromic acid oxi-dation of the corresponding alcohol at 0° - 10°C. (23,24). This method, however, has a low yield and the added disadvantage of using an expensive starting material. Isomerization of vinyl oxide on a catalyst surface is known to produce some 3-butenal along with other products (31) but no attempt was made to u t i l i z e this reaction in the present work, because of the poisonous nature of the starting mate-rial and the low yield. During the study of the thermal reaction of crotonaldehyde (Chapter III) i t was found that the only measurable process at temperatures around 200°C was isomerization to 3~butenal. It was further found that this i s a rever-sible reaction in which the equilibrium mixture contains only about 1% of the unconjugated isomer at 210°C. Extrapolation showed that the equilibrium 10 content i s about 6% at 380°C. Moreover the reaction i s heterogeneous, with the rate of both the forward and reverse reaction increasing linearly with the surface of the reaction vessel. A study of the vapour pressures of cro-tonaldehyde and 3-butenal showed the latter to be more volatile around room temperature (Figure l ) . With these data in mind, the apparatus shown in Figure 2 was con-structed. "A" was a 100 ml pyrex bulb. "B" was an electrically heated oven. The part of the pyrex side-tube passing through the oven was loosely packed with glass wool. The upper part of the condenser was packed with pyrex helices. "C" was a mercury bubbler with the pressure regulated by adjusting the position of tank "D". The bubbler "F" was keeping the pressure inside the trap "E" at about 2 mm Hg. Cold air was used as coolant for the con-denser. To this effect compressed air was passed through the copper c o i l , "G", which was kept in an ice bath, and fed to the top of the condenser. "H" and "I" were BIO sockets, the f i r s t f i t t e d with a thermometer and the second used as a sampling outlet while the optimum conditions for the op-eration of the apparatus were being set. A l l sockets, stopcocks and glass-to- rubber joints were greased with Apiezon N. Crotonaldehyde (CP. grade, not purified) was admitted to the ap-paratus at "H". Then the thermometer was set in place, the bulb, "A", cooled to dry ice temperature and the apparatus /degassed by opening the stopcocks' at "J" and "KM while the mercury level in "C" was adjusted to let free passage of gasses and "F" was t i l t e d for the same purpose. The oven was turned on and allowed to reach 380°C. After degassing, the mercury level at "C" was adjusted to n™ and the bubbler, "F", put to vertical position. Cold air was fed to the condenser at such a rate that the ther-mometer at "H" registered 10-l4°C and the bulb, "A", was heated to 50~55°C f 11 12 15 with the aid of a water-bath. At the same time the trap, "E", was cooled to dry ice temperature. Under these conditions the crotonaldehyde at "AM de-veloped a vapour pressure sufficient to cause passage of i t s vapour through the heated packed tube, but not strong enough to raise the level- of the liquid higher than the point 11L" in the condenser. On the hot glass-wool surface crotonaldehyde vapour reacted quickly and reached the equilibrium composition. The mixture, containing approximately 6% J-butenal, was led to the condenser above "L" and fractionated through the pyrex helices. Whenever pressure exceeded 37 mm Hg, bubbles of enriched product burst through the mercury and were trapped at "E". Gas chromatographic analysis showed that the product contained about 70$ 3-butenal and 1% volatile impurities, the rest being unchanged crotonaldehyde. 2 to J ml of i t were prepared after about two hours of reaction and further purified by gas chromatography. The rest of the crotonaldehyde was l e f t at "A" un t i l the next use of the apparatus, which was l e f t with the stopcock, "K", open, and the oven at 400°C overnight. This treatment ensured that any polymers deposited on the glasswool surface were destroyed leaving the surface active for the next run. The same column and conditions used in the case of crotonaldehyde (p.$) were used for the purification of 3-butenal, except that the material was passed twice through the column. After this treatment -it contained only about 0.2% crotonaldehyde and traces of more volatile impurities (~0.01%) as shown by chromatographic analysis using the flame ionization detector. 3- OXYGEN Cylinder oxygen was purified by liquifying and d i s t i l l i n g i t four times, each time rejecting approximately the last one tenth of the sample, and then d i s t i l l i n g i t into a 5 l i t r e receiving flask. 14 4. MATERIALS FOR CALIBRATION OF GAS CHROMATOGRAPH Except for crotonaldehyde and J-butenal, which were specially purified, a l l other materials used f o r quantitative and qualitative calibration of the gas chromatograph were A.R. or CP. grade compounds subjected when ap-plicable, only to degassing, and vacuum d i s t i l l a t i o n . These were: b i a l l y l , carbon monoxide, carbon dioxide, ethylene, propylene, acetaldehyde, acet-ylene, acrolein, 1,5-hexadiene and a l l y l alcohol. 5. SOLUTION OF AMMONIUM THIOCYANATE IN METHANOL A stock solution of ammonium thiocyanate was used for preparing a solution of ferrous thiocyanate, used in the determination of peracids by the method of Young et al ( J 2 ) . 5 gms of A.R. grade ammonium thiocyanate were dissolved in about 600 ml. of A.R. grade methanol, 0.5 ml. of concen-trated sulphuric acid was added and the solution was made up to one l i t r e . 6. SOLUTION OF FERROUS THIOCYANATE IN METHANOL A solution of ferrous thiocyanate is slowly oxidized by atmospheric oxygen with the formation of the dark red fe r r i c thiocyanate complex. For this reason this solution was alsays made up just prior to being used. 50 ml. of the stock solution of ammonium thiocyanate were shaken with 0.1 gm. of A.R. grade ferrous ammonium sulphate for one minute and allowed to stand for two minutes. The solution was then decanted from undissolved ferrous salt. 7. ACTINOMETRIC SOLUTIONS The only solution which needed special care in preparation was that of potassium ferrioxalate. Crystals of this complex salt were f i r s t prepared 15 by mixing under vigorous sti r r i n g 3 volumes of 1.5 M K2C2O4. and 1 volume 1.5 M FeClj. The resulting salt was recrystallized three times from warm water and dried at 45°C. Under these conditions i t s composition was K^Fe(C20^)j'3H 20 (33). A 0.006 M solution of the salt was prepared and stored in a dark-ccloured bottle. The other solutions used in actinometry were prepared by dissolving the A.R. grade compound in d i s t i l l e d water. They are described under "Actinometry" in Chapter IV. 16 III. THERMODYNAMICS AND MECHANISM OF THE THERMAL EQUILIBRIUM CROTONALDEHYDE j > 2-BUTENAL 1. APPARATUS The Vacuum System The vacuum system is shown in Figure 3. It was constructed of pyrex. A l l the stopcocks were lubricated with Apiezon N grease. The apparatus was evacuated by a one-stage mercury diffusion pump backed by a "Hyvac" rotary o i l pump. The cold traps were demountable and dry ice-acetone mixture was used always as refrigerant. Whenever gases had to be pumped away from the system an auxiliary "Hyvac" pump bypassing the traps was f i r s t used for some minutes, and then the vacuum was restored to the level of 10~5 mm Hg by using the diffusion pump-"Hyvac" unit. In this way no condensable materials with objectionable vapour pressure were retained in the traps, which con-tained only mercury droplets. "A" was an S13 socket to which various reaction vessels could be con-nected. These were either pyrex tubes equipped with break-seal, or a cylin-drical quartz vessel with f l a t optical windows. The latter was equipped with high vacuum stopcock,- and was placed permanently in a furnace, heated by means of a coil supplied by a manually controlled "Variac" transformer. The pressure of reactant to enter into a reaction vessel was measured by means of a spiral gauge (B) linked with an optical lever system as shown in Figure 3a. With the gauge, pressures could be measured with 0.2 mmHg accuracy. ( D ) was the sampler of the gas chromatography apparatus, (j) was a variable expansion volume used for mixing gases. (F) was a spiral trap 17 18 where the reagent was kept at -78°C. In the other trap, (G), a product could be isolated after being separated gas-chromatographically from the reaction mixture. Also i t was used in pair with (F) to degas a sample of material purified gas-chromatographically. The storage volume (C) and Toepler pump (E) were not used in this set of experiments. The Gas Chromatography Apparatus The gas chromatography apparatus, shown in Figure 4, was incorporated to the vacuum system and was constructed of pyrex, except for the metallic parts of the detectors. The carrier gas was helium. Its pressure was regulated by means of a Mathieson No. 70 regulator (A) and i t s flow rate was read on the flowmeter (B). The sampler (D) was used in a l l analytical uses of the apparatus and i t could be fed either by expansion or condensation of a sample. For puri-fication of materials the sampler (D-^ ) was used. It was shaped in such a way that the sample was subjected to heating only when i t was forced into the heated part of the sampler by helium bubbling through i t , retreating to the cold part below when helium stopped passing. The furnace (E) was heated by means of a coi l connected to the outlet of a manually regulated "Variac" transformer. For analytical work a 5-f°0',:' long column of 5 mm diameter packed with dinonyl phthalate on firebrick was used. The exit gases from the column were directed to a Perkin-Elmer Model 154 Vapour Fractometer equipped with flame ionization and thermo-conductivity detectors. In this set of experiments only the flame ionization detector was used. For purification of materials a 5-f'°°t long column of 6 mm diameter packed with dinonyl phthalate on firebrick was used. In this case the gases were directed to the ther mo conductivity detector (C). 19 I •a 1 w-F L O W HeAum To fexJCtin- €£me/c 5 T* l/mum U<nt 20 When a component was to be c o l l e c t e d , the vent was closed at the moment i n which the corresponding peak was appearing and the stream directed through the cold traps (F) or (G). 2. EXPERIMENTAL Determination of Reaction Products In preliminary experiments i t was found that the only measurable product when crotonaldehyde vapour i s heated i n a quartz vessel at the temperature range 150°C to 210°C i s 3-butenal. Moreover, i t was found that the rate of appearance of t h i s product declines r a p i d l y with time reaching eventually zero. This f a c t suggested a r e v e r s i b l e reaction, which was found to be the case by heating J-butenal vapour under the same conditions and detecting crotonaldehyde as the reaction product. It was decided to study f i r s t the thermodynamics of equilibrium and then the k i n e t i c s of both the forward and reverse reaction. In the thermo-dynamic studies the s t a r t i n g material was always crotonaldehyde, because the equilibrium i s overwhelmingly i n favour of t h i s compound. Experimental Procedure This procedure was followed to study the thermodynamics of the equilibrium crotonaldehyde < > J-butenal . Pyrex tubes about 15 cm long with a diameter of 1.0 cm. were prepared. One end was f i t t e d with a break-seal and the other with a stopcock equipped with a BIO cone. Some of the tubes were packed with a weighed amount of glass-wool. The tubes could be connected to the vacuum system i n quantities up to f i v e at a time by means of a manifold which could be f i t t e d at point (A) of the apparatus (Figure J ) . The tubes were evacuated f o r one hour at 21 room temperature and then f i l l e d with crotonaldehyde vapour up to a pressure measured by means of the spiral gauge. The stopcocks of the tubes were shut, the contents frozen to -78°C and the tubes sealed by melting the glass below each .stopcock. Then they were submerged into a stirred o i l bath kept at constant temperature by a combination of a "Variac" trans-former and a JUMO thermostat. After a measured time a tube was withdrawn from the bath, fit t e d to the vacuum system, the seal was broken and i t s contents were analyzed by gas chromatography. In the kinetic studies a quartz vessel was used with f l a t optical windows when the starting material was crotonaldehyde. This was the same vessel used subsequently for photochemical experiments and i t was placed permanently in an oven f i t t e d with quartz windows. Its volume was 1J5 ml. Prior to each run the vessel was heated under vacuum to 300°C for one hour and then allowed to cool to a chosen temperature. Crotonaldehyde was ad-mitted to the vessel at pressure measured by means of the spiral gauge. After a measured time the reaction mixture was expanded to the gas chroma-tography sampler and analyzed. With 2-butenal empty or packed pyrex tubes were used, as in the case of thermocynamic studies. Prior to each run the tubes were evacuated for one hour at room temperature. J...RESULTS Equilibrium Studies Experiments were carried out at four temperatures, from 150°C to 210°C. At 210°C and 1°0°C the pressure of crotonaldehyde was varied. At 170°C the pressure was kept constant and the surface-to-volume ratio of the tubes was varied by adding JO mg of glasswool to some of them. At 150°C 22 some of the -tubes were packed with JO mg of glasswool and the reaction conducted isothermally; some others were unpacked, but they were heated i n i t i a l l y to 210°C and then allowed to cool to 150°C, at which temperature the contents were allowed to reach equilibrium. The r e s u l t s are summarized i n Table I. TABLE I Determination of the Equilibrium Composition of Crotonaldehyde - 3-Butenal from 150°C to 210°C. Temperature °C Pressure Time mmHg at 25° C hours 3-butena,l/crotonaldehyde (a) x l O 4 210 4.7 44-51 110.5 + 1.5 ti 7.4 19-72 105.1 ±3.2 it 10.0 21-65 1 0 4 . 7i 1 . 8 II 15.0 42-115 1 0 7 . 0 i 4 . 0 II 25.8 17-115 9 9 . 3 i 5 . f i II 31.8 17-88 103.3 + 1.1 Average 104.1 + 4.2 190 10 .0 36-113 80.7+-4.4 II 15.0 32-112 76.3+2.8 II 23.7 47-188 79.8 + 2.3 n 30.8 40-116 76.2+5.6 Average 78.I + 3.8 170 20 .0 118-248 56.0+1.5 " (b) 11 40-136 56.8 + I . 5 Average 56.3+1.6 150 (b) ti 105-203 35 .0± 0.3 " (c) n 0 45.7 " (c) 11 2 44.7 " (c) 11 18-25 34.8 + 0.7 Average 34.9 + 0.7 (a) Uncertainties are mean deviations. (b) Tubes were packed with y0 mg of glasswool. (c) Tubes were i n i t i a l l y heated to 210°C. Time i s counted from the moment the temperature dropped to 150°C. 2J I t i s a p p a r e n t t h a t a t a g i v e n t e m p e r a t u r e t h e c o m p o s i t i o n o f t h e c r o t o n -a l d e h y d e - 3 - b u t e n a l m i x t u r e i s i n d e p e n d e n t o f p r e s s u r e , s u r f a c e - t o - v o l u m e r a t i o and t i m e . Moreover, i n t h e e x p e r i m e n t s a t 150°C, t h e c o m p o s i t i o n r e a c h e d f r o m b o t h s i d e s i s t h e same. I t i s , t h e r e f o r e , t h e case o f a s t a b l e e q u i l i b r i u m . The a v e r a g e s a t t h e f o u r t e m p e r a t u r e s a r e p l o t t e d i n F i g u r e 5 a c c o r d i n g t o t h e ( a p p r o x i m a t e ) i n t e g r a t e d v a n ' t H o f f e q u a t i o n A H 1 l o g K = I (1) 4.576 T The s o l i d l i n e r e p r e s e n t s l e a s t square f i t t i n g o f a l l t h e d a t a and c o r r e s p o n d s t o A H - 7.20 ± 0.09 K c a l / m o l e (2) I - 1.281 + 0.043 (3) The i n t e g r a t i o n c o n s t a n t " I " can be r e l a t e d t o t h e s t a n d a r d e n t r o p y o f t h e r e a c t i o n , A S ° , i f i t i s assumed t h a t t h e d e t e r m i n e d e n t h a l p y , A H , i s a p p r o x i m a t e l y e q u a l t o t h e s t a n d a r d e n t h a l p y , A H 0 . T . A S ° = A H ° - A F° , A H ° ~ A H (4) and - A F ° = 4 .576-T-logK (5) From e q u a t i o n ( l ) , (4) and (5) i t i s f o u n d t h a t A S 0 ^ 4.576-1 o r A S°=5.86+ 0.20 c a l - m o l e - 1 . °K~ 1 (6) 24 25 Kinetic Studies In one set of experiments crotonaldehyde vapour at five pressures,, ranging from 10 to y6 mm Hg was introduced into the quartz vessel, which was heated at 210°C. The vapour was l e f t 5 to 25 minutes in the vessel, then the contents expanded quickly to the gas chromatography sampler and analyzed. The results were expressed in % 3~butenal and the f i r s t order rate constant, k-, , was calculated using the formula 2.5 100 ki - • log (7) ( l + (X)t 100-(l + <*)x where t is the time- in seconds, x the % 3-butenal and c< the reciprocal of the equilibrium constant for the reaction crotonaldehyde < ' ' > J-butenal (8) k - l The results are summarized in Table II. TABLE II Thermal Isomerization of Crotonaldehyde Temperature °K Pressure mm Hg k (a) sec - 1 x 10 7 42J 20.0 0.987± O.65 443 20.0 2.89 i - O.30 46j 20.0 4.54 £ 0.49 483 20.0 8.86 + 1.22 II 36.0 9.40 ± 1.25 II 25.0 8.90 ± 0.98 ti 20.0 8.25 ±_ 0.93 II 15.0 9.O3 ± O.56 II 10.0 8.70 1 1.03 (a) Uncertainties are mean deviations. 26 It i s seen that k-^  remains virtually constant at the pressure range studied, therefore, reaction (8) is of the f i r s t order from l e f t to the right (forward reaction). The Arrhenius parameters of the forward reaction were estimated by determining the rate constant at four temperatures, the same ones for which the equilibrium constant was determined. The pressure was 20 mm Hg. The results are summarized in Table II and an Arrhenius plot appears in Figure 6. The solid line corresponds to k ^ 1 00.40±0.93 exp [-(14,220 + 1,920)/RT] and i t was found by applying the method of the least squares to a l l the data from runs at 20 mm Hg. The preexponential factor of this expression i s seen to be in complete disagreement with the 10 1? expected from a gas phase f i r s t order reaction. The rate constant, k_-^ , for the reverse reaction was f i r s t determined at 20 mm Hg and four temperatures, as i n the case of crotonaldehyde. The equation for a simple f i r s t order reaction was used 2-5 B 0 k , = • l o g — (9) t B where B o and B are the i n i t i a l and f i n a l concentration of 3-butenal respec-tively. Use of this relation i s justified because the opposite reaction i s much slower. The reaction was run under the same experimental conditions as in the case of the thermodynamic studies, i.e. sealed tubes in a stirred oil-bath were used. Least square treatment of the data gave k_x - 10-0.52 ±1.17 . e x p [-(7,050±2,400)/RT] 27 2 8 Again, i t i s seen that the preexponential factor is quite unexpected for a gas phase f i r s t order reaction. This, and the fact that the activation energies are rather too low for such a reaction, suggested that the reaction is surface catalyzed. In order to establish the influence of the surface on the reaction rate experiments were performed in which the reaction tubes were packed with weighed amounts of glasswool. The pressure was kept at 20 mm Hg. The data are summarized i n Table III. TABLE III Thermal Isomerization of 5 -Butenal (20 mm Hg) Temperature Glasswool 1 k l «s (a) °K mg sec - 1 x 10-^  425 _ 6 . 8 2 + 1 . 1 9 425 100 87 .9 £12.4 445 - 11.2 + 1.52 tt 50 42.5 £ 6.5 n 60 65.2 + 2.2 ti 100 104 £ 8.4 465 - 16.4 £ 4.4 II 100 177 £ 7.5 485 - 18.1 + 5.8 ti. 100 250 £ 1 0 . 1 (a) Uncertainties are mean deviations. It i s apparent from the Table that packing of the vessel increases the rate of the reaction. Least square treatment of the data for vessels packed with 100 mg glass-wool gave "• k_ 1 ( l 0 0 mg glasswool)-10°«58 i 0 r 6 6 . exp [-(7,140 * 1,55O )/RT] It i s seen that the preexponential factor in this case is one order of mag-nitude larger than i n the case of the unpacked vessels, but the activation 29 4.00 3 5 0 3.00 o e o e o e _L o e 2..100 2.200 23oo 2.400 -1 50 energy i s the same within experimental error. This i s apparent also from the Arrhenius plots on Figure 7» where the lines for the two cases are almost parallel. In order to correlate the rate constants at different conditions of packing, a knowledge i s necessary of the surface and volume of the reaction vessels and of the surface per mg of glasswool. For the vessels, which were cylinders 15 cm long with a diameter of 1.0 cm, the volume was 11.8 cm^  and the geometric surface 48 cm . For the glasswool, the surface was estimated by the standard procedure of gas- adsorption using the Brunaurer-Emmet-Teller equation (54,55)« The form used was 1 (C-l) P +• (10) V ( P S - P ) v mc v mc P S where P i s the pressure of the adsorbate, P S i t s saturated vapor pressure at the temperature of adsorption, V the total adsorbed volume at S.T .P. , V M the volume necessary to form a complete monolayer and C - exp [(qi~q 2)/HT], where q-^  i s the heat of adsorption in the f i r s t layer and qg is the heat of liquefaction of the adsorbate and assumed to be the heat of adsorption in a l l layers except the f i r s t . A plot of P/v(P - P ) versus P / P A gives a s s straight line with slope (0-l ) / ( V mC) and intercept l / ( V m C ) , which permits V M to be estimated. The adsorbate used was krypton. If an atomic radius of 1.87A for the close packed krypton atoms i s assumed, i t can be shown that the surface of adsorbent i n (metre) i s obtained by multiplying V M by 2.24 . In this way i t was estimated that the surface of glasswool i s 0.222 (metre)2/gram. ^Adsorption experiments were performed in Dr.L.G.Harrison's laboratory with the help of Dr.R.J.Adams to whom the author i s grateful. 51 Fixate & Dependence o£ Me Raie IrometifiaMef) 52 Given these data, i t i s now possible to calculate the surface/volume r a t i o f o r a vessel unpacked or packed with a weighed amount of glasswool. In Figure 8 the rate constant i s plotted against t h i s quantity. The four points correspond to 0 , JO, 6 0 and 100 mg of glasswool packing. I t i s evident that the rate i s proportional to surface, or the reaction i s f i r s t order with respect to surface. In p r i n c i p l e extrapolation to zero surface would give the extent of the reaction, i f any, which i s homogeneous. This operation, however, to be meaningful requires an accurate knowledge of the r e l a t i o n between B.E.T. and geometrical surface f o r glasswool. As i t i s , extrapolation shows a negative homogeneous rate. We may consider i t as zero. For crotonaldehyde no systematic study of the r e a c t i o n i n packed vessels was performed. Two runs i n tubes packed with 6 0 and 100 mg of glasswool at 170°C showed rate constants 7«5I x 1 0 ~ 7 and 15*2 x 1 0 " ^ sec" 1 respec-t i v e l y , as compared with 2 . 8 9 x 1 0 " ^ sec"! f o r runs i n the unpacked vessel.. This f a c t was considered as s u f f i c i e n t evidence that the forward reaction i s a lso heterogeneous. 4. DISCUSSION Thermodynamic Studies As stated e a r l i e r (Introduction), e q u i l i b r i a between and Afyj isomers of unsaturated carboxylic acids, esters and n i t r i l e s are well known ( 3 0 ) . I t has been found that equilibrium i s very favorable to the 0 ( , ^ -isomer. One ^-substituent i s e s s e n t i a l to give some j?,^ s t a b i l i t y , while a second ^-substituent makes the |3 ,y -isomer the major component of the equilibrium mixture; ^ - s u b s t i t u t i o n favors the fS,^ -isomer but not so powerfully as does ^ - s u b s t i t u t i o n . The present r e s u l t s are the f i r s t , to the author's knowledge, con-55 cerning an unsaturated aldehyde. It i s apparent that the generalizations about the equilibrium composition of acids, esters and n i t f i l e s with no substituents in ^ or ^ position are true also in the present case. As a matter of comparison, Linstead and Noble ( j 6 ) have found that in the equilibrium (crotonic acid) ~c—» (butenoic acid) at 150°C the |5, ^ -isomer constitutes 2% of the mixture. At this temperature the -isomer i n the present experiments constitutes O.J5^« Given the difference i n the nature of the functional groups, and the fact that the equilibrium for the acids was determined i n alkaline solution, the agreement in order of magnitude is satisfactory. The enthalpy found for the reaction, 7*20 + 0 . 0 9 kcal/mole, is a meas-ure .of the resonance energy of the conjugated system -CH:CHCH:0. There is no way of comparing this value with calorimetric measurements for lack of such data in the case of J-butenal. The heat of formation of this aldehyde is estimated by Roberts and Skinner ( 3 7 ) to be - 2 0 . 6 Kcal. The heat_of for-mation of crotonaldehyde was recently ( 3 8 ) found calorimetrically to be -34.45+O.O9 Kcal/mole. This would lead to AH = 1 3 . 8 Kcal/mole for the isomerization. It should be noted, however, that Roberts and Skinner must have underestimated the heat of formation of 5-butenal by 4 - 5 Kcal, because in the same work { 3 7 ) they estimated the same quantity for 5 -butenal to be - 5 2 . 5 Kcal/mole, while the recent ( 3 8 ) calorimetrically established value i s - 5 7 . 0 6 ± 0 . 1 7 Kcal/mole. Then A . H - 8 . 8 - 9 . 8 Kcal. Using the enthalpy of isomerization found in the present work (7«20 ± 0 . 0 9 kcal/mole) and the heat of formation of crotonaldehyde ( - 3 4 . 4 5 + 0 . 0 9 kcal/mole). ( 3 8 ) the heat of formation of 3-butenal i s estimated to be - 2 7 . 2 5 + 0 . 1 3 kcal/mole. In a study of photolysis of crotonaldehyde and 3-hutenal ( 2 3 , 2 4 ) the 54 radical dissociation CH2:CHCH2CH0 > CHgtCHCH^ + CHO was postulated. It i s interesting to determine the longest wavelength of light that can bring about this dissociation from the thermodynamic point of vi^ w, given the heat of formation of 3"butenal found here and the heats of formation of the a l l y l and formyl radicals found in the literature. The heat of formation of the a l l y l radical, CH^iCHCh^ , i s estimated to be 3& +_ 1 kcal/mole based on a l l y l resonance energy of 1J.1 kcal/mole (39) and a primary C-H bond strength in propane of 98 kcal/mole (40) found by Benson and his co-workers. That of formyl radical, CHO , was calculated recently from mass-spectrometric data (4l) to be equal to -0.3+ 3 kcal/mole, while kinetic data lead to two widely different values, -2.8 (42) and 11.2 kcal/mole (43)« If we accept the value -0.3+ 3 kcal/mole (which i s also in essential agreement with the lower value derived from kinetic data) we find for the bond dissociation energy, D(CH2:CHCH2 CHO) D(CH2:CHCH2 CHO) r AH^,( CH^ : CHCH£ ) + AH f(CH0) - AH f(CH z:CHCH 2CH0) = 65.O + 3.I kcal/mole This value sets the longest wavelength of radiation capable to disso-ciate 3-butenal into a l l y l and formyl radicals as equal to 4900 A, well above the wavelength used i n the photolysis of crotonaldehyde and 3-butenal. (23,24). 55 Kinetic Studies The experimental results described in the previous section point to the conclusion that hoth the isomerization of crotonaldehyde to 3-butenal and the reverse reaction are heterogeneous with the rate expressed by the law of the following type: rater S-^kc( crotonaldehyde) -k^(3-butenal)j ( l l ) where S is the surface area. The experimentally determined constants, and k_p are equal to k cS and k^S respectively. The primary scope of this study was to provide information about the extent that a thermal reaction would interfere with photochemical isomer-ization at higher temperatures. The results for crotonaldehyde, obtained in the same quartz vessel i n which photochemical experiments were subse-quently run, show that at 150°C k^= 9.87 x l C f 8 s e c - 1 . It corresponds to a mole-fraction of 3 -butenal equal to 9.55 x 10 r5 i n 1000 sec. In photochemical experiments (Chapter IV) i t was found that at l40°C (the highest temperature studied) the mole fraction of 3 _butenal i n the same length of time i s 3*5^ x 10"5 at 366O A and 2.80 x 10" ^  at 3I3O A. These values are the lowest obtained at this temperdture. It i s evident that the corrections due to thermal reaction are less than k% and can be applied without d i f f i c u l t y (see Chapter IV). It i s interesting to compare the experimental Arrhenius parameters with those predicted by the theory of absolute rates as applied to surface reactions. Laidler, Glasstone and Eyring (44-46) have shown that for a surface reaction obeying f i r s t order kinetics the rate law is given by the expression 36 h5 Rate = Cp.L~ 7^  e-^«/kT m o i e c u i e c m - 3 sec--'- (12) S ^8TT 2 ( 8 T T 5 A B C ) 1 / 2 ( 2 1Tm)5/ 2( kT ) 2 where 0" and are the symmetry numbers of the molecules of reactant and activated complex, m is the mass of the reacting molecule and A, B, C, i t s three moments of inertia. L i s the number of surface sites per cm5 and C g the number of reactant molecules per cm?. The interatomic distances i n the molecule of crotonaldehyde are -known (47, 48) and the three moments of inertia were estimated by a graphical method. They are: ' A = 3 .58 x 10~5 8 g.cm2 B - 3 .42 x 10-38 n C = 1.29 x 10-59 " The symmetry numbers may be assumed to be the same i n both the reactant molecule and the activated complex. The number of surface sites per cm? can be approximated by the number of cm2 per cm^ , or the surface/volume ratio of the reaction vessel, multiplied by 10*5 (46). Thi s ratio i s about 1.2 for the quartz vessel used. Application of equation (12) gives for the predicted rate: Rate = 1.3 10~A- e" ^ k ^ - c _ molecule cm"? sec -* © The quantity 1.3 l O - ^ sec"l corresponds to the preexponential factor. It i s about three orders of magnitude smaller than the factor found in this study (10-0-52 +1.17) Another approach to theoretical estimation of the Arrhenius parameters is as follows. It is assumed that the adsorption-desorption equilibrium is 57 a much faster process than the chemical reaction occuring at the surface. For a unimolecular reaction the relevant equations are k a A + S c > AS ( 1 3 ) AS product (14) If (13) is fast, the concentration of adsorbed molecules, (AS), i s not disturbed by the occurrence of reaction ( l 4 ) . It follows that (AS) = Ka-(A)-S (15) where K a= kg/k^ is the equilibrium constant of adsorption. The surface, S, can be expressed as a function of the total surface area, S°, i f a type of adsorption i s assumed. For a Langmuir adsorption S = S°/\l+ K a-(A)] Substitution into (15) gives (AS) = Ka- (A)-S°/[l +- Ka- (A)] which leads to the following expression for the rate of the reaction Rate = kr.Ka-S°-(A)/[l + K a-(A)] ( l 6 ) If f i r s t order kinetics are obeyed, K-(k) and the rate expression simplifies to (17). Rate - k r-K a-S°-(A) = k e x p-(A) (17) where k Q Xp •= kj.- Ka- S° is the experimental rate constant. 38 Since k f is a unimolecular rate constant, i t is equal to k r - Ar.exp(-E/RT) ( l 8 ) where A. - 1015 sec"-1- . E is the activation energy for reaction ( l 4 ) . K&, being an equilibrium constant, can be expressed as a function of the standard entropy of adsorption, A S ° , and the enthalpy of adsorption, AHa. K a = exp(AS°/R -4H a/RT) (19) The approximations involved i n (19) were discussed i n relation with the estimation of the enthalpy and standard entropy of the isomerization. From relations ( l 7 ) > ( 1 8 ) and ( 1 9 ) i t follows that the experimental preexponential factor, A e Xp, and activation energy, E e Xp, are i n fact: A e x p = A r-S°.exp&S°/R) ( 2 0 ) Eexp = E +4H a ( 2 1 ) A more general treatment, leading to relations ( 2 0 ) and ( 2 1) i n the special case of f i r s t order kinetics, has been made by Schuit and van Reijen ( 4 9 , 5 0 ) « The entropy of adsorption of crotonaldehyde on quartz is not known. As a rough approximation i t can be considered as equal to - 2 3 cal-mole" which i s the entropy of liquefaction for an unassociated compound (Trouton's constant). Before this quantity i s used into ( 2 0 ) , however, i t vnust be multiplied by a conversion factor i n order to be expressed in the correct units. Equation ( 1 9 ) is true for concentrations i n the equilibrium constant expressed as atmospheres, i.e. for K a having dimensions atm*"!. In the rate law, equation ( 1 7 ) , the surface, S°, is expressed as site •cm""?. It is clear that K a must be i n units of molecule"-*- cm? for k to have dimensions 59 s e c - 1 . (The units "site" and "molecule" are considered equivalent and cancel out). In order to convert from pressure into concentration units, a tempe-rature must be chosen. Since the standard state of gases is customarily considered to be at 1 atm and 25°C, this temperature i s chosen. The conversion factor then is 4.06 x 1 0 - 2 0 atm-molecule-1 cm?. A r ~ 10 1? sec" 1 and S ° ~ 1.2 lO 1^ site cm-?. Substitution into (20) gives Aexp - l o 1 5 ' 1'2'1°1'>' 4.06'10 - 2 0. exp(-23/l.98) 21 4 .5- io5 sec" 1 It is seen that the value of the preexponential factor calculated i n this way is about four orders of magnitude larger than the value found experimentally. The uncertainty involved i n assuning a value for the stan-dard entropy of adsorption could account for the discrepancy. For complete agreement with the experimental preexponential factor the entropy should be -42 e.u. This value is not improbable, given, the differences i n magni-tude between entropy of liquefaction and adsorption which are known to exist (5l)« It is seen from equation (21) that the activation energy of reaction (l4) i s equal to the experimental quantity minus the enthalpy of adsorption. Since the latter quantity is usually negative, the "true" activation energy i s larger than the experimental one. For AH &^AH(liquefaction) = 8.60 kcal/mole (47) the "true" activation energy must, therefor^ be of the order of E ~ 22.8 kcal/mole 4o IV. DETAILED STUDY OF THE MECHANISM OF PHOTOCHEMICAL ISOMERIZATION OF CROTONALDEHYDE 1. APPARATUS The vacuum system and gas chromatography apparatus described i n the previous chapter were used also i n the present study. The cylindrical quartz vessel with f l a t windows was f i t t e d at point (A) of the apparatus (Figure 3) and used throughout the photochemical experiments. The Optical System The optical system is shown in Figure 9« The lamp (A) was a 250 Watt medium pressure BTH ME mercury arc. The positions of the lamp and the quartz lenses, (B) and (E), were adjusted i n such a way that a parallel light beam f i l l e d the reaction vessel as much as possible without reflection on the side wall. (G) was an RCA 935 photocell which i s known to have i t s maximum response at 3400 + 500 A. The photocell was shielded by a neutral density f i l t e r (a fine wire mesh) so that the light intensity f a l l i n g onto i t never exceeded i t s region of linear response. Linearity was checked by means of a set of calibrated neutral density f i l t e r s . (C) and (D) were f i l t e r combinations recommended by Kasha (52) with some modifications. For isolation of a narrow band around 3^30 A the double vessel shown at (C) was used. The f i r s t compartment, 5 0 1 1 1 l°ng, was f i l l e d with an aqueous solution of nickelous sulphate, containing 200 g NiSO^^R^O per l i t r e . The second compartment, 1 cm long, contained an aqueous solution of potassium chromate containing 0.200 g ^CrO^ per l i t r e . On (D) a Corex 9863 f i l t e r was placed i n front of a f i l t e r c e l l 1 cm long containing an aqueous solution of acid potassium phthalate (5«0 g KHCgH^ O^  per l i t r e ) . 41 42 To i s o l a t e the band at 3 J 4 0 A a si n g l e 5 c m long f i l t e r c e l l was placed at (O) and i t was f i l l e d with an aqueous s o l u t i o n of nickelous sulphate containing 100 g per l i t r e . The Corex f i l t e r was l e f t at (D) and the 1 cm long c e l l was f i l l e d with a s o l u t i o n of naphthalene i n isooctane, containing 1 2 . 8 g C o^Hg P e r l i t r e . For work at $660 A a 5 P m long vessel f i l l e d with an aqueous sol u t i o n of copper sulphate ( 100 g CuS0^'5H2(-) P e r l i t r e ) was placed at (c) and a Pyrex 7580 f i l t e r was placed at (D) i n f r o n t of the Corex 9 8 6 j f i l t e r . No other f i l t e r c e l l was used. A l l f i l t e r c e l l s had quartz windows and the one nearest to the lamp was cooled by means of a rubber tube wound around i t through which tap water was flowing. Cooling was necessary, because i t was found that when the f i l t e r solutions were overheated t h e i r transmission c h a r a c t e r i s t i c s were appreciably a l t e r e d . The spectra of the f i l t e r combinations were determined against no blank i n a Carey 14 automatic recording spectrophotometer. The spectra are shown i n Figure 1 0 . I t i s seen that the three f i l t e r combinations have t h e i r adsorption minima near yly0, yyk0 and y660 A r e s p e c t i v e l y . The Photometer Unit The c i r c u i t diagram of the photometer u n i t i s shown i n Figure 11 . The photocell was an RCA 955 Phototube with a quartz envelope designed f o r work i n the u l t r a v i o l e t region. The photocell current passed through the re-s i s t o r chain (RI . A) and the required voltage was tapped o f f by the selector switch, (S-^). An opposing voltage was provided by the potentiometer c i r c u i t (B^) and (Rcj_i2)* (B-^) was a 2 v o l t battery and the voltage tapped from the potentiometer was adjustable to any value between 0 and 2 v o l t s with an ac-curacy of 2 x 1 0 " ^ v o l t s . The difference between the voltage tapped from 3000 StOO J ft 3 1 0 0 44 S.C-KEY: B 2 G. P s C o S.C. V. Hi R 2 R5 R4 >i R9,10 R l l , 1 2 2 v o l t accumulator. 120 v o l t h.t. battery Galvanometer Photocell: R C A 955 Weston Standard C e l l 6SC7 valve 5M : h.s.c. : 1W 1.5M : h.s.c. : 1W 500k : h.s.c. : IW 150k : h.s.c. : 1W dual decade II II 10k l k : 100 ohms " 10 ohms " Muirhead ti R 1 3 25k Ri4 2k R15 R l 6 R 1 7 R18 R19 R20 R21 50k 50k 2.2k 1M 100k 55k 10k R 2 2 5-5k R25 l k w.w. w. w. w.w. w.w. w.w. £w w.w. w.w. w.w. . w.w. w.w. R24 R25 550 ohms: 10k: w.w. 1W 1W 1W 1W 1W 1W 1W 1W w.w.: : 1W 1W Ftqutt II. T^lMmekt Unit 4 5 the r e s i s t o r chain (R^.^) and the opposing voltage from the potentiometer was applied to the double-triode amplifier valve (V). The second triode u n i t compensated f or supply voltage v a r i a t i o n . The output of the am p l i f i e r was fed either i n t o the galvanometer (G) or to the recorder (R) through the attenuator (R^p_24)- The high i n t e n s i t y f o r the unit was supplied by a 120 v o l t battery and the low i n t e n s i t y from a 6 v o l t transformer. In making a photometric measurement the following procedure was followed. The c i r c u i t was switched on by closing switches ( S 2 ) , (Sj) (to the r i g h t ) , (S^) and (Sy) and allowed to warm f o r 15 minutes. With no l i g h t f a l l i n g on the photocell and with the potentiometer reading zero, dark current from the photocell was balanced by adjusting (R^z.) and ( R ^ ) f o r zero galvonometer current. The c i r c u i t had to be rebalanced f o r each p o s i t i o n of the selector switch (S^). To ensure that the battery (B^) gave a constant voltage, switch ( S 5 ) was closed and the voltage from the standard c e l l (S.C. ) was balanced by increasing the voltage of the potentiometer. A reading was taken which was the same every time that the operation was repeated. When the voltage of the battery had decreased, t h i s reading was d i f f e r e n t and then the battery was recharged. Switch ( S 5 ) was opened and l i g h t allowed to f a l l on the photocell. The photocell current was balanced with the potentiometer and a reading taken. This reading was proportional to the l i g h t i n t e n s i t y . In p r a c t i s e the photocell current was only p a r t l y balanced and the resi d u a l voltage was measured by means of the recorder (R). For t h i s mode of operation, switch (Sy) was opened and switch (S^) was thrown to the l e f t . Conversion factors were determined f o r each s e t t i n g of the selector switches (S-^) and (S^) to convert m i l l i v o l t readings i n t o potentiometer u n i t s . Continuous recording of the l i g h t i n t e n s i t y had the advantage that v a r i a t i o n s 46 of the i n t e n s i t y due to imperfections of the lamp and s l i g h t photolysis of the f i l t e r solutions could be followed and corrections applied. The photometer unit was used f o r monitoring the output of the mercury arc and determining the absorption curves of crotonaldehyde. 2 . EXPERIMENTAL Determination of the Absorption Curves of Crotonaldehyde The absorption curves of crotonaldehyde were determined at 31J0 and 366O A at various temperatures. No special runs were needed f o r t h i s deter-mination. The runs f o r study of the photolysis at these two wavelengths • and at various temperatures provided the necessary data f o r the construction of the absorption curves. The procedure was as follows. The lamp was switched on and allowed to warm f o r about 30 minutes. At t h i s time i t s output was f a i r l y stable. Fresh f i l t e r solutions were put i n the f i l t e r c e l l s and the l i g h t allowed to pass through the evacuated reaction vessel on to the photocell. The output of the photocell was p a r t l y balanced by means of the potentiometer and the residual voltage was monitored by the recorder. Crotonaldehyde vapour was admitted into the reaction vessel at a pressure measured by means of the s p i r a l gauge. The recorder registered then a sharp drop i n voltage and the pen adjusted to a d i f f e r e n t reading. From the two readings i t was possible to calculate the percent absorption of l i g h t by the measured pressure of crotonaldehyde. The r e s u l t s are plotted according to Beer's equation i n Figures 12 and l j . The pressures are corrected to 25°C. It i s apparent that Beer's law holds under the conditions of the experiment. The e x t i n c t i o n c o e f f i c i e n t f o r l i g h t obtained by using the JIJO A f i l t e r combination (Figure 12) i s constant at the temperature range of 2 5 ° to l40° C and equal to 1 6 . 1 47 49 mole ^*lt«cm - 1 . The value found at J1J0 A by studying the spectrum of crotonaldehyde obtained by the Carey 14 instrument (Figure 14) i s 15*5 mole" 1 ,It*cm - 1 . The agreement i s s a t i s f a c t o r y , given that the e x t i n c t i o n c o e f f i c i e n t determined from Figure 12 i s for absorption of a band of con-siderable width (Figure 10). The e x t i n c t i o n c o e f f i c i e n t f o r l i g h t obtained by using the 366O A combination (Figure 15) varied with temperature; i t was equal to 5*27, 6.02 and 6.47 mole - 1'It*cm - 1 at 25, 100 and l40°C r e s p e c t i v e l y . The value obtained at 25 from the Carey 14 spectrum i s 5«15 mole - 1-lt«cm - 1. It i s thought that the increase of e x t i n c t i o n c o e f f i c i e n t with temperature f o r l i g h t around 566O A i s due to the f a c t that t h i s region of wavelength l i e s very near the end of the crotonaldehyde absorption region (Figure 14). Since the band used i n determining the e x t i n c t i o n c o e f f i c i e n t s from Figure 13 has a considerable width (Figure 10)> i t covers the very edge of the ab-sorption region of crotonaldehyde. This region corresponds to absorption • by molecules i n v i b r a t i o n a l l y excited l e v e l s of the ground state. As the temperature increases the population of these l e v e l s increases, r e s u l t i n g -in higher e x t i n c t i o n c o e f f i c i e n t . Once the e x t i n c t i o n c o e f f i c i e n t s were determined the percentage l i g h t absorbed i n each photochemical run could be calculated from the pressure of crotonaldehyde by using Beer's equation. For most runs t h i s percentage had already been found experimentally (points i n Figures 12 and 13). Never-theless, the calculated value was used i n estimating quantum y i e l d s because' i t was s t a t i s t i c a l l y more accurate. In some runs i t was d i f f i c u l t to obtain r e l i a b l e experimental value, due to r e l a t i v e l y wide f l u c t u a t i o n s of the l i g h t i n t e n s i t y . This was p a r t i c u l a r l y true i n runs at pressures below 5 mm Hg. In these cases use of the determined e x t i n c t i o n c o e f f i c i e n t s was the 50 51 only way to calculate the percentage of light absorbed. No absorption curve was constructed for work at 5540 A, because runs at this wavelength were at a single pressure, 20 mm Hg, and temperature, 25°C. At these conditions the extinction coefficient was found t.o be 17.8 mole ^'It'cm ^, as compared with 17*5 found by studying the spectrum produced by Carey 14. Actinometry The photometer unit was calibrated by using the potassium ferrioxalate actinometer. The procedure followed was essentially the one described by Hatchard and Parker (53)* It i - 3 based on the fact that dilute acidic solu-tions of potassium ferrioxalate when illuminated produce ferrous ions with an accurately known quantum yield. The concentration of ferrous ions pro-duced is determined by reacting an aliquot of the photolysate with a 0,1% aqueous solution of 1:10 phenanthroline and a buffer solution prepared by adding 5°0 ml of 1 N tig*®!), to 600 ml of 1 N sodium acetate and diluting to 1 l i t r e . A red color develops and the optical density of the mixture is measured at 510 W|A against a blank containing the same reagents, except for the fact that the potassium ferrioxalate has not been photolyzed. A Bausch and Lomb spectrophotometer was used. It was f i r s t calibrated against standardized solutions of FeSO^. It was found that the instrument gave an optical density reading proportional to the ferrous ion concen-tration in the O.D. region 0.200 to 0.750- The proportionality constant was 7.80 x 10~ 5 M(Feff) (O.D.) - 1. The photometer unit was calibrated for position 3 of the selector switch (S^) (Figure 1$). Light was allowed to f a l l onto the photocell and i t s intensity was recorded for some time. When a steady intensity was ob-tained the photocell was replaced by a quartz vessel containing 40 ml of 52 potassium f e r r i o x a l a t e s o l u t i o n and blackened on a l l sides except the one f a c i n g the l i g h t beam. After a measured time the vessel was withdrawn and the l i g h t i n t e n s i t y measured again by means of the photometer u n i t . The mean of the two readings was considered to be the i n t e n s i t y during the ex-posure of the actinometric s o l u t i o n . An aliquot of the s o l u t i o n was measured into a 5^ m l volumetric f l a s k , enough 0.1 N R^SO^ was added to make the volume 10 ml, and 10 ml each of 1:10 phenanthroline and buffer solutions were added. The f l a s k was then f i l l e d up to the mark with d i s t i l l e d water. A blank s o l u t i o n was prepared by using the same reagents. During the whole operation the room was darkened. Aft e r standing f o r JO minutes i n dark the o p t i c a l density of the red s o l u t i o n was meaeured against the blank s o l u t i o n at y10 w\|». From the O.D. the number of moles of Fe + + was calculated. The quantum y i e l d of production of Fe+* i s given by Hatchard and Parker (53) as 1»23 and 1.21 mole«einstein -! at 3130 and 366O A respectively, therefore the number of einsteins of l i g h t could be calculated and thence the f a c t o r converting potentiometer units (ohms) into l i g h t i n t e n s i t y units ( e i n s t e i n s e c - ! ) . The factors found i n t h i s way were 3 » H x 10~^ and 3*50 x 1 ° ~ e i n s t e i n * s e c - l o h m ~ * at 51?° and 366O A respectively. These factors had to be corrected f o r r e f l e c t i o n and absorption of l i g h t by the reaction vessel windows, the back window of the oven and the window of the actinometer c e l l . In the following discussion secondary r e f l e c t i o n e f f e c t s are ignored. When l i g h t of i n t e n s i t y I Q f a l l s onto the f r o n t window of the reaction vessel part of i t i s absorbed and r e f l e c t e d by the vessel windows. I f the i n t e n s i t y behind the vessel i s A Q= ( l 0 - I i ) / l 0 i s the absorption coef-f i c i e n t of the c e l l (including r e f l e c t i o n ) . Assuming that both walls have 5? x the same c o e f f i c i e n t the i n t e n s i t y inside the vessel i s I Q ( l - A c ) s . No account has been taken, however, of r e f l e c t i o n . I f each quartz surface r e f l e c t s i n t e n s i t y 1^ when i n t e n s i t y I Q f a l l s onto i t , the r e f l e c t i o n coef-f i c i e n t i s R=.Ip/l 0 and, since a window has two surfaces, the i n t e n s i t y i n s i d e the vessel i s I Q [ ( l ~ A c ) s + 2R] . I f the absorption c o e f f i c i e n t of the back window of the oven i s A o v , the i n t e n s i t y behind t h i s window i s I 0 ( l - A c ) ( l - A o v ) , and the' i n t e n s i t y i n the rea c t i o n vessel i s I 0 [ ( l - A c ) ¥ + 2R t 2R(l~Ac)? /' 2] due to r e f l e c t i o n by the oven window, cor-rected f o r absorption by the vessel window. F i n a l l y , i f the absorption c o e f f i c i e n t of the actinometer c e l l window i s A a c the i n t e n s i t y absorbed by the actinometric so l u t i o n i s I 0 ( l - A c ) ( l - A o v ) ( l - A a c ) . R e f l e c t i o n from t h i s window a f f e c t s the i n t e n s i t y i n the reaction vessel only when the l i g h t passing through the vessel i s measured by means of the actinometer, as the case was i n a set of experiments run before the recorder was f i t t e d to the photometer u n i t . In t h i s case the i n t e n s i t y i n the vessel i s (l-Ac)8 t 2R+ 2R(l-Ac)?/2 + 2R(l-Ac?/ 2( 1 - A Q V ) 2 J Normally, however, the i n t e n s i t y was recorded by means of the photometric u n i t . In t h i s case the l a s t term ins i d e the brackets i s dropped. A -and A Q V were measured and found to be O . I 6 5 and O.O76 respectively. A could not be measured and i t was "assumed to be equal to the reflectance, 2R, of a quartz window. R was estimated by using Fresnel's formula f o r 2 normal incidence R: n-1 n+1 where n=.1.48 i s the index of r e f r a c t i o n f o r quartz. I t i s found R=O . O J 7 5 Now the r a t i o of the l i g h t i n t e n s i t y inside the reaction vessel to that absorbed by the actinometric s o l u t i o n can be calculated. It i s found 54 to be 1.47 f o r normal runs. Therefore the corrected conversion f a c t o r s are 4.57 x 1CT 1 1 and 5 . 1 5 x 1 0 - 1 1 einstein-sec" 1«ohm~ 1 at 313O and 366O A r e s p e c t i v e l y . For runs i n which the l i g h t was measured by means of the actinometer the correction f a c t o r i s found to be 1.54 . Determination of Reaction Products It has been established (23 ,24)-that the only measurable product of the photolysis of crotonaldehyde at the near u l t r a v i o l e t region f o r conversions up to 1% i s 3-butenal. I f the photolysis i s extended i n time three other products can be measured, i . e . propylene, b i a l l y l and carbon monoxide. In the present work the re a c t i o n was confined to the isomerization stage, except f o r a short study, of the e f f e c t of wall conditions on the quantum y i e l d , i n which photolysis was allowed to proceed f u r t h e r . Even i n t h i s case, however, the amount of carbon monoxide formed was not enough to be measured gas-chromatographically, due to the r e l a t i v e i n s e n s i t i v i t y of the thermoconductivity detector. The reaction mixture was analyzed gas-chromatographically using the flame i o n i z a t i o n detector. The dinonyl phthalate column, used f o r analysis during the thermal experiments (Chapter I I I ) , was also used i n the present case under the same operating conditions. The peak of the chromatogram corresponding to crotonaldehyde was used as a reference, because the i n i t i a l pressure of t h i s compound was accurately known and corrections due t o reaction were small and could e a s i l y be applied. Conversion f a c t o r s transforming peak areas of 5-butenal, b i a l l y l and pro-pylene i n t o equivalent peak area of crotonaldehyde were determined by means of c a l i b r a t i n g chromatograms using mixtures of known composition. From the stoichiometry of the reaction i t i s found that one molecule of 55 3-butenal' or propylene i s produced from one molecule of crotonaldehyde, while one molecule of b i a l l y l i s produced from two molecules of the aldehyde. Therefore, if the areas of the peaks corresponding to crotonaldehyde, 3 -butenal, b i a l l y l and propylene are A c, A R, Ag-i and A re s p e c t i v e l y the area A Q, corresponding to the i n i t i a l amount of crotonaldehyde, C 0, i s A 0 = A c t Ag +• 2Ag]_ t Ap. (The areas of the products are expressed as equivalent areas of crotonaldehyde). The r a t i o 0Q/kQ i s then the f a c t o r converting A C J 2Ag^ and A p into the corresponding amounts of products. This method of analysis had the advantage that f l u c t u a t i o n s of the s e n s i t i v i t y of the i o n i z a t i o n detector from run to run do not a f f e c t the an a l y s i s . I t also made i t possible to use only a small amount of reaction mixture for analysis with the r e s u l t that the chromatography apparatus had a very low background l e v e l . I t was not neccessary to measure t h i s amount accurately, because the method u t i l i z e s only the t o t a l amount of croton-aldehyde which was accurately known. About 2 . 5 mm Hg of reaction mixture were admitted to the chromatography sampler f o r a n a l y s i s . 3. RESULTS E f f e c t of the Condition of the Wall and of Wavelength on the Photolysis A major d i f f i c u l t y i n studying the photolysis of crotonaldehyde i s presented by the slow formation of polymer during the reaction. The amount of polymer deposited on the walls of the vessel during a normal run (e.g. one i n which the measurable products amount to 2-3% of the reaction mixture) i s very small, as judged by the f a c t that the o p t i c a l density of the v e s s e l , measured before and a f t e r such a run, changes very l i t t l e . This polymer, however, cannot be removed by evacuating the vessel and as a r e s u l t i t ac-cumulates with successive runs. The rate of accumulation i s not i t s e l f 56 constant, but i t diminishes as the number of runs increases u n t i l no more polymer i s deposited. In t h i s steady-state condition the rate of polymer deposition must be equal to the rate of i t s destruction, probably by photo-l y s i s . The photolysis of crotonaldehyde was studied e a r l i e r (23,24) i n a re a c t i o n vessel which was allowed to reach t h i s steady-state condition. Such a vessel w i l l be c a l l e d i n the following discussion a "seasoned" ve s s e l . In the present work i t was thought necessary to investigate the e f f e c t which a "seasoned" vessel could have on the course of the photolysis. Radiation of 3150* 55^0 and 366O A was used, so that information of the e f f e c t of wavelength on the photolysis could be obtained at the same time. A "seasoned" vessel was obtained by photolyzing crotonaldehyde i n successive runs using the f u l l output of the lamp f i l t e r e d only through the Corex 9863 glass. This operation continued u n t i l the amount of r a d i a t i o n absorbed by the vessel a f t e r one hour of pumping out was steady. A "clean" vessel was obtained by heating the vessel i n the presence of about 10 mm Hg of oxygen to 700°C f o r ten minutes and then allowing the temperature of the oven to f a l l to 25°C. This operation was repeated a f t e r every run. Experiments were run i n both vessels at 25°C and 20 mm Hg pressure of crotonaldehyde at the three wavelengths mentioned above. At the time the photometric u n i t had not been equipped with a recorder and i t was found necessary to measure the transmitted r a d i a t i o n by means of an actinometer placed behind the reaction v e s s e l . The experimental r e s u l t s appear on Tables IV and V. In the l a s t column of the tables the amount of 3-butenal i s given corrected f o r secondary photolysis to propylene and b i a l l y l . Plots of ( 3 - b u t e n a l ) c o r against TABLE IV Photolysis of Crotonaldehyde i n a "Seasoned" Vessel Pressure of Crotonaldehyde: 20 mm Hg. Temperature: 25°C. Wave-length Time min Absorbed 111. 5-Butenal molexlO^ B i a l l y l molexlO" Propylene molexlO^ (3-Butenal , , ~ 7 cor molexlO' A E i n s t e i n X I C K . X y z x +2y + z JIJO 10 0.223 1.19 1.19 11 . 15 0.554 1.47 - - 1.47 n 20 0.819 4.29 4.21 7.85 4.46 n 30 0.945 4.77 7.50 8.18 4.98 11 50 1.45 8.88 17.6 20.6 8.86 11 50 1.76 10.1 21.0 51-7 10.8 11 60 2.08 12.2 29.8 45.5 13.2 11 60 2.08 13.0 34.1 46.5 14.1 11 70 2.61 15.0 46.5 70.0 16.2 11 120 4.72 27.4 149 248 32.9 3340 10 0.467 1.66 - - 1.66 n 15 0.786 3.84 - - 5.84 11 20 0.854 3.02 - - 5.02 n 30 1.26 5.12 1.62 i . 4 o 5.16 11 40 1.66 7.49 5.21 1.40 7.56 ti 50 1.99 10.6 5.27 3.82 10.7 n 56 2.05 9.95 4.26 2.78 10.1 11 65 5-17 14.9 11.0 7.57 15.2 11 90 5-87 18.1 17.2 12.6 18.6 n 105 4.89 25.2 30.4 21.4 26.0 11 110 4.94 24.1 27.4 20.8 24.8 3 6 6 O 5 0.521 1.52 - - 1.52 11 10 0.996 2.67 - - 2.67 ti 15 I.56 4.57 - - 4.57 it 20 1.90 5.84 - - 5.84 it 4o 2.62 6.94 - - 6.94 n 45 5-97 11.9 - - 11.9 11 60 5.54 19.2 19.2 absorbed i l l u m i n a t i o n are shown i n Figure 15. The following general conclusions are drawn from these p l o t s . (1) The quantum y i e l d of isomerization increases s l i g h t l y as the reaction proceeds i n both the "seasoned" and "clean" vessel. (2) The quantum y i e l d i s appreciably higher i n the "seasoned" than i n the "clean" v e s s e l . 58 O 3/30 /, ) Tfoee uflvez dvtae* A * * ^-W*^* / W 3660 /) j 7 ^ U c ^ ^ faw "Cfaw* Ifeuel o / o.o27 O 2 o.oo9o 4.0 8.0 esn * / o -59 TABLE V Photolysis of Crotonaldehyde i n a "Clean" Vessel Pressure of Crotonaldehyde: 20 mmHg. Temperature: 25°C Wave-length A Time min Absorbed 111. E i n s t e i n xlo5 3-Butenal mole 10' X B i a l l y l mole 10? y . Propylene mole 10 9 z ( 3 - B u t e n a l ) c o r mole 10' x + 2y + z 3130 25 0.957 0.767 M 0.767 n 52 2.14 2.59 8.66 15.1 2.91 it 120 4.15 5.63 44.1 61.0 7 . H it 190 6.45 7 .48 99.0 l 4 l 10.9 3340 45 1.87 2.50 - - 2.50 u 65 2.20 2.50 1.45 1.08 2.5? 11 80 2.71 2.78 2 .24 1.56 2.85 11 110 4.58 5.93 9.38 7.20 6.32 it 150 3.87 5 .40 8 .42 6 .84 5 .64 3660 20 2.92 1.66 - - 1.66 ti 35 4.67' 3.32 - - 5.52 11 75 8.54 5.70 mm 5.70 (3) The quantum y i e l d i n both vessels increases with decreasing wave-length. A further point of i n t e r e s t i s the f a c t that propylene and b i a l l y l do not appear as products of the photolysis at 366O A (Tables IV and V). This f a c t i s e a s i l y understood i n terms of the mechanism (23, 24) i n which these products a r i s e from photolysis of 3~butenal considering that 3-butenal does not absorb i l l u m i n a t i o n of t h i s wavelength (Figure 14). E f f e c t of Pressure and Temperature on the Quantum Y i e l d of Isomerigation Crotonaldehyde at pressures ranging from 0.4 to 34 mmHg was photolyzed i n a "clean" vessel at the temperature range 25 to l40°C and at the wave-lengths of 51?0 and 5660 A. The reaction was confined to the early stage where no appreciable amounts of propylene and b i a l l y l appear. Whenever these products did appear i n measurable amounts corrections were made to the amount 60 of J-butenal produced. These corrections i n no case accounted f o r more than 5% of the amount recorded (Tables VI and VII). TABLE VI Photolysis of Crotonaldehyde at JIJO A. Temperature Pressure Time Absorbed 3^Butenal(a) 1/<T (b) 111. °0 mmHg minutes E i n s t e i n 10 6 mole 10* ein»mole~ 25 5.9 25 2.56 86.8 29.5 11 7.5 20 4.32 122 55.4 11 9.5 25 5.45 117 46.6 11 12.9 30 6.69 125 55.5 11 14.0 30 9.88 190 52.0 n 18.4 30 U.7 177 66.1 •it 19.0 25 6.97 115 60.6 it 21.6 50 15.8 195 71.5 60 4.5 26 2.41 105 25.4 n 12.6 30 7.48 26l 28.6 ti 17.9 55 10.7 557 31.8 11 26.0 30 13.4 556 57.7 it 28.9 50 12.5 557 35.0 100 4.2 16 1.31 84.5 15.4 11 5-5 15 1.54 90.0 17.0 tt 9.6 20 5-73 184 20.1 11 15.5 20 5.62 229 24.4 11 15.5 30 4.60 207 22.0 n 17.4 20 4.55 231 18.7 it 26.J 25 11.3 471 23.8 11 28.1 20 11.6 450 25.6 140 2.9 15 0.68 62.8 10.7 n 5.8 16 2.00 162 12.3 tt 10.8 16 2.06 172 12.0 it 11.5 17 2.64 181 14.6 11 16.6 21 4.91 342 14.3 11 19.2 20 5.18 589 15.2 it 23.4 20 6.29 407 15.4 11 24.2 20 5.62 411 15.7 11 34.0 20 8.01 507 15.8 (a) Corrected f o r secondary photolysis. (b) Corrected f o r dark reaction. 61 TABLE VII Photolysis of Crotonaldehyde at y660 A. Temperature °C Pressure mmHg Time minutes Absorbed 111. Einstein 10° 3~Butenal(a) mole 10^ 1/f (b) ein'mole"! 25 0.4 30 0.732 21.5 34.0 11 2.5 30 4.74 76.0 62.4 11 2.6 20 2.70 48.5 55-6 it 7.4 28 11.4 . 120 95.0 tt 9.2 30 16.4 194 84.6 ti 12.4 30 19.5 143 136 11 15.5 30 27.0 184 147 11 18.9 30 33.2 192 173 11 20.5 30 36.9 190 194 100 2.8 18 2.51 57-0 42.9 It 6.0 20 5.52 113 48.6 tt 14.2 21 14.4 212 67.6 It 20.2 20 19.7 240 82.0 II 28.0 20 26.5 277 95-2 140 2.1 15 I.69 47.7 35-1 It 3.9 10 2.02 51.0 39*7 II 5.4 10 2.88 74.3 38.9 tl 8.7 10 5.04 114 44.5 II 10.0 15 7.80 179 43.3 II 1-12.4 15 9.74 216 44.9 II 16.8 10 8.76 174 50.5 II 19.2 15 150 286 53-5 II 23.2 17 20.5 407 50.0 II 28.7 15 22.0 391 56.2 II 31.0 16 24.0 403 59.9 (a) Corrected for secondary photolysis. (b) Corrected for dark reaction. A correction for concurrent dark reaction was also applied. If the rate of the thermal isomerization of crotonaldehyde i s k-^ A and that of the opposite reaction is k_^B, where A and B are the concentrations of crotonaldehyde and 3-butenal respectively, the overall rate is dB/dt = TI + k xA - k_xB (1) 62 <P i s the quantum y i e l d of the photo-isomerization and I i s the absorbed l i g h t i n t e n s i t y . For small conversion A cr A Q , the i n i t i a l concentration of crotonaldehyde, and under t h i s condition both I and k]A are constant. The integrated form of ( l ) i s then (TI + k ^ / O H +• k xA 0 - k_iB) = exp(k-lt) (2) For k_2«l , which i s true i n the present case, the exponential can be approximated by ( l -t- k _ ] t ) . It i s found *P - (B + k_ xtB - k 1 t A 0 ) / ( l t ) But B / ( l t ) i s the apparent quantum y i e l d , Then the true quantum y i e l d i s <fs <p a(l + k > i t ) — k ^ / l (3) Values of k _ i and k^ at the temperatures of the experiment were e s t i -mated by using the information gathered during the study of the thermal rea c t i o n (Chapter I I I ) , k-^  was calculated from the formula k]_ = 1 0 G « 4 0 e x p ( _ 1 4 < 2 2 0 / R T ) k_-^  was assumed to be equal to k]/K , where K i s the equilibrium constant of the thermal reaction and i s given by the formula K = 10 1«28l e x p(_7,200/RT) Actual c a l c u l a t i o n showed that corrections of the quantum y i e l d f o r dark re a c t i o n are le s s than 0.y% below 70°C. Therefore, such corrections were applied only at experiments run at 100 and l 4 0°C. In no case were they higher than 2%. 6 5 The inverse of the quantum y i e l d , l / ^ , i s plotted against pressure on Figure 16 (Stern-Volmer p l o t s ) . I t i s seen that l/<3> increases l i n e a r l y with the pressure and that the rate of increase i s highest at low temper-ature and long wavelength. The quantum y i e l d at the l i m i t of zero pressure increases generally with increase i n temperature and decrease i n wavelength, but i t i s always smaller than unity. Slopes and intercepts calculated by le a s t squares approximation appear i n Table VIII. TABLE VIII Least Squares Estimation of Slopes and Intercepts of the Stern-Volmer Plots of Crotonaldehyde Wavelength Temp. Slope (a) Intercept (a) K l n(a,b) K 5 n(b,c) A °C mmHg~1(25°C) ein'mole - 1 s e c - 1 x l 0 ~ 7 s e c - 1 x l 0 ~ 8 5130 25 2.31 t 0.46 21.11 6.6 0.7921 0.158 1.59±0.52 11 60 0.6021 0.528 21.61 5.6 5.20 ± 1.74 6 . 6 0t5 . 5 9 11 100 0.439t 0.274 15-515-9 4.66 1 2.91 6.761 4.22 11 140 0.192 t 0.120 11.511.7 11.2 £ 7.0 11.5 1 7.2 5660 25 7.50 t 1.06 34.9H2."{ 0.245 to. 055 O .85OIO.5II ti 100 2 67 0 32 36.9 ± 4.4 0.765 t o . 092 2.7510.-54 11 i4o i.o4 1 0.20 3 5 . 9 1 2 . 5 2.07 +0 .40 7.23 ±.1.40 (a) Uncertainties were determined f o r 95% confidence i n t e r v a l (b) For der i v a t i o n of X, , K, see DISCUSSION (c) Uncertainties were taken-^as equal to the uncertainty f o r 95% confidence of the le a s t accurately known component i n c a l c u l a t i n g . 4. DISCUSSION Surface E f f e c t I t was seen i n the previous section that "seasoning" of the r e a c t i o n vessel r e s u l t s i n a considerable increase of the quantum y i e l d of the isomerization. No unambiguous explanation f o r t h i s e f f e c t can be given on the basis of the a v a i l a b l e data. I t could be that, i n p a r a l l e l to the 64 O 3660 fil 9 ,1 loo'C e O / / a M0°C o o / O - e 10.0 2o.o 65 reaction i n the gas phase, photoisomerization takes place i n the polymer layer deposited on the walls of the seasoned v e s s e l . The l i g h t absorbed by t h i s layer, about ^>0% of the t o t a l r a d i a t i o n , was not taken i n t o account i n computing quantum y i e l d s . I f i t i s assumed that t h i s l i g h t brings about isomerization of crotonaldehyde with the same quantum e f f i c i e n c y as the l i g h t absorbed by the gas phase, the r e s u l t i n g quantum y i e l d s are of h a l f the values appearing on Figure 15 f o r the runs i n a "seasoned" vessel. This reduced value, however, i s s t i l l more than twice larger than the value of quantum y i e l d at the same wavelength i n a "clean" v e s s e l . Therefore, i f photoisomerization takes place i n a polymer layer, i t must be an operation more e f f i c i e n t than the one taking place i n the gas phase. It w i l l be assumed that photoisomerization i n a "clean" vessel' takes place i n the gas phase. This assumption i s based on the f a c t that the quantum y i e l d of isomerization i n such a v e s s e l , although much smaller than unity, i s , nevertheless, too large to be due to r e a c t i o n by surface molecules alone. As an upper l i m i t , a monolayer coverage of surface corresponds to about l O 1 ^ molecule'cm 2 or 2.5*lO 1^ molecules f o r both illuminated sur-faces of the v e s s e l . At a pressure of 20 mm Hg there are about 8 x molecule*cm"? i n the gas phase, or $000 times the number of the surface molecules. I t i s apparent that even i f the quantum e f f i c i e n c y of isomer-i z a t i o n i s unity on the surface, i t can only account f o r a small part of the observed quantum y i e l d . In the following discussion surface e f f e c t s are ignored. The Excited State It i s generally thought (55""56) that the absorption of l i g h t i n the near u l t r a v i o l e t by aldehydes and ketones corresponds to an e x c i t a t i o n process i n which a non-bonding oxygen electron i s promoted to the antibonding 6 6 TT-orbital. The resulting state i s a singlet, "''(n, TY*)* A molecule in the ^ (n, "TT* ) state possesses electronic energy whose magnitude in excess of that of the ground state is characterized by the 0-0 energy, i.e. the energy separating the lowest vibronic level of the ground state from the lowest vibronic level of the excited state. It possesses also vibrational energy whose magnitude, in contrast to that of electronic energy, i s not a constant of the state but can only be described by means of a probability distribution which w i l l be a function of the following factors. (i) the vibronic distribution of the ground state, P(c.) ( i i ) the energy of the exciting radiation, hy ( i i i ) the relative a b i l i t y of molecules in a vibronic level of the ground state to absorb radiation of a given wavelength} this could be thought of as a selection rule in a broad sense. (iv) the relative a b i l i t y of molecules in a vibronic level of the ex-cited state to undergo various processes which wi l l be described later. These factors w i l l now be discussed briefly. Vibronic Distribution of the Ground State, P(£.) If the normal frequencies of a molecule are known P(E.) can be eval-uated by using the formula developed by Marcus and Rice (57) for the case of quantized molecules: P(£)= ( £ + U)3'1 /Rsffrkv. (4) \-\ s where Ez is the zero-point energy and the i t n normal frequency. If the normal frequencies are not known, as the case is with croton-aldehyde, some other approximation i s needed. Consider the molecule to be 67 composed of s harmonic o s c i l l a t o r s . The f r a c t i o n of molecules with energy i n the range £ to £ t d£ present i n these o s c i l l a t o r s i s P (e )de = , L _ (5) T . e x p ( - 2 A T ) d £ 0"* where g i s the degeneracy of the states with energy i n the range £ to £ +• d£ . Since the molecule consists of s o s c i l l a t o r s , and considering that the energy i n the range £ to £. + d£ corresponds to j quanta, g i s equi-valent to the t o t a l number of ways i n which j objects can be d i s t r i b u t e d i n s boxes, which i s <j+»-D! g -2 / ( s - l ) I In the c l a s s i c a l l i m i t of continuous energy (or i n f i n i t e s i m a l quanta) j » s-1 and ( j + s - l ) ! ~ j / j 8 " 1 . Then g r g(£ ,s) r e S - V ( s - l ) ! The i n t e g r a l i n equation (5) becomes the gamma function i f t h i s value of g(E,s) i s used. Therefore f S - l e x p ( - E / k T ) HO- (6) ( k T ) s ( s - l ) / where £ can vary from zero to i n f i n i t y . Energy of the E x c i t i n g Radiation, hy The e x c i t i n g r a d i a t i o n can be considered approximately as monochromatic since the l i g h t source used i n the present work was a medium pressure mercury lamp i n which the continuum is-comparatively weak with r e l a t i o n to the l i n e s at JIJO and 366O A. Using t h i s approximation the energy i s 31950 cm" 1 and 68 27?20 cm"-'- a t t h e s h o r t and t h e l o n g w a v e l e n t h r e s p e c t i v e l y . S e l e c t i o n R u l e s There a r e no s i m p l e s e l e c t i o n r u l e s g o v e r n i n g t h e e l e c t r o n i c t r a n -s i t i o n f r o m a v i b r o n i c l e v e l o f t h e ground s t a t e t o a v i b r o n i c l e v e l o f t h e e x c i t e d s t a t e . Moreover, t h e r e i s an i n f i n i t y o f v i b r o n i c l e v e l s i n t h e model chosen f o r t h e ground s t a t e . T h e r e f o r e i t w i l l be assumed t h a t t h e s o l e f a c t o r d e t e r m i n i n g t h e amount o f r a d i a t i o n a b s o r b e d by a v i b r o n i c l e v e l o f t h e ground s t a t e i s t h e p o p u l a t i o n o f t h a t l e v e l . Or I a(£) = I a-P(£) (7) where I a i s t h e t o t a l a b s o r b e d i n t e n s i t y and I a(£) i s t h e i n t e n s i t y a b s o r b e d by m o l e c u l e s o f v i b r a t i o n a l energy £ . T h i s r e l a t i o n i m m e d i a t e l y l e a d s t o t h e c o n c l u s i o n t h a t , b a r r i n g f u r t h e r c o n s i d e r a t i o n s , t h e v i b r o n i c d i s t r i b u t i o n o f t h e e x c i t e d s t a t e w i l l be a r e p l i c a o f t h e v i b r o n i c d i s t r i -b u t i o n o f t h e ground s t a t e w i t h t h e d i f f e r e n c e t h a t a l l t h e l e v e l s w i l l be d i s p l a c e d by a c o n s t a n t amount o f energy. T h i s amount w i l l be e q u a l t o t h e d i f f e r e n c e o f t h e 0-0 energy f r o m t h e energy o f t h e e x c i t i n g r a d i a t i o n . I f t h i s d i f f e r e n c e i s £ Q , t h e v i b r o n i c d i s t r i b u t i o n o f t h e e x c i t e d s t a t e w i l l be (€ - S o ) 9 " 1 e - (£ - £ c ) A T P * ( E ) = ( 8 ) (kT) s.(s-l)« where £ can v a r y f r o m £ c t o i n f i n i t y . P r o c e s s e s Open t o t h e M o l e c u l e s i n t h e E x c i t e d S t a t e I f we denote by a m o l e c u l e o f t h e ^(n, TT*) s t a t e w i t h n v i b r a t i o n a l e n e r g y , t h e f o l l o w i n g r e a c t i o n s a r e p o s s i b l e (58)* ( l ) D i s s o c i a t i o n o f t h e ^-W1 m o l e c u l e t o r a d i c a l f r a g m e n t s and/or s t a b l e 69 molecules or isomerization: X M N > product(s); rate = k-^CV1) ( 2 ) C o l l i s i o n a l degradation of 1 M N + M > 1 M ° + M; rate = k2n(1^)W As written, ( 2 ) assumes that a l l the v i b r a t i o n a l energy of 1 l l n i s removed i n one c o l l i s i o n . We w i l l discuss l a t e r the ' v a l i d i t y of t h i s statement, ( j ) Internal conversion: 1 M N • M ; rate = k ^ N ( 1 M N ) The i n c l u s i o n of t h i s process becomes necessary when, as i n the present case, at pressures approaching zero. (4) Fluorescence and/or i n t e r n a l conversion: 1M° > M ; rate = k ^ V ) (5) Intersystem crossing: 1 M ° > V ; rate = k ^ V ) or more accurately lM° > V followed by 5 M m + M » ?M° + M with the second step much f a s t e r than the f i r s t . (6) Phosphorescence and/or i n t e r n a l conversion: 70 ?M° i. M ; rate = k g C - V ) ( 7 ) D i s s o c i a t i o n of t r i p l e t to r a d i c a l frafments and/or stable molecules or isomerization: ?M° —> product(s) rate =. ky(?M°) The quantum y i e l d measured i n the present study i s that of isomeri-zation. Since t h i s y i e l d i s s u b s t a n t i a l l y smaller than unity at the l i m i t of zero pressure, i n t e r n a l conversion must be postulated. Estimation of k- K^ and k^ n f o r a "Strong C o l l i s i o n Mechanism" Assuming a c o l l i s i o n a l degradation i n which the energy l e f t a f t e r one c o l l i s i o n i s less than the energy needed to cause isomerization, the following general formula can be derived from the above reaction scheme by steady-state appraximation: 9 = [ k l n + a . b . k 2 n - ( M ) ] / [ k l n + k 5 n + k l n.(M)] ( 9 ) where a = k^/(k^ •+• k^) , the f r a c t i o n of ^M0 molecules which cross to the t r i p l e t state, and b = k 7/(kg + k^) , the f r a c t i o n of ?M° molecules which d i s s o c i a t e . I f k^ = 0 , t h i s expression becomes: 1/9 ^ 1 + k 5 n / k l n + k 2 n . ( M ) / k l n ( 1 0 ) i . e . a plot of l/-P versus (M) gives a s t r a i g h t l i n e with slope k ^ r / k ^ and intercept 1 +- ^r/^in ' The r e s u l t s i n the present work agree i n form with equation ( 1 0 ) . I t i s i n t e r e s t i n g , however, to examine i f equation ( 9 ) could not y i e l d a p l o t of s i m i l a r form f o r c e r t a i n values of a»b . Equation ( 1 0 ) derives from equation ( 9 ) f o r a«b = 0 . For a«b ^ 0 i t i s obtained: 71 1/9 r [1 + k 5 n / k l n f k 2 n ( M ) / k l n ] / [l + a.b-k 2.(M)/k l n] ( l l ) I t i s clear that equation ( l l ) gives the same intercept as equation (10) when l/*-? i s plotted against (M) . The shape of the l i n e can only be seen i f values are assigned to the parameters. Even for a-b^ 0 equation (10) gives the correct value of k^ n/k^ n and the order of magnitude of k 2 n / k ^ n . From Table VIII i t i s seen that at 25°C and 366O A k ^ n / k l n = 34.9 and k ^ / k ^ - 7-50 mmHg-^. Using t h i s value of k 2 n / k ^ n the plots appearing i n Figure 17 can be drawn f o r various values of a-b . Inspection of the plots shows that f o r a-b> 0.002 the curvature of the l i n e s becomes larger than experimental errors i n the Stern-Volmer plots could conceal (Figure 16). Therefore t r i p l e t contribution must be n e g l i g i b l e and the slopes appearing i n Table VIII are equal to the corresponding r a t i o s , k 2 n / k ^ n . The numerator, k 2 n , i s the rate constant of the c o l l i s i o n a l d eactivation i n v o l v i n g -^M1 molecules and molecules i n the ground state. It can be equated to the c o l l i s i o n frequency constant, Z , which i s e s t i -mated to be 1.97* ( T ) 1 / 2 * ^ 1 0 It-mole-L s e c - 1 i f a c o l l i s i o n a l diameter of 6.5 A i s assumed f o r crotonaldehyde. The values of k- n^ estimated i n t h i s way are shown along with the values of k, i n the l a s t two columns of Table VIII. k, was calculated from the 3n 5 n intercepts of the Stern-Volmer p l o t s , which are equal to 1 +• k ^ / k ^ , using the estimated values of k- n^ . It i s apparent that both k^ n and k^ n have larger values at 3130 A than at 566O A at the same temperature and that they increase with increasing temperature at constant wavelength. Increase i n temperature increases the vibronic energy of the molecules i n the ground state and, i n d i r e c t l y , the vibronic energy i n the e l e c t r o n i c -a l l y excited state. Decrease i n the wavelenth of the e x c i t i n g r a d i a t i o n increases d i r e c t l y the v i b r o n i c energy i n the excited state. The dependence 72 looY-1 l5o\ O IO.O QL£ - O a.@ = o.ooZ a£- o.oos <xS = O.O/O (a.4 - o.ol a.4 - o.ol 3o.o '3 fl^u/ce 17. Gt&u&Jexl Ste/ty-&m&t P&h ^tfi (&tt(Wo bcdu&> GL$ and ^in/tin 75 of k- n^ and k^ n on wavelenth and temperature i s , therefore, qualitatively correct. It w i l l be now investigated i f a simple model based on the assum-ption of "strong collision" can give quantitative agreement. Theoretical Estimation of k^ n for a "Strong Collision Mechanism" It was shown earlier (equation (8)) that the energy distribution function for molecules arriving i n the electronically excited state is (c - g - 1 ^ - ^ P*(g) ; (8) (kT) s (s-1)! If i t is assumed that an excited molecule upon colliding with a molecule in the ground state is l e f t with insufficient energy to react unimolecular-Iy> P (£) is approximately the distribution function of reacting molecules. It i s only an approximation because P (£.) is in fact modified by the rela-tive rate of reaction of molecules with energy £ • The rate constant for unimolecular reaction of a molecule with vibra-tional energy £• is given by the three theories of unimolecular reaction (Kassel, Eyring, Slater) as equal to k. , v(l - c a Ai)3"1 (12) where £ a is the activation energy and s the number of vibrational degrees of freedom. According to Slater ( 5 9 ) V is the root-mean-square of the normal frequencies of the molecule and according to Eyring ( 4 5 ) is equal to kT/h. The overall rate constant is k x= J k(£)-P*(£).d£ Assuming the validity of equations (8) and (12), k-^  i s : ( 1 5 ) 74 f V l ( k T ) S (8-1)! [(I" Kfi )8_1(£ -f0 ) S- 1exp(£ 0-£ )/kT] • d£ (14) In equation (l4) i t has been assumed that the e f f e c t i v e number of v i b r a t i o n a l degrees of freedom of crotonaldehyde i s the same i n both the ground and the e l e c t r o n i c a l l y excited state. This number can be at most equal to 3N-6 , where N , the number of atoms i n the molecule, i s 11 i n the case of croto-naldehyde. The energy £ Q , representing the difference of energy between the ex c i t i n g r a d i a t i o n and the 0-0 energy, was estimated to be equal to 5455 and 828 cnf •'"mole-"'' at 3130 and y660 A resp e c t i v e l y . The c a l c u l a t i o n was based on the value 5774.5 A (26494 cm-''") given by Rao and Rao (60) f o r the 0-0 band of crotonaldehyde. Values of k^ n/v>j were computed f o r the two wavelenths and at the temperatures of 25 and l40°C by i n s e r t i n g various values of s and into equation (l4). A 1620 IBM computer was used to evaluate the i n t e g r a l numerically. Results of the computation are shown gr a p h i c a l l y i n Figure 18. On the l e f t h a l f of the Figure the r a t i o k1(l40°C)/k1(25°C) i s shown f o r - 700 and 1300 ciiflmole"""1". The value of t h i s r a t i o calculated from a l Table VIII i s 14.1 and 8.5 at 3130 and $660 A resp e c t i v e l y . I t can be seen from Figure 18 that the predicted values are quite smaller than these. On the r i g h t h a l f of the Figure the r a t i o k 1 ( 3 1 3 0 A)/k 1 ( 3 6 6 0 A ) i s shown for the same values of F . The value calculated from Table VIII i s 3*24 a and 5«4l at 25 and l40°C r e s p e c t i v e l y . I t i s seen that there i s some agreement with the experimental values f o r £& = 1300 cm'^mole * at l40°C, but the value predicted f o r 25°C using t h i s £ i s too large. 9. I t i s clear from the study of Figure 18 that the theory used predicts 75 76 a much larger increase of i n going from 3660 to yly0 A at constant temperature than i n going from 25 to l40°C at constant wavelength. The opposite trend i s apparent from the data i n Table VIII. I t i s concluded that the model used i s too crude. Refinement of the "Strong C o l l i s i o n Mechanism" In the present refinement a "strong c o l l i s i o n " between two molecules of equal mass w i l l be defined as a c o l l i s i o n a f t e r which the molecules are l e f t with equal v i b r a t i o n a l energies. In the case of the e l e c t r o n i c a l l y ex-c i t e d molecule, i f t h i s energy i s greater than £ a the molecule w i l l - s t i l l be able to react unimolecularly or be further deactivated by c o l l i s i o n . This process w i l l continue u n t i l the molecule has e i t h e r reacted or l o s t i t s v i b r a t i o n a l energy to a l e v e l below £ a . An accurate mathematical imple-mentation of the above p r i n c i p l e s would involve the energy d i s t r i b u t i o n functions of both the ground and e l e c t r o n i c a l l y excited state. C o l l i s i o n s i n which the v i b r a t i o n a l energy of the excited molecule i s 1 increased would have to be taken i n t o account. The computational d i f f i c u l t i e s would be awsome. As a necessary approximation i t w i l l be assumed that a l l the molecules i n the ground state have a unique v i b r a t i o n a l energy, the average v i b r a t i o n a l energy of the state. Using the relevant d i s t r i b u t i o n function (equation ( 6 ) ) i t can be shown that t h i s energy i s skT . C o l l i s i o n s with e l e c t r o n i c a l l y excited molecules are not considered to a l t e r t h i s value, because of the large pool of ground state molecules and the f a c t that the re a c t i o n i s stu-died isothermally. I f a molecule i n the e l e c t r o n i c a l l y excited state has i n i t i a l l y v i b r a -t i o n a l energy £ , a f t e r a c o l l i s i o n with a molecule i n the ground state i t w i l l be l e f t with energy (£ + skT)/2 . A f t e r a second c o l l i s i o n i t w i l l be l e f t with energy (fc +• 5skT)/4 , a f t e r a t h i r d with energy (£, + 7skT)/8 and so on. I t i s clear that i t s v i b r a t i o n a l energy tends to skT , the 77 average v i b r a t i o n a l energy i n the ground state, and w i l l never f a l l below t h i s value. As a consequence, £ must be greater than skT ; otherwise a l l 8L excited molecules would either react or s u f f e r i n t e r n a l conversion and the quantum y i e l d would be almost constant. The above considerations lead to the following reaction scheme: M + hv -^M^l, ; rate - I & ^ 1 > product " k1(£1)-(1M£l) 1 M £1 > M " k ^ ) . ^ 1 ) 1M £1 + M > 1Mf2+ M " Z-CM)^1!^!) l j ^ n - 1 + M „ l M € n ^ M n z . ( M ) . ( l j ^ n - 1 ) 1M £ n deactivation where 6 2 = (E1 + skT)/2 Hn-1 = (e i+(2 n _ 2 - l)s k T ) / 2 n " 2 j (15) En = ( E l t (2 n _ 1 - l ) s k T ) / 2 n _ 1 j Steady-state approximation gives f o r the concentration of M n ~ l : = I a[z(Mf -V lT [^(8.) + k 5(£i) Y Z(M)] The rate of product appearence i s equal to the difference between the rate of i n i t i a t i o n , I , and the rate of termination , Z(M)(1M^n~l). Using the a steady-state concentration i t i s found: 78 rate = I ^ f l - [z(M)] 7 f f t ^ C f . ) + k ^ . ) +- Z(M)]j ^ i * l For computational purposes t h i s expresion can be s i m p l i f i e d to the following: -ate r i j i - j y i / f i + [ ^ ( 5 . ) + k 5(£.)]/ Z(M)jj l e i (16) This i s the rate f o r molecules with i n i t i a l v i b r a t i o n a l energy £ ^  . In order to obtain the o v e r a l l rate , equation ( l 6 ) must be m u l t i p l i e d by the energy d i s t r i b u t i o n function, equation (8) , and integrated over a l l energies above £ • The r e s u l t i n g expression f o r the quantum y i e l d , , i s ( k T ( s - l ) ! ,-(e-e.)/kT. s-1 . kT 1 +• 4 — ' * Z(M) (17) In (17) k-, (£. ) has been substituted by V, ( l ~ E /Z\ ) where 1 s are 1 a^ J- ' given by r e l a t i o n s (15)' The function k^(£^) could probably be approxima-ted by a si m i l a r expression, i . e . k, c V i ( l - SQz/6.:) > although t h i s view \> 7 a? 1 has been c r i t i c i z e d ( 6 l ) . In any case the r e s u l t i n g expression would contain too many parameters to be subjected to numerical analysis. We w i l l abandon, therefore, the model i n t h i s stage. A l t e r n a t i v e Mechanism Up to t h i s moment i t was assumed that J-butenal i s produced by unimole-cular reaction of molecules i n the 1(n-TT*) state. An a l t e r n a t i v e mechanism w i l l be discussed now i n some d e t a i l . Photochemical isomerizations of the type 79 I I •C-C:C-C:0 I I I H -4> — C:C-C-C:0 M l H have been shown by means of deuterium tracer techniques (62,63) to proceed through the enol I - C:C-C:C-OH I I I The t r a n s i t i o n state leading to enol i s thought to be a six-membered r i n g (63). In the case of crotonaldehyde the o v e r a l l reaction mechanism would be i t / H CRV CH ^ / CH 0 II < CH CH, CH CH S / CH H •* 2*2 \ CH CH "1 CH2:CHCH2CH0 It i s seen that crotonaldehyde must have the c i s - c o n f i g u r a t i o n i n order to form the t r a n s i t i o n state. It i s known, however, that t h i s aldehyde e x i s t s l a r g e l y i n the trans form (48,64). It i s possible that when l i g h t i s absorbed by the f r a c t i o n of molecules which are c i s the r e s u l t i n g excited molecules react to form ult i m a t e l y 3-butenal with no other competing process except c o l l i s i o n a l deactivation. Light absorbed by trans-molecules may r e s u l t i n excited molecules which have an unfavorable configuration to form the enol and f i n a l l y 3-butenal. These molecules would then preferably f a l l back into the ground state. The following equations describe t h i s mechanism: M ± M t 5 C < r M c+ hV J equilibrium constant = K et rate = I. 80 M T + hV M^J rate = I 1M n ^product " k. ( V ) c r lnc c V+-M > 1M°+ M " Z C M X V ) c c ^M™ *• product k ( M™) t r l n t t ' iMf+M . V t M " Z(M)(V) t t V * M " k (V) t pnt t Subscripts c and t mean c i s and trans respectively. Steady-state approximation y i e l d s the expression < ? = 7 " i n / l V * Z ( M ) 1 + 7 k l n t / [ k l n t * V + Z<M>] <18> where I = I. + I i s the t o t a l l i g h t i n t e n s i t y absorbed. If i t i s assumed that klnt<<( k^ n c and ki n+^« k ^ n t ' t h e second term of equation (18) can be dropped and we have 1/9= -i . Z(M) 1 + | (19) k, lnc l / l c , the r a t i o of t o t a l absorbed intensity over thei- i n t e n s i t y absorbed by cis-crotonaldehyde, can be approximated by the inverse mole-fraction of the cis-form I / I c = (Mc + Mt)/Mc = 1 + K c t 81 where Kc^ . is the equilibrium constant of the cis-trans conversion. Then Z lAp r (1 + K c t) + (1 -f K c t) (M) (20) k l n c According to equation (20) the intercepts of the Stern-Volmer plots are equal to 1 +• . K ^ would be expected to decrease with increasing temperature since cis-crotonaldehyde i s the least stable isomer. This trend is observed in the intercepts i n Table VIII at JIJO A. The intercept remains constant within experimental error at J660 A and also shows a dependence on wavelenth. These facts show that i t cannot be equal to 1 + K , and that the approximations used i n deriving equation (l°) are not valid. The whole argument serves to demonstrate, however, that the intercepts and slopes of the Stern-Volmer plots of crotonaldehyde can have a very uncertain meaning even when a single-step deactivation is considered to be true. 82 V. PHOTOLYSIS AND PHOTOCHEMICAL OXIDATION OF 5-BUTENAL 1. APPARATUS The apparatus used i n the experiments with crotonaldehyde was also used i n the present experiments. The bulb (c) (Figure j ) was f i l l e d with oxygen. Light of wavelengths of JIJO and 5540 A was i s o l a t e d by means of the f i l t e r combinations described i n Chapter IV. In a l l the experiments described i n the present chapter a "clean" quartz reaction vessel with i l -luminated volume equal to 155 ml was used. In order to i d e n t i f y products a f i v e l i t r e pyrex bulb f i t t e d . w i t h a quartz window was used as a reaction vessel f o r a few runs. 2. EXPERIMENTAL Determination of the Absorption Curves f o r 5-Butenal at 5150 and 5540 A The e x t i n c t i o n c o e f f i c i e n t s of J-butenal at these wavelengths and at the temperatures of 25° and l40°C were determined using the experimental technique described i n Chapter IV f o r the case of crotonaldehyde. The res u l t s are plotted according to Beer's law on Figure 19. As i n the case of croton-aldehyde, the e x t i n c t i o n c o e f f i c i e n t at JIJO A i s constant at the temperature i n t e r v a l of 25 to l40°C, while at 5540 A i t increases from 6.60 mole - 1* lt*cm " l a t 25°C to ~J.6h m o l e - 1 , I t * c m - 1 at l40°C. Here again the dependence of the e x t i n c t i o n c o e f f i c i e n t on temperature may be due to the f a c t that t h i s wavelength region i s located at the very edge of the absorption spec-trum of 5-butenal (Figure 14) where l i g h t i s pa r t l y absorbed by molecules i n the higher v i b r o n i c l e v e l s of the ground state. The population of these l e v e l s increases with temperature and as a r e s u l t the e x t i n c t i o n c o e f f i c i e n t 85 84 increases. Determination of Reaction Products It has been established that the products of photolysis of J-butenal i n the near u l t r a v i o l e t region are propylene, carbon monoxice and b i a l l y l . The i d e n t i f i c a t i o n of the l a t t e r hydrocarbon was based on gas chromato-graphy and U.V. spectrum evidence (24). In the present work i t was possible to obtain the gas phase I.R. spectrum of t h i s product, by photolyzing 10 mm Hg J—butenal i n the f i v e l i t r e r eaction vessel at JIJO A and room tem-perature f o r 24 hours, subjecting the reaction mixture to analysis by gas chromatography and trapping the product at the vent of the apparatus. The spectrum of the product was i d e n t i c a l \to that of b i a l l y l . Crotonaldehyde has been stated as a possible product of the photolysis of J-butenal, but no a n a l y t i c a l r e s u l t s could be given, because the s t a r t i n g material contained about 1% of the aldehyde as an impurity (24). In the present work J-butenal was obtained i n a r e l a t i v e l y high state of purity ( l e s s than 0.3% crotonaldehyde). At the same time the thermal isomerization was f a i r l y well understood (Chapter I I I ) . It was, therefore, decided to determine crotonaldehyde along with the other products i n order to decide i f the photochemical isomerization of J-butenal i s a r e v e r s i b l e process. A f i v e foot long dinonyl phthalate column was used f o r analysis of the products. Approximately 2.5 mm Hg of the reaction mixture was admitted into the sampler of the gas^-chromatography apparatus. Both the flame ion-i z a t i o n and thermoconductivity detectors of the Perkin-Elmer Vapor Fractometer were used i n s e r i e s . In t h i s way i t was possible to obtain also a signal f o r carbon monoxide, to which the flame i o n i z a t i o n detector i s i n s e n s i t i v e . This detector gave strong signals f or a l l other products. In the case of the photooxidation of J-butenal i t was necessary to 85 e s t a b l i s h f i r s t the products of the reaction. For t h i s purpose 10 mm. Hg 3-butenal and 5 vam Eg oxygen were illuminated at 313O A and room temperature i n the f i v e l i t r e vessel f o r 24 hours. Analysis of a portion of the reaction mixture by the dinonyl phthalate column using the flame i o n i z a t i o n detector-showed major peaks at 0.5, 0.7, 5«6 a n ( i 15 minutes, along with the peaks at 7.5 and 20 minutes corresponding to 3-butenal and crotonaldehyde respectively. A much smaller peak appeared at 1.6 minutes. The peaks at 0.5 and 0.7 minutes were i d e n t i f i e d as riethylene and propylene res p e c t i v e l y by trapping at the vent of chromatography apparatus and repeating the analysis using a six foot S i l i c a G-el Type 15 column at 75°C and 8.5 p s i . Use of t h i s column revealed also the existence of small. amounts of acetylene. When the thermoconductivity detector was used, carbon dioxide was also revealed i n amounts comparable to those of propylene. No add i t i o n a l i d e n t i f i c a t i o n of these products was attempted, because the S i l i c a Gel column separates hydrocarbons up to Cj unambiguously. For carbon d i -oxide i d e n t i f i c a t i o n the add i t i o n a l evidence was the f a c t that i t gave a signal only at the thermoconductivity detector. The products with retention times 1.6, y.6 and 15 minutes were iden-t i f i e d as acetaldehyde, a c r o l e i n and a l l y l alcohol r e s p e c t i v e l y by taking the I.R. spectra of the corresponding f r a c t i o n s trapped at the e x i t of the chromatography apparatus. In the case of the product with retention time 3.6 i t was also necessary to carry out an analysis i n a HMPA (Hexa Methyl Phosphor Amide) Column to determine i f i t was not a mixture of a c r o l e i n and b i a l l y l . These two compounds have the same retention time i n the dinonyl phthalate column. It was found that the product i s pure a c r o l e i n . Analysis of gases not l i q u e f i a b l e at l i q u i d nitrogen temperature by means of a ten foot long molecular sieves column operated at 80°C and 2 psi 86 showed that carbon monoxide was also a product. A peroxide t e s t by the method of Young et a l (32) run on the part of the reaction mixture which could not d i s t i l over melting ethyl ether (about -120°C), was p o s i t i v e . I t was found, however, that contact with Hg vapor or standing i n the dark f o r some hours destroyed the peroxide. During one run i n the quartz reaction vessel, at 515^  A and room temperaturejthe course of the reaction was followed by taking the u l t r a v i o l e t spectrum of the contents of the reaction vessel at short time i n t e r -v a l s . This was done by removing the reaction vessel from the l i g h t beam a f t e r a measured time of photolysis and taking the spectrum by means of a Carey 14 spectrophotometer. Then the vessel was returned to the l i g h t beam fo r another length of time and the spectrum was taken again. The operation was repeated several times. The successive spectra taken i n t h i s way appear on Figure 20. I t can be seen that a new absorption maximum appears around 2600 A and that i t disappears on standing i n the dark for 24 hours. A si m i l a r absorption max-imum has been observed during the thermal oxidation of butane and higher hydrocarbons and i t i s believed to be caused by a c y c l i c peroxide of unknown structure (65,66). Open-chain peroxides absorb i n a continuous way s t a r t i n g at about J000 A with no maximum being reached down to 2300 A (65-68). An attempt was made to i d e n t i f y the peroxide, or peroxides, present by the paper chromatographic method developed by Cartlidge and Tipper (69). The stationary phase was Whatman No. 5 paper treated with a 20% g l y c o l s o l u t i o n i n 80-100° petroleum ether. The moving phase was 50'»50 chloroform i n 80-100°.petroleum ether. The paper was clamped by large spring c l i p s between two glass plates to prevent evaporation of any v o l a t i l e components. A saturated ferrous thiocyanate so l u t i o n i n methanol was used as developer. 87 1 3000 3500 88 Under these conditions t-butyl peroxide had an Rp value of 0.40, as compared with the authors' 0.35* The product of 3-butenal photooxidation gave on development an elongated spot with Rp, taken at the center, equal to 0.57« The shape of the spot suggested the presence of a mixture, but no further attempts were made to resolve i t . I t can be deduced, however, that per - 3 -butenoic acid probably was not present, because the Rp values of percrotonic and per-n- and per-iso-butyric acids are given by Cartlidge and Tipper as 0.23 and O.37 respectively and per-3-butenoic acid would be expected to have a comparable value. The other probable peroxide, a l l y l hydroperoxide, was not available for comparison and i t s synthesis was not attempted as i t i s a rather dangerous operation (70). Experimental Procedure The experimental procedure followed during the study of the photolysis of 5-butenal was the same as the procedure followed i n the case of the photolysis of crotonaldehyde, with the difference that the, thermoconduc-t i v i t y detector was used i n the analysis i n series with the flame i o n i z a t i o n detector i n order to obtain a peak for carbon monoxide. Evaluation of the various components of the reaction mixture was by r a t i o s , taking as reference the peak of 3-butenal corrected f o r reaction. In the case of the photooxidation the procedure was as follows: 5-butenal was admitted f i r s t at a measured pressure into the reaction vessel and then oxygen was admit'ted. The t o t a l pressure was measured and the pres-sure of oxygen was taken as equal to the difference of the two pressures. The gases were l e f t i n the dark f o r f i v e minutes to ensure mixing and l i g h t was then shone through the vessel. The intensity of the transmitted light.' was recorded. The l i g h t absorbed by the reaction mixture was calculated by using the extinction c o e f f i c i e n t of J-butenal for the conditions of the run. This method i s legitimate, because i t was found experimentally that addition 8 9 of oxygen does not a l t e r the absorption c h a r a c t e r i s t i c s of J-butenal. After a length of time the l i g h t was shut o f f and the contents of the reaction vessel were subjected to analysis. Two methods were used, designated as "A" and "B". "A" Method: In t h i s method of analysis the sampling was done by expansion into the sampler of the chromatography apparatus. The same dinonyl phthalate column used f o r analysis of the photolysis products was used but without the thermoconductivity detector because the large excess of oxygen i n t e r -fered with the estimation of carbon monoxide. In t h i s way ethylene, pro-pylene, acetaldehyde, a c r o l e i n and a l l y l alcohol were estimated with respect to J-butenal. "B" Method: In t h i s method the contents of the reaction vessel were expanded int o a " B e l l " trap (71) cooled with l i q u i d nitrogen and connected to the Toepler pump. The non-liquefiable gases, oxygen and carbon monoxide, were toeplered into the gas burette and thence expanded to the gas chromato-graphy sampler. The molecular sieves column was used f o r an a l y s i s . A f t e r these gases were pumped out, melting ether was substituted f o r nitrogen as a coolant of the " B e l l " trap and further gases were pumped. These were carbon dioxide, ethylene, acetylene and propylene. They were analyzed by means of the S i l i c a Gel Type 15 column using the two detectors i n s e r i e s . Since no reference peak was av a i l a b l e , the chromatography apparatus was c a l -ibrated f o r absolute amounts of carbon monoxide, carbon dioxide, ethylene and propylene. No c a l i b r a t i o n was done f o r acetylene, because of the very small quantities formed, but the apparatus was assumed to have f o r t h i s hydrocarbon the same response as f o r ethylene. The residue i n the " B e l l " trap was -analyzed f o r peroxide using the co l o r i m e t r i c method of Young et a l (52). Since the i d e n t i t y of the peroxide was not known, the Bosch and Lomb 90 spectrophotometer was c a l i b r a t e d by using known amounts of hydrogen peroxide. The two methods of analysis were run exclusive of each other, but the presence of propylene and ethylene i n both methods ensured a way of comparison. 3. RESULTS Photolysis J-butenal at pressures ranging from 2 to JO mm Hg was photolyzed at two wavelengths, 5150 and 5540 A, and two temperatures, 25 and l40°C. The experiments were ca r r i e d out at constant incident l i g h t i n t e n s i t y . The quantum y i e l d s of propylene, b i a l l y l and carbon monoxide as well as the percentage of 5 -butenal isomerized to crotonaldehyde, are summarized i n Tables IX and X. TABLE IX Photolysis of 5-Butenal at 515O A 1 a Time =, 10 minutes. Light Intensity I 0 - 1«5 x 10 u ein«sec Temp °C Pressure mm Hg T(propene) X 9 ( b i a l l y l ) y <p(co) l / f t o t a l l / ( x +2y) Crot.% 25 1.8 0.557 0.195 _ 1.08 0.18 it 2.4 0.554 0.191 - 1.07 0.25 11 9.2 0.480 0.172 0.81 1.21 -11 9.2 0.474 0.169 0.76 1.25 -11 10.5 0.442 0.170 0.75 1.28 0.08 11 10.8 0.481 . 0.176 0.78 1.20 0.19 11 27.4 0.566 0.155 0.66 1.47 0.05 11 50.8 0.551 0.151 0.65 1.55 0.16 l4o 5.5 0.651 0.160 - 1.05 0.45 11 14.7 0.579 0.155 0.85 1.15 0.52 11 22.1 0.508 0.169 0.85 1.18 o.4i it 28.0 0.497 O.I65 0.81 1.21 0,60 11 36.3 0.458 0.155 0.78 1.50 0.51 91 TABLE X Photolysis of 3-Butenal at 3340 A Time - 15 minutes. Light Intensity I 0 ~ 1.6 x 1 0 - 8 ein/sec Temp °C Pressure mm Hg (propene) X ^ ( b i a l l y l ) y 9 (co) t o t a l Crot. : 25 5.0 0.545 0.173 1.45 0.06 ti 5.8 0.557 0.181 - 1.43 -ti 11.6 0.285 0.158 0.65 1.66 0.18 II 18.4 0.259 o . i 4 l 0.52 I.85 0.12 II 25-3 0.210 0.121 0.46 2.22 0.15 140 6.6 0.479 0.177 - 1.20 0.48 II 12.9 0.458 0.175 0.80 1.27 0.64 M 20.0 O.398 0.180 0.77 1.52 0.51 II 27.9 O.359 O.I65 0.71 1.45 0 . 6 l It i s apparent that the quantum y i e l d s of a l l products decrease with i n -creasing pressure except f o r the quantum y i e l d of b i a l l y l at l40°C, which remains p r a c t i c a l l y constant. The quantum y i e l d s at 313^ A and 25°G are plotted against pressure i n Figure 21. It i s clear that the y i e l d of pro-pylene i s affected most by pressure change. The quantum y i e l d of carbon monoxide i s almost equal to the y i e l d of the combined products containing the a l l y l group. This y i e l d extrapolates very near unity at zero pressure. Stern-Volmer plots of the combined products containing the a l l y l group appear on Figure 22. The s t r a i g h t l i n e s correspond to the l e a s t square f i t t i n g of the data. The relevant parameters are shown i n Table XI. It i s seen that as i n the case of crotonaldehyde photolysis the slopes decrease with increasing temperature at constant wavelength and are smaller at the shorter wavelength at constant temperature. The r a t i o s p r o p y l e n e / b i a l l y l are plotted against pressure i n Figure 23. I t i s apparent that propylene i s favored over b i a l l y l at low pressure, high temperature and short wavelength. 92 9? 94 95 TABLE XI Least Square Estimation of Slopes and Intercepts f o r the Stern-Volmer Plots of 3 -Butenal Wavelength A Temp °C Slope (a) Intercept (a) Quantum Y i e l d mm Hg--*- x 10 at zero pressure (a) 3540 5150 t 11 25 i4o 25 140 1.53+ 0.27 1.064 ± 0.044 0.940 i 0.039 1.15 ±0.25 0.994 ± 0.043 1.007+0.043 3.75 +- 0.95 1.226 ± 0.145 0.816 * 0.097 1.55 t 0.76 1.121 ±0.103 0.892 t 0.082 (a) Uncertainties were calculated f o r 95$ confidence i n t e r v a l . Photooxidation Mixtures of 3 -butenal and oxygen at various p a r t i a l pressures were illuminated at the wavelength of 31?0 A and at two temperatures, 25 and l40°C. The rea c t i o n mixture was analyzed according to methods "A" and "B" described i n the experimental part. A few experiments were carried out i n the dark i n order to e s t a b l i s h the extent of any thermal reaction taking place. No products could be detected by either of the a n a l y t i c a l methods when 10 mm Hg 5 -butenal and 5 mm Hg oxygen were l e f t i n the "clean" reaction vessel f o r 15 minutes at 25°C. When the mixture was l e f t i n the vessel at l40°C for the same length of time, analysis by method "A" revealed the existence of very small amounts of ethylene (0.0021$), propylene (0.0014$) and a c r o l e i n (0.0060%). These amounts are les s than 1% of the amounts formed i n the photochemical reaction and consequently no correction was applied f o r dark reaction to the r e s u l t s obtained by method "A". Analysis by method "B", however, showed that con-siderable amounts of peroxide were formed i n a dark reaction. A b r i e f study of t h i s reaction was undertaken i n order to e s t a b l i s h the extent of the 96 correction needed f o r the r e s u l t s of the photooxidation. The experimental r e s u l t s are summarized i n Table XII. TABLE XII Formation of Peroxide i n Thermal Oxidation of 3-Butenal Temperature : l40°C Time : 15 minutes (a) 3-butenal mm Hg Oxygen mm Hg Peroxide mole x 108 6.9 6.8 I.56 10.5 7.0 2.58 10.2 19.5 2.60 18.4 7.0 5.25 21.8 6.9 7.56 26.5 7.2 8.00 (a) Time was counted from the moment oxygen was introduced into the reaction v e s s e l : 3~butenal was introduced f i r s t . I t i s apparent that the rate of formation of peroxide increases with i n -creasing pressure of 3-butenal. Increase of oxygen pressure does not a f f e c t the rate. A graph of peroxide versus pressure was constructed (Figure 24 ) by means of which corrections f o r thermal reaction could be applied to the photochemical r e s u l t s presented subsequently. It i s i n t e r e s t i n g to note that the dependence of peroxide formation on pressure of aldehyde i s more than l i n e a r . A logarithmic plot of the r e s u l t s gave the order as equal to 1.4. The r e s u l t s of the photochemical experiments are presented i n Tables XIII and XIV. Inspection of the data and c o r r e l a t i o n with the data of photo-l y s i s (Table IX) lead to the following conclusions: (a) The quantum y i e l d of propylene i s v i r t u a l l y unaffected by the presence of oxygen. This i s g r a p h i c a l l y demonstrated i n Figure 21, where the points f o r propylene from photolysis and photooxidation are seen to follow the same 97 |o.o 2o.o P > mmH^ 98 TABLE XIII Photooxodation of 3-Butenal; Analysis by Method "A" T ~ 25°Cj 5-but. = 10 mmHg; 0 2 - 5 mmHg Time min. ''"abs -1 i n l ° e in°sec *10 Quantum Yield s ethene propene acetylene a c r o l e i n . x l O 4 a l l y l alcohol 5 1 8 . 4 0 . 2 4 9 0 . 4 8 5 58 1.61 2 . 4 8 10 16 . J 0 . 2 4 9 0 . 4 8 1 54 1 .67 2.50 10 16 .0 0.225 0 . 4 8 2 57 1 .74 2.52 22 1 9 - 4 0 . 2 6 0 0.500 59 1.50 2.40 20 0 . 9 2 0 . 5 3 4 0.524 28 1 .64 1 .95 " 2 . 4 4 0.265 0 . 4 5 7 50 1 .42 2 . 1 7 4 . 2 0 0.265 0 . 4 8 6 51 1.70 2.58 4 . 2 2 0.275 0 . 4 8 5 26 1.54 2 . 5 5 " 8.14 0.249 0 . 4 7 1 50 1.42 1 .98 1 0 . 4 0.254 0 . 4 5 5 51 1.54 2.09 T = 25°C; time = 10 minj I ~ 3.2'10~9 ein'sec" 5-butenal oxygen mmHg mmHg 10.0 0 .9 0.195 0.450 20 f a i l e d 2.21 11 2..5 0.205 0.412 22 1.54 1.92 11 8 . 9 0.255 0.445 25 1.52 1.49 11 11.5 0.264 0.470 28 1.49 1 .46 11 20.0 0.279 0.462 29 1.61 1 .46 1.5 5.2 O.556 0.585 20 1.17 1.15 2.2 5.6 0.268 0.552 23 1.25 1.10 5.0 5-6 0 .296 0.566 27 1.44 1.60 8.1 5.0 0.272 1 0.515 27 1.51 I.83 10.2 5.0 0 . 268 0.492 31 1.-52 1.55 15.4 5.4 0.26! 0 . 464 28 1.44 1.94 16.6 5-0 0.255 0.444 54 1.70 2.09 22.3 5.2 O.252 0.582 52 1.89 2.11 99 TABLE XI11 -; continued Photooxidation of 3-Butenal; Analysis by Method "A" -T = l40°C; time ~ 10 min; I Q ~ 3.2-10" •9 • -1 7 ein-sec  3-butenal oxygen • Quantum Y i e l d mmHg • mmHg ethene propene acelylene a c r o l e i n a l l y l alcohol xio4 (a) 7.1 6.8 0.460 0.590 51 O.623 0.822 12.9 7-1 0.400 0.511 46 0.567 0.781 13-9 6.9 0.514 0.641 63 0.710 0.955 17.1 7.0 0.433 0.570 58 0.675 0.871 21.3 7-0 0.501 0.626 50 0.641 0.750 26.3 7-3 0.428 0.562 52 0.666 O.858 • 31.0 7-0 0.454 O.57O 45 0.645 O.833 (a) The quantum y i e l d of a c r o l e i n .at l40°C i s overestimated by about y % due to the presence of small amounts of b i a l l y l . curve. (b) B i a l l y l , a product of the photolysis, does not appear among the products o of the photooxidation at 25 C. In a run with 10 mmHg 3 -butenal and 0.24 mmHg oxygen at t h i s temperature i t was found by using the HMPA column that b i a l l y l was formed i n an amount equal to about 20% of a c r o l e i n (peak areas r a t i o ) . At pressures of oxygen above 1 mmHg, however, no b i a l l y l could be detected among the products.In two runs at l40°C small amounts of b i a l l y l (about 5$ of the amount of ac r o l e i n ) were-detected even with oxygen pressure equal to 7 mmHg. N o systematic study o f b i a l l y l formation was undertaken because the HMPA column "bleeds" excessively at the temperature used f o r analysis (32°C) and no accurate r e s u l t s could be obtained. The quantum y i e l d s appearing i n Table XIII under the heading "acr o l e i n " at l40°C are overestimated to about 5% due to the contribution of b i a l l y l to the peak of the chromatogram corres-ponding to a c r o l e i n . 100 TABLE XIV Photooxidation of J-Butenal; Analysis by Method "B" I ~ 5«2'10~9 ein«sec" o y T = 25°C; J-but. - 10 mmHg Quantum Y i e l d l i n . mmHg CO peroxide ethene propene acetylene x l O 4 c o 2 5 5 5.02 0.255 No analysis 10 II 5.14 0.260 11 n it 5.00 0.267 t i 15 II 5.52 0.254 t i 10 1.6 5.20 0.271 0.201 0.458 26 0.571 10 2.9 5.17 0.265 0.215 0.445 51 0.518 12 9.2 f a i l e d 0.250 0.265 0.456 40 0.816 10 15-5 5.58 0.205 0.265 0.442 58 1.05 T =. 25 C; oxygen = 5 mmHg Time min. 5-butenal mmHg 10 6.5. 5.22 0.274 0.260 O.515 40 O.655 II 11.6 5.46 f a i l e d 0.288 0.502 59 0.770 II 14.5 5.44 11 0.222 0.421 26 0.584 I I 20.0 5.55 0.248 0.251 0.450 18 0.700 I t 27.9 5-20 0.252 0.254 O.598 54 0.592 l40°C; oxygen r 7 mmHg Time min. 5-butenal mmHg 10 5.5 5.27 0.505 0.481 0.551 55 0.145 I I 7.4 5.01 0.501 0.416 0.488 49 O.156 I I 9.7 5.12 O.518 0.495 0.496 48 0.159 I t 24.2 6.5O 0.425 0.476 0.407 26 0.151 I I 29.1 6.15 0.409 0.465 0.586 51 0.147 101 (c) The major products of the photooxidation are acrolein, a l l y l alcohol and carbon monoxide. These products have a quantum yield higher than unity at 25 U C . Le sser products are carbon dioxide, ethylene, propylene and peroxide. These have a quantum yield below unity but higher than 0.2 . Two products, acetaldehyde and acetylene,• have a quantum yield of the order of 0.004. (d) Variation of the experimental conditions at constant temperature has very l i t t l e effect on the quantum y i e l d of the products except in the case of carbon dioxide, where the quantum yield increases almost threefold with a tenfold increase of oxygen pressure. The quantum yield of ethylene also increases with increasing oxygen pressure, but to a much smaller extent. (e) Increase of temperature from 25 to l40°0 has the result that the quantum yields of acrolein, a l l y l alcohol and carbon dioxide decrease to less than one half of their respective values at the low temperature, while those of the rest of the products increase. Another effect of the temperature increase is that small amounts of b i a l l y l appear among the products. Photooxidation of 3-Butenal with Oxygen-18 In one experiment y0 mmHg 3""butenal and 15 mmHg oxygen-18, in the form of molecular oxygen (purity 98.35%» obtained from the Isotope Department, Weismann Institute of Science, Rehovoth, Israel) were illuminated for y0 o -9 -1 minutes at 25 C with light of yly0 A (I ~L 2.0*10 ein«sec ). The reaction a. mixture was fractionated by means of the Bell trap and Toepler pump using melting ether as the cold bath. The gases which could not be retained at the trap were analyzed by mass spectrometry. An AEI MS9 Mass Spectrometer was used. The spectrum, shown in Figure 25, confirms the essential purity of oxygen-18 used. The residue in the trap was d i s t i l l e d into the gas chromatography sampler D (Figure 4) and analyzed by means of the dinonyl phthalate column. Acrolein 102 and a l l y l alcohol were trapped separately at the e x i t of the gas chromato-graphy system and t h e i r mass spectra were taken. From the study of the mass spectra (Figures 25-27) the following con-clusions can be drawn: (a) 0xygen-l8 and oxygen- l6 are found i n both the carbon monoxide and carbon dioxide produced during the reaction. In carbon monoxide the predominent species i s CO while i n carbon dioxide i t i s CO 0 . The abundance of a l l the other possible species ( CO 1^ , CO^O 1^ , CO^O 1^ ) i s , however, quite s u b s t a n t i a l . I t i s known that carbon monoxide, carbon dioxide and molecular oxygen do not exchange oxygen at temperatures lower than 400°C (72). The contribution of d i s s o c i a t i n g carbon dioxide to the signal of carbon mon-oxide must be very small given the higher abundance of the monoxide i n the react i o n mixture ( Table XIV) and the f a c t that the r a t i o CO/COg i n the spectrum of the dioxide i s about 0.09 (75)* A l l four species found i n the mass spectrogram (Figure 25) must be, therefore, produced during the photo-oxidation of 5-butenal. (b) The spectrogram of a c r o l e i n (Figure 26) shows that about 70% of the aldehyde contains oxygen-18, while that of a l l y l alcohol (Figure 27) shows that t h i s product contains almost e x c l u s i v e l y oxygen-18. loj 25-0,kO* J 6 CO V CHt*Cttz CO \ _l_ l/Z ccro fa/8 CHt-CHCH% C.o"o" i l l I ?(, £8 30 32. 3** 36 3g >*>/e co'h /|0 >M MH 46 HS 104 C/fz=CH 1 CH^CHCHO fam fJUoxlcUiw cuitf Ol80'& C H ^ C H C B O 1 * C H O 16 aj=CHcd*\ C H T - - C H C H O * C H X = C H C O V CHO 16. CH%--CtfCH0'1 J L , 1,111 ei n JO tn/e 55 56 Mat)** ^fwcitm cr^ ftourieivi ^ cem Kefccxielation u)i<t4 0/S0/8 105 50 100 18 J7 3o Si CHz*CHC%dsH i i CfrCHCHiO* CHt0^H CHt~-CHCHlO,6H 56 5"? 106 4. DISCUSSION Photolysis The r e s u l t s presented i n the previous section show that 5-butenal upon absorption of u l t r a v i o l e t r a d i a t i o n y i e l d s propylene, b i a l l y l and carbon monoxide i n stoichiometric quantities obeying the r e l a t i o n CO ^ propylene + 2 - b i a l l y l . ( l ) The f a c t that addition of oxygen does not a f f e c t the quantum y i e l d of propylene i s an evidence that t h i s product i s formed i n a molecular reaction CH2:CHCH2CH0 t hV j>CH2:CHCH^ •+• CO (2) The same f a c t suggests that the excited state from which propylene originates i s a s i n g l e t , because oxygen i s known to quench t r i p l e t states (56,74). Formation of b i a l l y l implies the presence of a l l y l r a d i c a l s , therefore the following process must also occur (5a) either CHO CH2:CHCH2CH0 + hV >CH2:CHCH2 + and/or H + CO (5b) Then two a l l y l r a d i c a l s can combine to y i e l d one molecule of b i a l l y l 2CH 2:CHCH 2 > CH2:CHCRgCHgCH:CH2 (4) The heat of d i s s o c i a t i o n of 3~butenal into a l l y l and formyl r a d i c a l s was estimated to be 65.0 + 5»1 kcal/mole (Chapter I I I ) . The C-H bond strength of the formyl r a d i c a l has been found to be 27 kcal/mole (42). Therefore, reactions (5a) and (5b) need a minimum energy of 65+5*1 a n d 92 +; 3.1 kcal/mole r e s p e c t i v e l y supplied by the absorbed r a d i a t i o n i n order to be thermodynamically f e a s i b l e at room temperature. Light at 515^ and 107 3J40 A is equivalent to 91-2 and 85-5 kcal/mole respectively. It i s seen that the energy of radiation at the longer wavelength i s less than the re-quired minimum for reaction (jb), therefore reaction (ja) must be the only mode of radical dissociation at room temperature. The fact that b i a l l y l is not formed in the presence of oxygen at room temperature could be interpreted as an indication that a l l y l radicals o r i -ginate i n a t r i p l e t excited state of J-butenal. This conclusion is incompa-ti b l e , however, with the fact that small amounts of b i a l l y l are formed at l40 ° C . Also, the linear form of the Stern-Volmer plots of the overall quantum yield (Figure 22) shows that t r i p l e t particapation i n the formation of pro-ducts must be negligible (see Discussion of Chapter IV). It w i l l be assumed, therefore, that a l l y l radicals originate i n an excited singlet state of 3-butenal and that oxygen eliminates these radicals by reaction after they have been formed. This reaction w i l l be discussed in a following section, where the mechanism of the photooxidation is discussed. Relation ( l ) implies that most of the formyl radicals produced in reaction (ja) form ultimately carbon monoxide. No mechanism wi l l be presented for this transformation. It has been seen (Figure 23) that propylene is formed in preference to b i a l l y l at low pressure, high temperature and short wavelength. The following simple mechanism can account, at least qualitatively, for these results. M -i- hv — * !Mn rate =• Jabs l j p i } C H 2 : C H C H j + CO n k 2 n ( V ) V > C H 2 : C H C H 2 + CHO II k , (XMn) 3n 1M n + M > 1M m f M II Z( 1M n)(M) > C H V . C H C H , + ^ 3 CO it 108 lM m >CH2;CH0H2 4 CHO rate = k.^m(lvT) 1M m + M » deactivation " Z( 1M m)(M) A 3-butenal molecule i n the ground state, M, upon absorption of radiation of energy hy is excited- to the n'th vibrational level of the excited singlet, 1Mn. Then i t can either decompose to CH2:CHCHjT CO or dissociate to CH2:CHCH2 +- CHO or be deactivated to the m'th vibrational level by co l l i s i o n with a molecule i n the ground state. The resulting molecule, -^ M1, faces the same alternatives. The process may continue for several collisions, but for simplicity i t wi l l be assumed that after the second col l i s i o n the molecule i s completely deactivated. Stationary state treatment, considering also reaction (4) , gives the following expressions for the quantum yields of propylene and b i a l l y l : (4) ~. k 0 v i k,m • Z(M) CP I n \ 2 n ^ T (.propylene) - *2n * k 3 n +- Z W [k2n + k 3 „ + Z( M)J { k 2 m +" k > ^ Z ^ M ) | T ( b i a l l y l ) - — — + — —r =, 2 [ k 2 n f k 3 n + ZM] 2 [ k2n + k 5n+ ZC^)} fern + k Jm * Z ( M > ] The ratio propylene/biallyl can now be written as a function of pressure Equation (6) shows that (propylene/biallyl ) M — > 0 =: 2 k 2 n / k j n ( 6 a) Examination of Figure 23 shows that the ratio (propylene/biallyl) extra-polates at zero pressure to a higher value at l40°C than at 25°C at constant 109 wavelength and at 31J0A than at 3J40 A at constant temperature. Therefore, according to (6a), k 2 n increases faster than k^ n with increase of supplied energy, either thermal cr radiational. Or the activation energy E 2 is larger than E,. 3 At higher pressures the terms 2k_ Z(M), in the numerator of (6), and 2m k^fflA(M), in the denominator, become significant. Clearly ( k ^ / k ^ ) ^ ( k2n^Jn^ and consequently the ratio propylene/biallyl decreases with increasing pressure. It was seen i n Chapter IV that a classical approximation of the energy distribution function of the excited state cannot account quantitatively for the variation of the quantum yield of isomerization of crotonaldehyde with change in temperature and wavelength. The test of the classical dis-tribution function, however, employed the slopes of the Stern-Volmer plots to which no clear meaning can be given i f more than one collision i s neces-sary i n order to deactivate completely an excited molecule. Moreover, crotonaldehyde photolysis may be a poor testing case, given the uncertainty existing about the meaning of the very large intercepts of these plots. A different approach w i l l be attempted in the case of J-butenal, independent of uncertainties concerning the mechanism of deactivation. It can be seen from Table XI that the overall quantum yield of the photolysis of 3-butenal extrapolates to unity only at 3130 A and l40°C, namely under conditions of high radiational and thermal energy. Under a l l other conditions the extrapolated yield i s less than unity. This fact can be interpreted as meaning that at 3130 and l40°C a l l the 3-butenal molecules arrive to the excited .state with enough energy to dissociate while under a l l other conditions only a fraction possess this energy. The classical energy distribution function of the excited state i s 110 (Chapter IV) 1 8-1 P(£)-- e RT (7) (B-1)< RT where E 0 i s the difference of the zero-zero energy, E 0-_ 0, from the energy of the e x c i t i n g r a d i a t i o n , hv , and s i s the number of v i b r a t i o n a l degrees of freedom of the molecule.- This function i s plotted i n Figure 28 f o r s = 27, which i s the maximum number of v i b r a t i o n a l degrees of freedom of 5 _butenal. integrated form of (7) i s plotted. The lowest extrapolated quantum y i e l d (Table XI) i s 0.816 + 0.097 at 3340 A and 25°C, or (l8.4i9«7)% of the molecules a r r i v i n g at the excited state have energy les s than the minimum energy, E , required f o r reaction. With the aid of Figure 28 i t i s found that E A - E D r (21.9 + 1.7)RT Since E q= hv - E 0 _ 0 and at the conditions of the experiment hv1 - 91 • 2 kcal/mole, T -298°K, i t i s found that E a + ' E o - o ~ 1 0 2«1 t l . 2 kcal/mole In the same way values of E A-^ E Q _ 0 f o r the rest of the experimental con-d i t i o n s were found. They are shown i n Table XV, along with the values c a l -culated f o r s=:21. I t i s seen that the values of E A + E Q _ 0 at the two wave-lengths d i f f e r more than the stated experimental e r r o r . The disagreement i s more pronounced at s=:21, therefore i t w i l l be assumed that i n 3 -butenal a l l v i b r a t i o n a l degrees of freedom are a c t i v e . In computing the error, however, the e x c i t i n g r a d i a t i o n was considered to be completely monoenergetic, an assumption which i s probably not j u s t i f i e d , considering the f i n i t e width of The abscissa i s the difference E ~ E Q i n u n i t s of RT. In the same Figure the I l l 28. C&AUca/ GwWj £>is hidden d 1-$uMm£ 112 TABLE XV Values of E a+ E 0 _ 0 Estimated from the Intercepts of Stern-Volmer Plots Wavelength Temperature kcal/mole  A °C B - 2 1 s^ 2 7 31 20 25 99:6 ±0.7 1 0 2 . 1 ± 1 . 2 " i4o 4102.5 4105.9 5540 25 9 5 . 5 t l . O 98.4 11.0 " 140 9 7 . 9 1 1 . 6 101.612.2 the bands used. Moreover, inspection of the absorption spectrum of 5-butenal (Figure 14) shows that at 5540A the extinction coefficient rises sharply in the direction of shorter wavelengths. It i s possible then that the effective wavelength absorbed at the experiments carried out at "5540 A" is shorter than this wavelength. This could account for the lower values of E a f E Q _ 0 obtained from the data of these experiments. It can be said, therefore, that the classical approximation of the energy distribution function of the electronically excited state of a poly-atomic molecule is essentially correct and i t may be used to evaluate a c t i -vation energies of reactions of excited molecules i f the uncertainties involved i n collisional deactivation can be eliminated and the zero-zero energy is known. Unfortunately in the case of 5 -butenal this quantity is not known, therefore no absolute value of the activation energy can be obtained. Since the electronically excited 5-butenal molecules dissociate ac-cording to two competing reactions ((2) and (5a.)) i t is necessary to define E a more precisely. It i s f e l t that the method used to evaluate E a justifies i t s definition as the activation energy of the process with the' lowest energy barrier. This is process (5a), therefore E a - E z . It was shown earlier that 113 the r a t i o s k/p/k^ can be evaluated from the extrapolated values of the r a t i o p r o p y l e n e / b i a l l y l at zero pressure and that E2}Ey Using the c l a s s i c a l energy d i s t r i b u t i o n function and the expression given by the theories of unimolecular r e a c t i o n f o r the rate constant (see Chapter IV) i t can be e a s i l y shown that E-hv + E, •3 * 0 - C RT •|E-hv TE 0_ 0)/RT] s-1 E-E, dE oo s-1 E-hy+ E n_ o-o RT -j(E-hV+- E O _ O ) / R T ] fe-I s-1 dE (8) I f E Q _ 0 i s known Ez, can be calculated as shown above and equation (8) may be used to estimate E2 by p l o t t i n g the r a t i o k 2 / k j against E2 f o r the various experimental conditions. This would be an a d d i t i o n a l t e s t of the c l a s s i c a l energy d i s t r i b u t i o n function and of the proposed mechanism. There i s a further conclusion to be drawn from the r e s u l t s of the photolysis of 5 -butenal. It can be seen from Tables IX and X that the amount of crotonaldehyde formed at l40°C and 10 minutes of photolysis i s about 0.5% of the reaction mixture. Using the data obtained i n Chapter III i t can be shown that under the same conditions the thermal reaction would produce approximately 1.5% crotonaldehyde. I f i t i s assumed that 5-butenal isomerizes thermally i n the presence of l i g h t to the same extent as i t does i n the dark, i t follows that the discrepancy i n the experimentally found quantities must be due to croton-aldehyde photolyzing back into 5-butenal. Assuming that the l i g h t i n t e n s i t y absorbed by each species i n a gas mixture i s approximately equal to 114 l a i - I 0 ( M . ) f i . i (9) where I Q i s the incident l i g h t i n t e n s i t y , (M^) the concentration of species i , £^ i t s absorption c o e f f i c i e n t and 1 the length of the l i g h t path, i t can be shown that the quantum y i e l d of isomerization of croton-aldehyde i s given by the following r e l a t i o n k _ i * t B 0 (10) In (10) C and B stand f o r crotonaldehyde and J-butenal r e s p e c t i v e l y , t i s the time and the rate constant of the thermal isomerization of J-butenal. The values of the various parameters are: I 0 = 9.5 x 10" 8 e i n . l t _ 1 s e c " l £ c = 16.1 mole - 1•It*cm - 1 1 - 10.0 cm t - 600 sec k_ 2 = 2.5 x 10~5 s e c - 1 C/B0 = 5 x 10" 5 I t i s calculated ^ r 145 mole e i n ~ l I t i s apparent that the proposed simple mechanism of production and elimin-a t i o n of crotonaldehyde cannot be true. An a l t e r n a t i v e w i l l be the following. It i s possible that i n the presence of u l t r a v i o l e t r a d i a t i o n most of the crotonaldehyde which would be formed thermally i n the dark does not ap-pear at a l l i n the gas phase. Thermal isomerization i s - a heterogeneous process. Light can hinder i t i n two ways. (a) By destroying the activated complex of the r e a c t i o n on the surface. (b) By isomerizing f r e s h l y formed crotonaldehyde back i n t o J-butenal before 115 i t has time to desorb from the surface into the gas phase. If the thermal isomerization takes place through the enol form of 3-butenal (II) the activated complex ( i l l ) i n the presence of light may rather react to the le f t than to the right. Moreover, the crotonaldehyde molecule formed (IV) has the cis-configuration and i t may be favourably disposed to react to the le f t when illuminated. CH^CHCH^CHO. H \ CH, i <-CH CH i II II2 CH ' CH III f H / CH2 I CH X , / IV 0 II CH CH,CH:CHCHO (trans) 3 Photooxidation It i s not possible to present a mechanism which w i l l account quanti-tatively for a l l the experimental results of the photooxidation, given the great number of products and their complex interrelation. The following suggestions w i l l be made: (a) Propylene is formed by molecular photodecomposition of 3-butenal, as in the case of photolysis without the presence of oxygen (reaction (2)). CH2:CHCH2CH0 + hv > OE^CHCH^ + CO (2) 116 The argument for the occurrence of this reaction is that the quantum yield of propylene i s unaffected by the presence of oxygen. (b) The a l l y l radical i s s t i l l formed by means of reaction (ja). CH2: CHCH2CH0-r hv >CH2:CHCH2 + CHO (5a) Since no b i a l l y l is found among the products at room temperature, i t w i l l be assumed that the a l l y l radical is completely scavenged by oxygen CH 2:CHCH 2 r 0 2 s»-CH2: CHCHgOO- (11) (c) The presence of b i a l l y l at l40°C can be explained i f i t is assumed that at this temperature reaction ( l l ) is reversed'to a considerable extent CH2:CHCH200« *CH2:CHCH2 f 0 2 (12) Benson estimated recently (75) that at l40°C and oxygen pressure equal to 7'6 mm Hg the rate of the forward reaction ( l l ) must be equal to the rate of the reverse reaction (12).within 1.5 power of 10. The behaviour of the a l l y l radical is exceptional in this case as compared with the behaviour of alkyl radicals, for which the rate of the reverse reaction reaches that of the forward reaction only around 400°C, and i t is due to the high resonance energy of the a l l y l radical. The small amount of b i a l l y l found at l40°C shows that reaction (12) proceeds at a rate about one order of magnitude smaller than that of reaction ( l l ) and of one or more competing reaction. (d) The presence of oxygen-18 in the products shows that almost a l l of the a l l y l alcohol and most of the acrolein produced during the photooxidation of 5-butenal arise by means of further reactions of the allylperoxy radicals. The fact that the quantum yield of these products is higher than unity at 117 room temperature leads to the conclusion that a chain process takes place. The allylperoxy radical may decompose according to CH2: CHCH200 . > CH2: CHCHO + OH (13 ) A reaction sequence similar to (ll)-(l3) has been postulated recently by Norrish and Porter (76) for the reaction of propyl radicals with oxygen in the thermal oxidation of 1-butene. A chain reaction may be started by hydrogen abstraction by the a l l y l -peroxy or OH radicals. In the molecule of 3-butenal there are two kinds of hydrogen which can be readily abstracted by these radicals. One is the aldehydic hydrogen with bond strength 86 kcal (77,78); the other is either of the two a l l y l i c hydrogens. The bond strength of the latter i s estimated to be about 85 kcal based on a l l y l resonance energy of 13 kcal (39) and a primary C-H bond strength i n propane'of 98 kcal (40). The relevant reactions are _^___^ ?RH tCH2:CHCH2C0 (l4) R -I- CH2:CHCH2CH0 -*RH f CH2:CHCHCH0 (15) where R i s CH2:CHCH2o6 or OH. The radical produced in reaction (l4) may dissociate readily according to CH2:CHCH2C0 > CH2: CHCH2 +- 00 (l6) given the high resonance energy of the resulting a l l y l radical. It may also peroxidize 0H2:CHCH2C0 + 0 2 ? CH2:GHCHgC'oo- ( 1 7) and the peroxy radical dissociate 118 CH2:CHCH2Cfg_0. >CH2:CHCH20 -l- C0 2 (18) Reaction (18) w i l l probably be favoured over the a l t e r n a t i v e path, reactions (l4) or (15) with R r CH2:CHCH2C0z, , because the t r a n s i t i o n state i s a s i x -membered r i n g CO / \ CHc 0 CH 0 ^CH 2 C0v \ CHp 0 I: CH 0 \ / CH£ f CH, CH +• 00; CH, Jits s e l e c t i o n i s also dictated by the absence of peroxy-J-bytenoic acid from the products. The a l l y l o x y r a d i c a l produced i n t h i s reaction may pro-pagate the chain by reacting according to the process ( l 4 ) - ( l 5 ) . A l l y l alcohol i s produced i n t h i s step. Since C0 2 i s produced with a quantum y i e l d quite lower than that of a l l y l a l c ohol, there must be another source of a l l y l o x y r a d i c a l s . A probable re a c t i o n i s 2CH2:CHCH200 -> 2CH2:CHCH20 + 0 2 (19) There i s considerable evidence f o r the occurrence of reactions of t h i s type i n both gas phase (79-81) and solu t i o n (82,8j) studies. The chain may terminate by either recombination and/or disproportion-a t i o n of a l l y l o x y r a d i c a l s 2CH2:CHCH20 •(CH2:CHCH2)202 >CH2:CHCH0+ CH2:CHCH20H (20) (21) Another chain i s propagated by the r a d i c a l produced i n reaction (15). This r a d i c a l i s a resonance hybrid of three mesomeric structures 119. CH2:CH~CH-CH:0 <H—* CH2~CH:CH-CH:0 < > CH2:CH-CH:CH-6 VI VII VIII Structures (VI.) and (VII) may easily peroxidize CH2:CHCHCH0 +• 0 2 > CH2:CHCHCH0 (22) 6-0- • CH2-CH:CHCH0 +. 0 2 — » CH2CH:CHCH0 (2j) . • 6-0-The resulting radicals possess a reactive aldehydic hydrogen each and may be subject to internal hydrogen abstraction. Rust (84) has shown in his work on the liquid phase oxidation of hydrocarbons of the type C C c - c - ( c ) x - c - c , x - 0 to J at 120°C that intramolecular abstraction of a tertiary hydrogen by the -0-6 group is highly efficient at the £-position, somewhat less so at the "^-position and apparently of l i t t l e or no significance at either the o< -or T-positions. Then the peroxy radical produced in reaction (23) may react according to CH?-CH:CHC:0 » CH9GH:CHC:0 (24) I I I 0-0- H . 0-OH The resulting hydroperoxide radical may decompose to ethylene}oxygen and the radical CH:C:0. CH5CH:CHC:0 « * CHnCH-CH:C:0 CH5:CH9 + 05+ CH:C:0 (25) I 1 0-OH 0-0-H The latter may react with oxygen, as follows 120 CH:C:0 +• 0 2 > CHO T C0 2 (26) The radical produced in reaction (22) cannot follow the same sequence because the aldehydic hydrogen is not suitably positioned. It may react with another peroxy radical in a way similar to reaction (19)• CH?:CHCHCH0 + R'OO »R ' 6 +• 0 P + CH9:CHCHCH:0 (2?) I I 0-0* 0' where R' i s CH2:CHCHCH0 or CH2:CHCH2 . The radical formed may decompose as follows: CH2:CH-CH-CH:0 > CH2:CH-CH0 f CHO (28) I 0' The formyl radicals produced i n reactions (26) and (28) and also in the i n i t i a t i o n reaction (5a) have been known to react readily with oxygen. The reaction HCO + 0 2 > CO + H02 (29) has been postulated by many authors (85-88). McKellar and Norrish (88) have shown that the alternate reaction HCO +- 0 2 >C02 +• OH ( 5 0 ) postulated by Marcotte and Noyes (89) as being predominant at low temperature is of less importance than reaction (29). Their conclusion was based on the observation that the intensity of the OH radical spectrum i n the flash photolysis of formic acid i s the same whether oxygen is added to the reaction vessel or not. 121 Another view i s that the formyl r a d i c a l peroxidizes with oxygen: HCO t 0 2 > HCQ 0 0. (51) This reaction was postulated by Bone and Gardner (90) i n order to explain the detection, of performic acid i n the thermal oxidation of formaldehyde. The a n a l y t i c a l method used has been c r i t i c i z e d , however, (9l) and i t has been shown by Markevich and F i l i p p o v a (92) that appreciable amounts of hydrogen peroxide are formed i n t h i s reaction. ..They suggest that although the peroxy r a d i c a l i s formed i n i t i a l l y i t isomerizes and decomposes rap i d l y : (52a) (52b) The r e s u l t i s k i n e t i c a l l y equivalent to the competing reactions (29) and (50). McKellar and Norrish (88) point out that theMow y i e l d of COg shows that reaction (52a), or i t s equivalent (29), i s of more importance. With t h i s reaction one more chain i n i t i a t o r , H0 2 , i s added to the ones presented e a r l i e r : H0 2 + CH2:CHCH2CHO — > H 20 2 +• CH2:CHCH2C0 (55) The combined quantum y i e l d of the photooxidation of 5-butenal., taken as equal to the sum of the quantum y i e l d s of a l l y l a l c o h o l , a c r o l e i n and ethy-lene or a l t e r n a t i v e l y the sum of the y i e l d s of CO and CO,, i s about 5*5 to 4 at 25°C. The quantum y i e l d of the i n i t i a t i o n reaction, based on the y i e l d of b i a l l y l during the photolysis, i s about 0.55« Therefore, the chain length i s about 10. At l40°C the quantum y i e l d s of a l l y l alcohol and a c r o l e i n f a l l to less than have t h e i r value at the low temperature. This e f f e c t may be due p a r t l y 122 to the fact that the dissociation of allylperoxy radical into a l l y l radical and oxygen is significant at this temperature. The increase of the quantum yield of ethylene could be explained by assuming that reaction (25) is favored at this temperature were i t not for the fact that the quantum yield of C0 2 decreases under the same conditions. There must be some other source of ethylene. The very high yield of CO at l40°C, being more than 3 times the com-bined yield of a l l y l alcohol and acrolein, can be explained only i f a further attack by oxygen molecules on the hydroperoxide radicals i s postulated to take place at this temperature: CH9CH:CHC:0 f 0 9 •CH9CH:CHC:0 —2—•CHO + CO (34) I I I 0-0-H 00H .00 In the experiment with oxygen-18 i t was found that the heavy oxygen enters into the molecules CO and CO2 i n a l l possible combinations with oxygen-l6, although the predominant species are 00-^ and CO^O*8. These species are the expected products of reactions (16), (18), (29) and (30), or the equivalent for the last two reactions (32a) and (32b). The species CO*8 can be produced i f the performyl radical, HCOs;, is capable of another mode of isomerization, an oxygen tautomerism. 0 l 6 0 l 6 0 1 8 H-C^ „ * H-C ( n > CO 1 8 +. HO l 60 1 8 \ 0 l 8 - o 1 8 * 0 1 8 A similar mechanism may be operative also with the radical CHg^HCHgCO^ . The presence of the species C0 1^0 1^ and C 0 l 8 0 l 8 can be explained i f an oxygen exchange is assumed between RCOj and O2. The amount of 0 0 found i n the mass spectrum i n Figure 25 is too small, however, to justify such an assumption. 12? BIBLIOGRAPHY 1. BONE,W.A. and HILL,S.GProc.Roy.Soc.(London)A129,445(1950) 2. BONE,W.A. and ALLUM,R.E.:Proc.Roy.Soc.(London)A154, 578(1931) 5. NAYL0R,0.A. and WHEELER,R.V.:J.Chem.Soc.2456(1931) 4. N0RRISH,R.G.W.:Discussions Farad.Soc.10,269(1951) 5. D0BRISKAYA,A.,NEIMAN,M. and RUDNEVSKII,N.:Zhur.Fiz.Khim.27,l622(l955) 6. MALMBERG,F.W., SMITH,M.L.,BIGLER,J.E. and ROBBIT,J.A.:Fifth Symposium on Combustion, 1954, 385-92(1955) 7. 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