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The photochemistry of aldehydes in the gaseous phase Sifniades, Stylianos 1962

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THE PHOTOCHEMISTRY OF ALDEHYDES IN THE GASEOUS PHASE STYLIANOS SIFNLADES ( DIPLOMA OF CHEMISTRY, The University of Athena, 1957 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of Chemistry. We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA October, 1968 In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department o r by h i s r e p r e s e n t a t i v e s . I t i s understood t h a t copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be allowed without my w r i t t e n permission. Department of flrTTCMTSTBY  The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8, Canada. Date ABSTRACT The object of the present work was the investigation of two separate topics, the investigation of the r a d i c a l -r a d i c a l termination step i n the photochemical oxidation of acetaldehyde and thg. investigation of the photolysis of -' orotonaldehyde, both in the 'gas phase and at room tempera-ture. A t h i r d topic, the photolysis of 3-butene-l-ral, was investigated i n the course of the work, i n order to provide evidence i n favour of the mechanism proposed for the photo-l y s i s of orotonaldehyde. .In the f i r s t part of the work, mixtures of acetaldehyde and i s o t o p i c a l l y enriched oxygen containing argon as a refe-rence gas were i r r a d i a t e d at 25°C with l i g h t at a wavelength of 3130 SI and the reaction products were analyzed by gas chromatography and mass spectrometry.. The major products were found to be peracetic acid and d i a c e t y l peroxide. By following the rate of formation of these products, as well as the con-centrations of the three possible kinds of oxygen i n the re-action mixture, i t was possible to v e r i f y McDowell and Farmer's mechanism for the photooxidation of acetaldehyde and e s t a b l i s h a s a t i s f a c t o r y mechanism for the chain terminating step of the reaction. In the second part of the work, orotonaldehyde was i r r a d i a t e d at 30°C with l i g h t at a wavelength of 2450-4000 i , and the products analyzed by gas chromatography and i i i i n f ra-red spectroscopy. The only product i n experiments of short duration was found to "be 3-butene- 1-al, thus e s t a b l i s h -ing isomerization as the primary process of the reaction. In l a t e r stages C O , propylene and 1,5-hexadiene were formed. The following experimental laws were found to hold for the rate of formation of these products: d(3-butene-l-al)/dt r ? - I a t s , ^ z. 0.095 i 0.005 2 d ( l , 5-hexadiene)/dt ~ ( I ^ g . t)/(crotonaldehyde ) d(propylene )/dt (I a- b|.t)/(crotonaldehyde)l» 3" 1* 5 No detailed law was established for the rate of formation of C O , but whenever measured.it was found to follow c l o s e l y the propylene formation. These experimental laws were interpreted by means of a mechanism, i n which crotonaldehyde participates in so far as i t iaomerizes to 3-bujrene-l-al. The l a t t e r was assumed to y i e l d the three other products of the reaction through a mixed radical-molecular photochemical d i s s o c i a t i o n . Evidence was provided i n favour of the proposed mecha-nism by studying the photolysis of 3-butene-l-al at 30°C and a wavelength of 2450-4000 1. The products of the reaction were found to be propylene, 1,5-hexadiene and C O , with their rates of formation obeying the following laws: d(1,5-hexadiene)/dt r %exad. , ; Eabs . ^hexad - ° * 1 3 5 ~ ° « 0 3 i 8 i v d(propylene)/dt ~ 9 Prop , ] Cabs . ^ prop ~0.79t0.05 CO formation was found to be approximately equal to propylene formation. These experimental laws were interpreted by means of a mixed radical-molecular photochemical d i s s o c i a t i o n of 3-bu-t e n e - l - a l , thus j u s t i f y i n g the mechanism proposed for the photolysis of orotonaldehyde. ACKNOWI^ GSMENT The present work haa been conducted under the supervi-sion of Professor CA. McDowell to whom I wish to express my gratitude for his lasting interest and enlightening dis-cussions throughout the course of the work. I also wish to thank Dr. D.C. Frost for his helpful guidance in the part of this work connected with mass spectrometry. I am grateful to the University of British Columbia for Teaching Assistantships during the 1960-61 and 1961-62 sessions and to the National Research Counoil for a Student-ship during the 1961-62 summer session. Finally, I wish to thank the glassblowlng, electronic and workshop staff for their assistance in the construction of parts of the apparatus. C ON T E N T S Page. CHAPTER I. INTRODUCTION I. 1. PHOTOCHEMICAL OXIDATION OF ACETALDEHYDE 1 I. 2. PHOTOLYSIS OF C ROT ON ALDEHYDE 9 CHAPTER I I . PREPARATION OF MATERIALS II . 1. ACETALDEHYDE 14 II. 2. OXYGEN 15 II. 1 5 3 . OXYGEN-18 16 II. 4. ARGON 16 II. 5. OXYGEN-ARGON MIXTURE 16 II. 6. PERACETIC ACID 17 II. 7. DIACETYL PEROXIDE 17 II. 8. LIGHT FILTER SOLUTIONS 18 II . 9. CROTONALDEHYDE 18 11.10. 3-BUTENE-1-AL 19 11.11. PROPYLENE, CO, and 1,5-HEXADIENE 21 11.12. SILVERING SOLUTIONS 21 11.13. ACTINOMBTRIC SOLUTIONS 22 CHAPTER III . MECHANISM OF THE TERMINATION STEP IN THE PHOTOOXIDATION OF GASEOUS ACET ALDEHYDE AT 25°C III. 1. APPARATUS § ; 3 I I I . 2. EXPERIMENTAL 35 v i Page III* 3. RESULTS 41 III, 4. DISCUSSION 43 CHAPTER 17. MECHANISM OP THE PHOTOLYSIS OF GASEOUS CROTONALDEHYDE AT 30°C AND 2450-4000 &, IV. 1. APPARATUS 56 IV. 2. BXPERIMENTAL 57 IV. 3. RESULTS 7 6 IV. 4. DISCUSSION 89 CHAPTER V. MECHANISM OF THE PHOTOLYSIS OF GASEOUS 3-BUTENE-l-AL AT 30°C AND 2450-4000 ft, V. 1. EXPERIMENTAL 111 V. 2. RESULTS 111 V. 3. DISCUSSION 120 REFERENCES 125 CHAPTER I. INTRODUCTION I. INTRODUCTION 1. PHOTOCHEMICAL OXIDATION OF ACETALDEHYDE The Interest of the research workers i n the oxidation of lower a l i p h a t i c aldehydes i s well understood i f one remembers that they are among the most e a s i l y oxidized organic compounds known. This property makes t h e i r oxidat-ion a reaction e a s i l y followed at comparatively low temp-eratures, where techniques are simple and side reactions easy to control. It i s important to investigate the mode of oxidation of aldehydes because there i s strong evidence that they are formed as intermediates during the oxidation of hydrocarb-ons. Therefore an understanding of the elementary processes which occur i n aldehyde oxidations w i l l probably makes poss-ib l e an inter p r e t a t i o n of the res u l t s i n the more complic-ated hydrocarbon oxidations. That aldehydes play a part during hydrocarbon oxidat-ions was noticed as early as 1930 by Bone et a l (1,2) and Naylor and Wheeler (3). Their e f f e c t of reducing the i n -duction period of these reactions was studied and many imaginative mechanisms were postulated over the years to explain t h i s e f f e c t as well as their formation during the reaction. Norrish (4) postulated that aldehydes aris e z through - O H attae:k on the hydrocarbons, although he did not propose a precise mechanism for their p a r t i c i p a t i o n i n the reaction. Sven i f the part played by aldehydes i n hydrocarbon oxidations i s not completely understood, i t i s clear that investigation of aldehyde oxidations may be proved h e l p f u l i n e lucidating certain aspects of these complicated react-ions, e.g. by evaluating rate constants and a c t i v a t i o n energies of elementary prooesses. The easiest way to obtain r e l i a b l e data of t h i s nature i s by running the oxidation i n the gag phase, and at low temperatures, where complicating factors such as uneven d i s t r i b u t i o n of oxygen, and react-ions between products and f i r s t material are minimized. The aldehyde most thoroughly investigated i n this respect i s acetaldehyde. A low b o i l i n g l i q u i d , i t can a t t a i n con-siderable pressure at room temperature, a fact which i s a d i s t i n c t advantage i n r e l a t i o n with the high b o i l i n g alde-hydes. Moreover, i t can undergo oxidation without polymer-i z i n g , as the case i s with formaldehyde. These merits have been f u l l y recognized by research workers, as a short review of the work done on acetaldehyde oxidation w i l l -show. In 1930, Bowen and Teitz (5) studied the photochemic-a l l y i n i t i a t e d oxidation at 20°0. They found peroxides to be the main reaction products and measured the rate of the reaction by iodometrieally estimating them. Their data 3 showed that the rate was d i r e c t l y proportional to the alde-hyde concentration and square root of the absorbed illum-ination, and independent of the oxygen concentration. How-ever the study was not a rigorous one, because no correcti o n was made for the v a r i a t i o n of the absorbed l i g h t at d i f f e r e n t aldehyde pressures and the lowest oxygen pressure used was approximately one quarter of an atmosphere. They presented evidence that d i a c e t y l peroxide was the major peroxide formed, along with some peracetic acid, and interpreted their r e s u l t s by means of a mechanism involving activated molecules. In 1931, Bodenstein (6) investigated the thermal oxid-ation at 60-90°C and found peracetic acid as the p r i n c i p a l product. Its rate of formation was proportional to the square of the aldehyde concentration and inversely prop-o r t i o n a l to the oxygen concentration. He suggested a mech-anism based on activated molecules. Later, Hatcher et a l (7) and Pease ( 8 ) showed again that peracetic acid was the primary product, but that the rate was proportional to the square of the aldehyde concentration and independent of the oxygen concentration. They proposed various activated molecule mechanisms to explain the r e s u l t s . At about the same time, Semenoff (9) reviewed the reaction and showed that the experimental facts could be explained i f i t was assumed that an i n i t i a l reaction product decomposed to give chain branching and. that termination was due to interaction 4 of two ohain propagating r a d i c a l s . In 1936, Carruthers and Norrish (10) made a b r i e f study of the photochemical oxidation at 30°C. The oxidation was taken to completion. The gaseous products were CO, C H 4 , Hg and CgH^ and condensed products were mainly peroxides. The peroxide was assumed to be diaoetyl peroxide because the r a t i o of aldehyde to oxygen used up was 1:0.7. The rate of the re-action was measured by oxygen uptake. No mechanism was pro-posed. In 1941, Mignolet (11) studied the photochemical oxida-t i o n at 35°C and measured the rate by following o v e r a l l pressure change. He found that the rate was proportional to I a t j S £ , Aid and 0 2 and concluded that the primary oxidation product was diaoetstl peroxide. It was not u n t i l McDowell and co-workers reinvestigated both the thermal and photochemical oxidation that a s a t i s f a -ctory mechanism was postulated. In 1949, McDowell and Thomas (IE) made a rigorous investigation of the thermal oxidation and proved unambiguously what e a r l i e r workers had obserareii, namely, that peraoid was formed via a chain at a rate inde-pendent; of oxygen concentration and proportional to the aquare of the aldehyde concentration. For the f i r s t time a r a d i c a l ohain mechanism was proposed to interpret the r e s u l t s . The mechanism was as follows: 5 CHgCHO + Og a. CH300' + HOg* GHgCO'+Og s. GH 3C0 3' CHgCOg'+CHgCHO » CH3C:03H + CHgCO' CHgCOg.+ Og * H0 2»+ CHgCOg. ^ g c O + CO This mechanism explained most of the experimental r e s u l t s , but i t was s t i l l not quite s a t i s f a c t o r y , due to the u n l i k e l y termination step. Also, i t did not quite explain why the rate of the aldehyde oxidation remains almost constant a f t e r a considerable amount of i t was already consumed. Hiolause (13')) suggested a branohing chain step, but. i t did not agree with the experimental r e s u l t s . F i n a l l y , McDowell and Farmer (14) explained the data convincingly by suggesting that the branching step was uni-m&lecular decomposition of peraceti® acid and chain termi-nation was by r a d i o a l - r a d i o a l disproportionation, as follows: GHgCHO + 0 2 v CHgOO* + H0p* CHgCH* + Og » CHgCOg* CH 3C0 3 ,+ CHgCHO » CHgCOgH + CHgCO' CH 3C0 2H ± CHgCOg* + OH* 2CH 3C0 3' > (CHgCO)gOg + Og This mechanism explained f u l l y the r e s u l t s and agreed with the views that Semenoff (9) had expressed many years e a r l i e r . In the same work (14) they investigated the photochemical oxidation using modern techniques at 20°C. Peracetic acid was measured by the ferrous thiocyanate method, and the rate, when only a small percentage of the reactants was consumed, 6 was found to be accurately proportional to the concentration of the aldehyde and the square root of the in t e n s i t y of the absorbed illumination, being independent of the oxygen con-centration. E s s e n t i a l l y the same mechanism proposed for the thermal oxidation was used to explain the r e s u l t s , except that the i n i t i a t i o n step was altered to take account of the illumination and the branching step omitted because the ex-perimental conditions rendered i t n e g l i g i b l e . The whole mechanism was as follows: CH 3 C H 0 f hv : >CH3* +• CHO* CH3. + 0 2 >") CHO* t 0 2 ^ R* 4- C H 3 C H O »RH + CHgCO* 9 s l a b s CH 3 C0* -I- 0 2 ? C H g C 0 3 « i i C H 3 C 0 3 * * GHgOHO > CHgCOgH + CHgCO* i i i 2 CHgGOg* : ^ ( C H 3 C 0 ) 2 0 2 4- 0 2 i v Application of the stationary state hypothesis to the above scheme gave the expression a(CH 3 C0 3H)/dt (K 1 1 1/K l v * ' ) ( < P gI a b a ) * {0H a CH0) whioh agreed w e l l with the experimental r e s u l t s . McDowell and Farmer's conclusions were substantiated by subsequent work. Hanst et a l (15) provided favorable evidence by studying the ozone i n i t i a t e d oxidation of acetaldehyde. And McDowell and Sharpies (16,17,18,19) investigated once more the photochemical reaction, along with that of prop-7 ionaldehyde and confirmed the re s u l t s over a wide range of experimental conditions. By using the intermittent i l l u m i n -ation technique, they were able to estimate the absolute values for the propagating and terminating steps i n the photooxidation for both aldehydes. They also computed the ac t i v a t i o n energy for the propagation step. The above review on the gas phase oxidation of acet-aldehyde shows quite well that the reaction was almost completely understood when the present research was under-taken. McDowell and Farmer's mechanism had been repeatedly tested and proved correct. There were, however, some aspects of t h i s mechanism which needed further elucidation. One was due to the fact that the two peroxides formed in the reac-tio n , peracetic acid and d i a c e t y l peroxide, could not be characterized and determined with accuracy i n presence of each other with the a n a l y t i c a l means available at the time. E a r l i e r workers r e l i e d on simple t i t r a t i o n s to estimate the amount of each peroxide. This led, for a time, to confusion, as i t can be seen from the fact that h a l f of the early workers maintained that peracetic acid was the main peroxide while the other half maintained that d i a c e t y l peroxide was. McDowell and Farmer's mechanism showed how both the peroxides could be formed and implied that the r a t i o (diaoetyl per-oxide/peracetic acid) would be d i r e c t l y proportional to the square root of the inte n s i t y of the absorbed illumination. The presence of d i a c e t y l peroxide i n the reaction products 8 was confirmed "by paper chromatographic methods. However, no unambiguous proof of the postulate was possible because of the uncertainty inherent i n the a n a l y t i c a l method. It was, therefore, an object of the present work to reinvestigate the low temperature photochemical oxidation of acetaldehyde and estimate the rate of formation of both peroxides. It was hoped that modern techniques of analysis, e s p e c i a l l y gas chromatography, would make the fulfilment of t h i s aim possible. Another aspect of McDowell and Farmer's mechanism which needed further investigation was the r a d i c a l - r a d i c a l termination step. It w i l l be r e c a l l e d that t h i s step i s the following: £GH3C03 > ( C H 3 C 0 ) 2 0 2 + (Dg There are two obvious ways by which th i s reaction can take place. One p o s s i b i l i t y i s that the oxygen molecule which i s formed i s produced by elimination of an oxygen atom from each of the i n t e r a c t i n g radicals,, according to the scheme: 2 C H g C 0 3 ' > C H 3 C \ 0 J P C H 3 * C H 3 ° \ 0 0 / C C H 3 * °2 The other p o s s i b i l i t y i s that the oxygen atom which i s eliminated comes wholly from one of the interacting r a d i c a l s . This could be understood i f the mechanism were the following: 2 C H 3 C 0 3 . _ C H 3 C ^ n - , p ; ^ _ , CRzcf Q OO...C-00 H 3 y C H 3 C * 0 9 McDowell and Sharplea (18) envisioned these two possib-i l i t i e s and suggested that use of i s o t o p i c a l l y enriched oxygen might enable the chemistry of the process to be elucidated. It was another obje:ct of the work described here to carry out such experiments, i n the hope that these fine d e t a i l s of a well established mechanism would be understood. 2. PHOTOLYSIS OF CROTONALDEHYDE Whereas acetaldehyde oxidation i n the gas phase has been so widely investigated, very l i t t l e work: has been done on higher aldehydes, with the exception of propionaldehyde. This i s due mainly to the fact that higher aldehydes have too small vapour pressures to be studied conveniently at lower temperatures. However such studies could be of value in understanding c e r t a i n aspects of hydrocarbon oxidation, because there i s evidence of the existence of higher alde-hydes i n the cool flames i n the i g n i t i o n of higher hydro-carbons. Orotonaldehyde i n p a r t i c u l a r has been detected i n the cool flames of butene ( 2 0 ) , pentane ( 2 1 ) , and four iso-meric hexanes ( 2 2 ) . Various imaginative mechanisms have been proposed for i t s appearance but without r e a l substantiation. The work: done on gaseous orotonaldehyde oxidation and decomposition, both photochemical and thermal, can be summ-arized as follows. 10 In 1936, Blacet and Roof (23) studied the photolysis of gaseous orotonaldehyde at 30°C at seven wavelengths, ranging from 3660 to 2399 A 0 and found no detectable decom-posi t i o n . The only a l t e r a t i o n which they were able to observe was polymerization at 3660 A 0 with a quantum y i e l d 0.02 based on the number of molecules disappearing from the gas-eous phase per quantum of l i g h t absorbed. They proposed a predominating reverse reaction of the type CH3CH:CHCH0 c > CHgCHjCH- f CHO* to account for the fact that no products were detected even i n the region of the l i g h t where absorption of il l u m i n a t i o n by er.otonaldehyde i s apparently continuous. In the same work they recorded b r i e f l y that oxidation of orotonaldehyde by oxygen does not proceed at a measureable rate i n the dark, but i t i s accelerated by l i g h t . At the same time, D e l i s l e et a l (24) studied the thermal decomposition at 430-482°C and found 60 and a mixture of hydrocarbons, mostly propylene, to be the main products. The reaction was heterogeneous and i n general of the second order. Ho mechanism was proposed. Three years l a t e r Blacet and his co-workers studied the photolysis at elevated temperatures (25) and the photochem-i c a l oxidation at 30°0 (26). In the photolysis at 150-400°C the main products were found to be 60$ CO, 27$ unsaturated hydrooarbons, 6$ hydrogen and 7$ methane. The rate of decom-pos i t i o n increased with increase i n temperature and decrease 11 In wavelength. No mechanism was proposed. In the same work • they studied b r i e f l y the thermal decomposition and found that i t i s not appreciable below 150°C and very slow below 275°C. At higher temperatures they confirmed the res u l t s found by DeLisle et a l (24). In the photochemical oxidation at 30°C they followed the course of the reaction by recording pressure decrease and postulated that the s o l i d accumulated i n the c e l l during reaction was crotonic acid. They suggested that the o v e r a l l reaction i s 2CH3GH:CH:CH0 + 0 2 ?-2CH3GHjCHCOOH but also noticed the p o s s i b i l i t y of a chain mechanism, because . the quantum y i e l d could be as high as 3.2 under ce r t a i n c i r -cumstances. Introduction of nitrogen caused a marked deorease of quantum y i e l d , therefore they concluded that reaction takes place mainly through activated molecules, l a t e r Volman et a l (27) used the mirror technique and detected free r a d i -cals i n illuminated crotonaldehyde. They suggested that a rupture of the type (CH3CH:CHCH0)* > CH 3CH:CH« CHO* must occur too. In recent years Tosberg and P i t t s (28) reinvestigated the high temperature photolysis of crotonaldehyde and i n geniera. 1 confirmed the re s u l t s found',by Blacer et a l (25). They i d e n t i f i e d the- unsaturated hydrocarbon as being mainly but-2-ene and suggested that i t was formed through a r a d i c a l IE displacement process: CHg 4 CH3CH:CHCH0 * CH3GH:CH0H3 4- CHO ' Harrison and Lossing (29) studied the mercury-photosen-s i t i z e d low temperature decomposition, im an e f f o r t to define the primary process talcing place ; i n orotonaldehyde photolysis. They used a mass spectrometer to follow the reaction products which were mainly CO and propylene. Added methyl radicals,C H Y , 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 for the amount of products formed. They concluded that the main process taking place was molecular rearrangement to CO and. propylene, accompanied hy some r a d i c a l s p l i t t i n g , as follows: Recently Oshorne and Skirrow (30) reinvestigated the thermal oxidation using modern techniques of analysis to i d e n t i f y and determine the products. They found peroxyero-tonic acid to he the main product at 166°C and adopted McDowell and Parmer's mechanism for the oxidation of acetaldehyde to explain the r e s u l t s . Above 200°C the peracid was quickly de-composing and only low molecular weight fragments could be detected. From the proceeding survey i t can be seen that the reactions taking place when orotonaldehyde, alone or i n mixture with oxygen, i s heated or illuminated, are far from main process processes minor 1 3 being unambiguously -understood. The low temperature photoly-s i s , which could provide information about the primary pro-cesses taking plaoe without the l a t t e r being masked by secondary reactions, was abandoned early because the analy-t i c a l tools available at the time made detection and deter-mination of any products impossible. In Harrison and lossing's work ( 2 9 ) i t was possible to gather evidence i n favour of a primary process, but, unfortunately, no kinetic study of the reaction was undertaken to confirm the proposed scheme. It was, therefore, an object of the present work to reinvestigate the low temperature photolysis of crotonal-dehyde, with the hope that modern techniques of analysis would make i t possible to gather evidence for a convincing primary process. CHAPTER I I . PREPARATION OF MATERIALS 1 4 II PREPARATION OF MATERIALS 1 . ACETALDEHYDE Acetaldehyde was p u r i f i e d gas-chromatographically by passing the commercial material through a 5 foot long column packed with dinonyl phthalate on f i r e b r i c k . The c a r r i e r gas was helium. The apparatus was the same gas-ohromatography unit used for the analysis of reaction mixture i n the photo-ohemleal oxidation of acetaldehyde and i t i s described on page 2&» A diagram i s shown i n Figure 2. Before a run, the trap F, i n which the p u r i f i e d a cetal-dehyde would be co l l e c t e d , was flamed and evacuated to better than 10"^ mm Hg for several hours. At the same time helium was passing at a slow rate through the dinonyl phthalate column, which was heated at 40°C, to ensure that no impuri-t i e s from previous runs remained i n the system. After t h i s treatment the stopcocks leading to the vent and the vacuum line were both closed and the helium was allowed to pass through the trap F , which was at room temperature, and the bubbler to the atmosphere for one more hour. This ensured that no traces of oxygen were l e f t i n the trap. Then the flow of helium was adjusted by increasing the pressure to 4 p s i and the current of the gas directed to the vent, after the trap was cut by means o f the stopcock. The trap was then cooled-to -78°C with dry ice and acetone. 15 Acetaldehyde was d i s t i l l e d into the cooled sampling trap D with the aid of the vacuum l i n e . When approximately 2 ml were collected, the connection with the vacuum was closed, the trap surrounded with water at the temperature of 35°C and helium allowed to pass through i t . It took less than h a l f a minute for the acetaldehyde to evaporate comple-t e l y . The recorder of the apparatus was watched, and when the peak corresponding to acetaldehyde appeared, the current of the gases was directed to the cooled trap F, where the alde-hyde was retained. It was degassed by d i s t i l l i n g i t into trap G and back into trap F, each time r e j e c t i n g approxima-t e l y the f i r s t one tenth of the sample. The acetaldehyde p u r i f i e d i n t h i s way was kept at -78°G i n trap F. It lasted, usually, for 5-6 experiments, then another sample was prepared. To check for impurities, e s p e c i a l l y d i a c e t y l peroxide which was found to occur i n the commercial product, a small portion of each sample was analyzed by passing i t through the a n a l y t i c a l dinonyl phthalate column. No impurities were ever detected using the thermoconductivity detector of the apparatus. 2. OXYGEN Cylinder oxygen was p u r i f i e d by l i q u i f y i n g and d i s t i l l i n g i t four times, each time r e j e c t i n g 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 16 l i t r e receiving flask. 3. OXYGEN-18 0 1 8 , i n the form of molecular oxygen, of 98.35 per cent purity was obtained, from the Isotope Department, Weis-mann Institute of Science, Rehovoth, I s r a e l . 4. ARGON Cylinder argon was used without further p u r i f i c a t i o n . No impurities could be detected i n a mass spectrogram. 5. OXYGEN-ARGON MIXTURE Iso t o p i c a l l y enriched oxygen was needed i n order to study the r a d i c a l - r a d i c a l termination step i n the photo-chemical oxidation of acetaldehyde. Argon was added to i t so as to give a referenoe peak i n the mass spectrometer. This mixture was prepared by allowing oxygen-18, argon and p u r i f i e d natural oxygen to expand successively i n a 5 l i t r e bulb measuring the t o t a l pressure after each gas entered the bulb. The pressures were 6.13, 11.13 and 117.5 mmHg respectively. Therefore the composition of the gas was 0 2 3 2 : 0 2 3 6 : A r g o n =106.4 : 6.13 : 5.00 . Taking into account the fact that the 0 g 2 6 was 98.35$ pure and contained 0.4.97$ 0 1 7, and that the natural abun-dance of 0 1 8 i n ordinary oxygen i s 0.204$ (31), i t can be 17 calculated that the r e a l composition of the oxygen-argon mixture was Og :Og :Og :Argon ^106 : 0.442 : 6.03 : 5.00 . In deriving these numbers i t was assumed that 0-1-8 i s stat-i s t i c a l l y d i s t r i b u t e d with the other species i n natural oxygen. The same assumption was made for 0^ i n O g 3 ^ . 6. PERACETIC ACID -Peracetic acid was prepared by the method of D'Ans and Frey (32) as modified by Greenspan (33). To 10 ml of g l a c i a l acetic acid, 0.11 ml of sulphuric acid were added under cooling with water at 20°C. After standing for 12 hours at 20°C the mixture contained 40-50$ peracetic acid (33). A small amount of i t , used for c a l i b r a t i o n of the chromato-graphy apparatus, was p u r i f i e d gas chromatographically using a 2 foot long dinonyl phthalate column at 0°C. In order to inject the raw peroxide into the system, the heat-ed sampler (D-j_) of the apparatus was used (Figure 2), which was suited for high b o i l i n g materials. In t h i s case the sampler could not be heated higher than 35°C because of danger of explosion of the peraeid, but even under these conditions enough peraeid for c a l i b r a t i o n was prepared after two runs. 7. DIACETYL PEROXIDE -Diacetyl peroxide, i n the form of 25 per cent solution i n dimethyl phthalate, was obtained from Wallace and Tiernan Inc., l u o i d o l Division. 18 Small amounts of i t , used for quantitative c a l i b r a t i o n of the gas chromatography system, were freed from the s o l -vent by passing through the 2 foot dinonyl phthalate column at 0°C. In this case, too, the heated sampler D^ was used. 8. LIGHT FILTER SOLUTIONS The two l i g h t f i l t e r solutions used i n the photooxidation of acetaldehyde were those recommended by Baekstrom (34) to i s o l a t e a f a i r l y narrow wavelength band close to 3130 A 0 mercury l i n e . F i l t e r solution I comprised 46 grams of a n a l y t i c a l reagent grade NiS04'6H 20 and 14 grams of a n a l y t i c a l reagent grade GoS04*7H20 dissolved i n 100 ml of d i s t i l l e d water. This solution was stable to the action of l i g h t . F i l t e r solution II comprised 25 grams of a n a l y t i c a l reagent grade potassium hydrogen phthalate dissolved i n 500 ml of d i s t i l l e d water. This solution was not stable to the action of l i g h t . 9. OROTONALDEHYDE A n a l y t i c a l reagent grade orotonaldehyde was p u r i f i e d gas chromatographieslly. Since the sample prepared i n each run was enough for approximately f i f t e e n experiments only, a comparatively large quantity (^10 ml) was introduced into the sampler D]_. This amount lasted for 5-6 p u r i f i c a t i o n s . The same procedure was followed as i n the preparation of 19 acetaldehyde, except that the dinonyl phthalate column was heated to 55°C. Under these conditions the p u r i f i e d croton-aldehyde showed only i n f i n i t e s i m a l amounts of impurities (less than 0.02$) when analyzed gas-chromatographically using the i o n i z a t i o n detector of the apparatus. 10. 3-BUTENE-l-AL This isomer of crotonaldehyde was prepared by oxid-i z i n g the corresponding alcohol, 3-butene-l-o.l, by chromic acid at low temperature. This method of oxidation i s gener-a l l y recommended when an unsaturated aldehyde i s going to be prepared from the alcohol (35). The experimental d e t a i l s were the following: In a triple-necked 500 ml f l a s k immersed i n ice and s a l t , and equipped with thermometer, drop funnel and c a p i l l a r y , and connected to the vacuum line through a trap cooled at -78°C, a mixture of 12 ml H_,S04 and 40 ml d i s t i l l e d water was placed. After cooling to 0°C, 16 ml of 3-butene-l-ol were added. Then the pressure was reduced to 30-40 nimHg and a solution of 10.8 grams CrOg and 12 ml HgS0 4 i n 40 ml d i s t i l l e d water was added drop by drop i n such a way that the temperature i n the f l a s k was maintained between 2° and 10°C. ?/hen a l l the chromic acid had been added (in about 25 minutes) the pressure was lowered to 10 mm Hg and the f l a s k allowed to reach room temperature. The substance collected i n the trap separated into two layers after being allowed to thaw. The upper layer was introduced 20 in the sampler D-j_ and the mixture separated gas-chromatograph-i e a l l y using the same 5 foot long dinonyl phthalate column as for orotonaldehyde hut at 2 psi pressure and 40°G. The main impurities i n the reaction mixture were orot-onaldehyde and acrolein. After passing through the column, a c r o l e i n was almost completely eliminated, hut orotonaldehyde persisted. It seems that 3 T b u t e n e - l - a l cannot he freed com-p l e t e l y from orotonaldehyde, because i t isomerizes very e a s i l y to this aldehyde with the conjugated double bonds. Hoaglin et a l (36) noticed t h i s i n s t a b i l i t y when they t r i e d to prepare 3-butene-l-al by hydrolyzing i t s e t h y l acetal;' they obtained orotonaldehyde instead. Also i n the present work i t was attempted at the beginning to oxidize 3-butene-l - o l to the aldehyde by the Oppenaurer method using einn-amaldehyde as the oxygen donor. The f i n a l stage of t h i s method involves heating to 130-140°C to decompose the alum-inium alcoholate formed i n e a r l i e r stages and d i s t i l l the formed aldehyde. Again drotonaldehyde was obtained instead of 3-butene-l-al. Therefore i t i s understandable that any method used for p u r i f i c a t i o n of 3-butene-l-al w i l l f a i l th free i t from a small amount of orotonaldehyde, unless i t operates at very low temperature. Use of the d i n o l y l phthalate column for preparative purposes proved to be i n t o l e r a b l y i n e f f i c i e n t below 40°C. Analysis of the product p u r i f i e d under these conditions showed that i t contained s t i l l ifo orotonaldehyde. 21 The 2,4-dinitrophenylhydrazone of the p u r i f i e d compound, after r e c r y a t a l l i z i n g twice from alcohol, had a melting point of 175°C, i n good agreement with the 177°C found by Shostakovakii et a l (37). 11. P R O P Y L E N E , 00 and 1,5-HEXADIENE These aubatances were needed for c a l i b r a t i o n of the chromatography lystem. A n a l y t i c a l reagent grade materiala were used for this purpose without further p u r i f i c a t i o n . 12. SILVERING SOLUTIONS Si l v e r i n g solutions, used for preparation of neutral density f i l t e r s , were prepared according to the procedure desoribed by Strong (38) as follows: Solution A was made by dissol v i n g 5 grams of s i l v e r n i t r a t e i n 300 ml of water and adding ammonia u n t i l the s i l v e r oxide, prec i p i t a t e d at the beginning, had almost, but not completely, diaappeared. The l i q u i d waa f i l t e r e d and d i l -luted with water up to 500 ml. Solution B was ma.de by dis s o l v i n g 1 gram of s i l v e r n i t r a -te i n water, b o i l i n g the solution and then adding 0,83 grams of Roohelle s a l t . The b o i l i n g was continued u n t i l a gray pre-c i p i t a t e was deposited. The aolution was then f i l t e r e d hot and d i l l u t e d to 500 ml. These solutions ware stored i n a dark place and used zz whenever neutral density f i l t e r s were needed. They could he us6d for one month. 13. ACTINOMETRIC SOLUTIONS The only solution whioh needed special care i n prepara-t i o n was that of potassium f e r r i o x a l a t e . Crystals of t h i s complex salt were f i r s t prepared hy mixing under vigorous s t i r r i n g 3 volumes of 1 . 5 M K2C2O4 and 1 volume of 1.5 M FeCIg. The r e s u l t i n g salt was r e e r y s t a l i z e d three times from warm water and dried at 4 5 0 c . Under these conditions i t s composition was KgFe (C20 4)g'3H 2 0 (39). A ©.006 M solution of the salt was prepared and stored i n a dark>coloured b o t t l e . The other solutions used i n actinometry were prepared by dissolving the a n a l y t i c a l grade compounds i n d i s t i l l e d water. They are described under "Actinometry" on'page 71. CHAPTER I I I . MECHANISM OF THE TERMINATION STEP IN THE PHOT OOXIDAT ION OF GASEOUS ACETALDEHYDE AT 25°C I l l MECHANISM OF THE TERMINATION STEP IN THE PHOTOOXIDATION OF GASEOUS ACET ALDEHYDE AT 25°C. 1. APPARATUS The Vacuum System The vacuum system i s shown i n Figure 1. It was constru-cted of pyrex, apart from the reaction vessel which was con-structed of quartz. A l l the stopcocks were I r r i g a t e d with Ap&ezon N grease. The apparatus was evacuated t'o a one-stage mercury d i f f u s i o n pump of conventional design, backed by a "Hyvao" rotary o i l pump (not shown). A P2O5 trap was included. The cold trap was demountable and l i q u i d nitrogen was used always as r e f r i g e r a n t . Using t h i s trapT,i the system could be evacuated to better that 10" 4 mm Hg, as recorded by a Mcleod manometer (not shown). The quartz reaction vessel (A) was c y l i n d r i c a l with f l a t o p t i c a l ends and was connected to the vacuum system by a quartz-to-pyrex graded seal. It was 10 cm long and hai an Illuminated volume of 73 ml. The narrow tubing between the c e l l and the stopcock had a volume of B ml. and was not i l l u -minated. The c e l l was i n an alluminium-block furnaeiee with quartz windows to permit the passage of u l t r a - v i o l e t l i g h t . The furnance was e l e c t r i c a l l y heated by means of a c o i l FIGURE 1. HIGH VACUUM APPARATUS FIGURE lex. SPIRAL GAUGE 25 supplied by a manually regulated "Variao" transformer. The temperature could be kept constant within 0.2°G. The pressure of reactants to enter into the reaction vessel, i f not greater than 89 mm Hg, was measured by means of a spiral gauge (B) linked with an optical lever system as shown in Figure la. With the gauge, pressures could he rea-dily measured with 0.2 mm Hg accuracy. When a reaotant pressu-re greater than 80 mm Hg was required, the spiral gauge was zeroed and the pressure was measured with the Hg manometer. (C) was a five It. bulb where oxygen was kept, (F) was a spiral trap were acetaldehyde was kept at -78°C. In the other trap, (G), various products could be isolated after being separated gas-chromatographically from the reaction mixture. Also i t was used in pair with (F) to degasc a sample of material purified gas-chromatographically. After an experiment had been carried' out, the products were allowed to expand into trap (D), which was cooled with liquid nitrogen. The condensable products were retained there and the gases toeplerecL tinto the calibrated volume (H). The condensable products could be analyzed at once, because trap (D) was, at the same time, the sampler of the gas chromato-graphy, apparatus. The gases could be compressed from (H) to either the gas chromatography sampler or a previously evacu-ated collecting bulb, f i t t e d at (I), for further analysis. 26 The Gas Chromatography Apparatus The gas chromatography apparatus was incorporated to the vacuum system and was constructed of pyrex, except for the metallic parts of the detector. The c a r r i e r gas was helium. Before entering the system, i t was freed from condensihle impurities, mainly water, hy passing through two cold traps. When the apparatus was used for a n a l y t i c a l purposes, the traps were cooled with dry ice and acetone. Under these conditions small traces of water could he detected i n the passing helium, which did not affect the analysis, When the apparatus was used to pur i f y substan-ces, e.g. acetaldehyde, the traps were cooled with l i q u i d nitrogen. In t h i s way no traces of water could he detected i n the p u r i f i e d material. The pressure of helium was regula-ted hy means of a pressure regulator (A) and i t s flow rate was read on the fluometer (B). The sampler (D) of the apparatus was connected to the vacuum system through two taps. It was shaped i n such a way, that i t could be used as a trap for condensihle materials. Also, i t could be fed with gases by means of the Toepler pump. It was used i n a l l the a n a l y t i c a l uses of the appara-tus, as well as when acetald'ehyde was p u r i f i e d . Per p u r i f i c a -t i o n of high b o i l i n g materials, e.g. d i a c e t y l peroxide, the sampler (D^) was used. This sampler could be fed either by d d i s t i l l i n g into i t the substance to be p u r i f i e d , or by means of a removable stopper. I t was shaped i n such a way , that . 27 HEALING TAPE FRO/A He TANK T O RECORDING PAANOWETER ^ TO VACUUM LINE FIGURE 2. GAS CHROMATOGRAPHY A P P A R A T U S 28 the sample was su b j e c t e d to h e a t i n g only when i t was f o r c e d to the heated p a r t of the sampler by helium b u b b l i n g through i t , r e t r e a t i n g to the c o l d part below when helium stopped p a s s i n g . The furnace (E) c o n t a i n e d the chromatography column. I t was c o n s t r u c t e d of two pyrex tubes w i t h i n t e r n a l diameters 3 and 5 cm. The t h i n n e r tube was f i t t e d i n t o the l a r g e r one and i t was wound wit h an e l e c t r i c a l l y heated c o i l . The two ends of the furnace were packed w i t h g l a s s wool. The temp-? era t u r e was r e g u l a t e d by means of a " V a r i a c " transformer operated manually, and i t c o u l d remain constant w i t h i n 0.2°G. The furnace was removable and could be r e p l a c e d by a c o o l i n g system (a one l i t r e Dewar f i l l e d w i t h i c e ) . The column was a l s o removable. Two columns were used r e g u l a r l y i n t h i s p a r t o f the work.. Both of them were packed w i t h d i n o n y l p h t h a l a t e ooated on f i r e b r i c k , but they were d i f f e r i n g i n dimensions and o p e r a t i n g c o n d i t i o n s . One was 2-foot long with i n t e r n a l diam-ete r of 3 mm. I t was operated at 0°G and under a pressure of 8 p s i . The other was 5-foot long with i n t e r n a l diameter of 6 mm and was used o n l y f o r p u r i f y i n g m a t e r i a l s . I t s oper-a t i n g c o n d i t i o n s v a r i e d a c c o r d i n g to the m a t e r i a l to be p u r i f i e d . The d e t e c t o r (C) o f the apparatus was a thermoconductiv-i t y d e t e c t o r . I t was surrounded by e l e c t r i c a l l y heated tape s u p p l i e d by a manually a d j u s t e d " V a r i a c " transformer, and 29 was kept always at 40°G w i t h i n 0.2°C. The e l e c t r i c a l pot -r e n t i a l d i f f e r e n c e at the two r e s i s t o r s of the d e t e c t o r was recorded "by the autographic r e c o r d e r (H). When the apparatus was used f o r a n a l y s i s , the helium, a f t e r the d e t e c t o r , was d r i v e n to a vent through a r e s t r i c -t i o n . In t h i s way i t was ensured that the pressure i n the system was always gr e a t e r than the atmospheric pressure and no h a c k - d i f f u s i o n of a i r was p o s s i b l e . When a component of the r e a c t i o n mixture was to be c o l -l e c t e d , or when a substance was to be p u r i f i e d , the vent was c l o s e d at the moment i n which the peak corresponding to t h i s substance was appearing and the stream d i r e c t e d through the c o l d traps (F) o r r ( G ) . The O p t i c a l System The o p t i c a l system i s shown i n F i g u r e 3. The lamp (&") was a General E l e c t r i c , water c o o l e d , AH6 h i g h pressure mercury arc with a housing designed to give a 3 mm diamter source. The p o s i t i o n of lamp and quartz lenses was a d j u s t e d i n such a way that a p a r a l l e l l i g h t beam completely f i l l e d the r e a c t i o n v e s s e l . The quartz„lense, a f t e r the c e l l , f o e u s s e d the l i g h t onto the p h o t o c e l l (D). The l i g h t i n t e n s i t y f i l t e r s were s i l v e r semi-mirrors on q u a r t z . They were prepared by a slow s i l v e r i n g process des-c r i b e d by Strong (38) as f o l l o w s . The quartz p l a t e to be s i l -vered was f i r s t c leaned by b o i l i n g with c o n c e n t r a t e d n i t r i c a c i d , then thoroughly r i n s e d with water and d i s t i l l e d water NEUTRAL FILTERS SHUTTER OPTICAL BENCH GURE 3. OPTICAL S v s i e n FOR ACFTALDEHYDE OXIDATION 3»oo 3160 e WAVELENGTH , A FIGURE 4. TRANSITION OF COMBINED FiLTtR SOLUTIONS 32 and immediately covered with 40 ml each of s i l v e r i n g s o l -utions A and B. After 5 to 15 minutes, the time depending on the thickness wished, the mirror was removed from the s i l -vering mixture, washed sucessively with water, d i s t i l l e d water and acetone and dried. (B) and (C) were quartz f i l t e r c e l l s , 3 cm and 1 cm respectively. The f i r s t was containing f i l t e r solution I, the second,filter solution I I . The spectrum of the f i l t e r solutions combination was measured against no blank i n an automatic recording spectrophotometer. The spectrum i s shown in Figure 4. It i s seen that the combination has an absorp-ti o n minimum at 3160 A 0, therefore the wavelength used was the 3130 l i n e . The Photometer Unit The c i r c u i t diagram of the photometer unit i s shown i n Figure 5. The photocell was i n a quartz envelope designed for work i n the u l t r a v i o l e t region. The output of the c e l l was accurately proportional to the il l u m i n a t i o n shone into i t for the illumination i n t e n s i t i e s used i n the experiments with acetaldehyde. The photocell current passed through the r e s i s t o r chain R]_4 and the required voltage was tapped o f f by the selectorrrgwitchiS|V-LAnSopposing voltage was provided by the potentiometer c i r c u i t B 1 and R5-.12 • B]_ was a 2 volt battery and the voltage tapped from the potentiometer was adjustable to any value between 0 and 2 vo l t s with an ac-Jg§Y;» B l G. P.O. s . c. v.. ?1 R6.6 R l l , 1 2 2 volt aooumulator. 120 volt h.t. battery. Galvanometer. Photoeell: 39. Weston Standard C®11. 6SC7 valv®. 311 : h.s.c. : if?. l.SH : h.s.c. . 1W. 500k ; h.s.o. : If. 150k i h . e . o * : Iff. 10k : dual a©cade: Muirhead. 100 ohms n 10 ohm© H I Hx3 25k : w.w. E^g 50k : w.w. i Rig 50k : w.w. j R17 2.2k* w.wv : Sl9 100k: w.w* : RgO 23k s w.w. : Rgl 10k : w.'w. : Rgg 3,3k: : Rgg lk : w,w.v : Rg4 330 ohms : w. Rg§ 10k : w.w. : AW* 1W. 1W. 1W. 11. FlQURE 5- P H O T O I A E T E R U N I T 34 curacy of 2xl0~ v o l t s . The difference between the voltage tapped from the r e s i s t o r chain R]__4 and the opposing voltage from the potentiometer was applied to the double-triode amp-l i f i e r valve V. The second t r i o d unit compensated for supply voltage v a r i a t i o n . The output of the amplifier was fed into the galvanometer, G, through the attenuator Ri9_24« T n e high i n t e n s i t y for the unit was supplied by a 120 volt battery and the low in t e n s i t y from a 6 volt transformer. To take a photometric measurement the following proc-edure was followed. The c i r c u i t .was switched on by cl o s i n g switches Sg, 3^ and S4 and allowed to warm for 15 minutes. With no light f a l l i n g on the photocell and with the potent-iometer reading zero, dark current from the photocell was balanced by adjusting R^g and R^4 for zero galvanometer cur-rent. The c i r c u i t had to be rebalanced for each position of the selector switch 3^. To ensure that the battery, B^, gave a constant voltage, switch 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 voltsge of the battery had decreased, t h i s reading was diffe r e n t and then the battery was recharged. Switch Sg was then opened and l i g h t was allowed to f a l l on the photo-c e l l . The photocell current was balanced with the potent-iometer and a reading taken. This reading was proportional to the lig h t i n t e n s i t y . 35 Only position 4 of the switch was used throughout t h i s part of the work. This was the less sensitive p o s i t i o n and was quite suited for the l i g h t i n t e n s i t i e s used. The photometer unit was used for monitoring the output of the mercury arc and measuring the percentage absorption of l i g h t by 150 mm Hg of acetaldehyde i n the reaction vessel. 2. EXPERIMENTAL • Determination of Percentage Absorption of Light by 150 mm  Hg of Acetaldehyde at 3130 A° In a l l the experiments, 150 mm Hg of acetaldehyde were used; therefore i t did not appear necessary to draw an absor-ption curve for a range of acetaldehyde pressures. Instead, three i n d i v i d u a l measurements of the li g h t absorbed by 150 mm Hg of acetaldehyde were taken. These measurements were spaced one week apart from each other and a l l showed that 30 per cent of the lig h t shone into the c e l l i s absorbed by acetaldehyde. The agreement between the measurements was 0.5 per cent. The following procedure was used. The AH6 lamp was switched on and allowed to warm for a few minutes, so that the v a r i a t i o n i n in t e n s i t y was less than Vfo.- A suitable i n -tensity was then selected and measured i n ohms using the photometer unit. 150 mm Hg of acetaldehyde were measured quickly into the c e l l £nd the new l i g h t i n t e n s i t y measured. 36 Addition of oxygen had no ef f e c t on the absorption of l i g h t by acetaldehyde. Determination of Reaction Products It i s well known (6,7,8,12) that peracetic acid i s the main reaction product formed i n the photochemical oxidation of acetaldehyde. Diacetyl peroxide i s another product, which becomes of increasing importance as the i n t e n s i t y of the absorbed illumination increases. Preliminary tests with these two peroxides showed that they could be separated gas chromatographically by a 2 foot long dinonyl phthalate c o l -umn operated at 0°C and under a pressure of 8 p s i . Under these conditions the retention time for di a c e t y l peroxide was 4 minutes and for peracetic acid, 21 minutes. Acetal-dehyde appeared at 1 minute, therefore i t was possible to measure the two products i n presence of unused aldehyde (Figure 6 ) . Ro attempt was made to measure the unused acetaldehyde, because such a measurement would require operation of the chromatography apparatus under various s e n s i t i v i t i e s i n the same analysis. The imperfections of the apparatus did not permit such an operation, because the e l e c t r i c a l system required some minutes to come into equilibuium after the s e n s i t i v i t y was changed, and the separation of d i a c e t y l peroxide from acetaldehyde was not long enough to allow for such delay. FIGURE 6. SE P A R A T I O N OF P E R O P U D E S By D I N O N Y L P H T H A L A T L C O L U M N 38 In order to decide about the mechanism o f the r a d i c a l -r a d i c a l c h a i n t e r m i n a t i o n step (see D i s c u s s i o n ) i t was import 34 ant that formation, i f any, of Og would be f a s t enough to be f o l l o w e d by a mass spectrometer with some accuracy. T h i s f a c t i m p l i e d t h a t d i a c e t y l peroxide should be formed at a reasonable r a t e . P r e l i m i n a r y experiments showed that when no l i g h t i n t e n s i t y f i l t e r s were used, t h i s peroxide was formed w i t h a r a t e comparable to that o f formation of the main product, p e r a c e t i c a c i d . When l i g h t i n t e n s i t y f i l t e r s were i n s e r t e d before the c e l l , the r a t e of formation of d i a c e t y l peroxide was found to decrease w i t h the same per-centage as the decrease of i l l u m i n a t i o n . I t was decided to work w i t h two l i g h t i n t e n s i t i e s , one w i t h no n e u t r a l f i l t e r on the path o f the l i g h t , the other w i t h a n e u t r a l f i l t e r p e r m i t t i n g 34.6 per cent of the l i g h t to pass. T h e r e f o r e , i f 100 a r b i t r a r y u n i t s o f I l l u m i n a t i o n were absorbed hy acet-aldehyde i n the f i r s t set o f experiments, 34.6 a r b i t r a r y u n i t s were absorbed i n the second set. C a l i b r a t i o n o f the Chromatography Apparatus f o r P e r a c e t i c A c i d and D i a c e t y l P e r oxide. C a l i b r a t i o n curves were c o n s t r u c t e d by p a s s i n g a known amount of the pure substance through the chromatography system and then measuring the area of the peak r e c o r d e d by the autographic r e c o r d e r . The peroxides c o u l d not be meas-ured by means of the gas b u r e t t e , because the mercury o f i$$e T o e p l e r was a t t a c k e d . Instead, a known pressure of the pure 39 substance was allowed, i n t o the r e a c t i o n v e s s e l , which had a t o t a l volume of 75 ml, and then d i s t i l l e d q u a n t i t a t i v e l y i n t o the sampler o f the gas chromatography apparatus. It was found that f o r both p e r o x i d e s , the area o f the peak was, w i t h i n experimental e r r o r , p r o p o r t i o n a l to the amount of the substance i n j e c t e d i n t o the apparatus. The areas were measured by means of an " A r i s t o " p l a n i m e t e r . The p r o p o r t i o n a l i t y c o e f f i c i e n t s were 2.1 x 10 moles of pera-c e t i o a c i d and 1.15 x 10 moles of d i a c e t y l peroxide per area u n i t (as measured by the planimeter) o f the r e s p e c t i v e peak, u s i n g the maximum s e n s i t i v i t y of the chromatography apparatus. These c o e f f i c i e n t s were checked f o r amounts of peroxides up to 6 x 10 moles. E x p e r i m e n t a l Procedure T h i s procedure was f o l l o w e d to study the photochemical o x i d a t i o n o f acetaldehyde. The r e a c t i o n v e s s e l was evacuated f o r two hours while heated at 200°C, then allowed to c o o l to 25°C. T h i s p r e c a u t i o n was necessary i n order to ensure than no products from a p r e v i o u s r e a c t i o n were l e f t i n the v e s s e l . The lamp and the photometer u n i t were switched on and allowed to s e t t l e down. In one set of experiments a n e u t r a l d e n s i t y f i l t e r was p l a c e d i n the l i g h t beam. The photometer u n i t was balanced w i t h no l i g h t f a l l i n g on the p h o t o c e l l and the temperature a d j u s t e d i f necessary. Then Og-Ar mixture was expanded i n t o the v e s s e l to 60 mm Hg 40 followed by eeetaldehyde u n t i l the t o t a l pressure was E10 mm Hg. In t h i s way the vessel contained Og-Ar mixture and acetal-dehyde with p a r t i a l pressure 60 and 150 mm Hg respectively. The reaotants were allowed to miw for 15 minutes, then l i g h t was shone into the reaction vessel for a given time, measured with a stopwatch, and the i n t e n s i t y measured using the pho-tometer unit. The absorbed i n t e n s i t y could be calculated, knowing that acetaldehyde absorbs 30 per cent of the i l l u -mination. The l i g h t was cut o f f and the condensihle material was condensed out quickly i n the chromatography sampler, which was immersed i n l i q u i d nitrogen. The non-condensible gases were then very slowly puped off-using the Toepler pump and driven into the gas burette. The refrigerant was removed from the sampler and a stram of helium allowed to pass phrough i t , which drove the contents into the chromato-graphy eoluiml. The gases i n the gas burette were compressed into a demountable bulb equipped with a brefek-seal. The bulb was sealed and stored u n t i l enough similar bulbs were co l l e c t e d to employ the mass spectrometer for one day. In the mass spectrometric analysis, the i n t e n s i t i e s of the peaks at masses 32,34, 36 and 40 were recorded. This last peak be-longed to the argon and since this gas passed unaltered the reaction i t was possible to calculate the amount of the other species, i . e . o|° , o| 4 , o | 2 , r e l a t i v e to i t . The amount of argon, bas&d on the o r i g i n a l composition of.the oxygen^argon 41 mixture at 25 C, was 1 . 0 1 x 10 moles; therefore, the abso-lute amounts of the other speoies could he calculated. 3. RESULTS Two sets of experiments were carried out using the same concentrations of reaotants ( i . e . 60 mm Hg oxygen-argon mixture and 150 mmHg acetaldehyde) hut at d i f f e r e n t l i g h t i n t e n s i t i e s . In one set, the l i g h t i n t e n s i t y was lOOj i n the other set the l i g h t i n t e n s i t y was 34.6 (arbitrary u n i t s ) . The r e s u l t s are given i n tables I and I I . TABLE I Analysis of Products During the Photochemical Oxidation of 150 mm Hg Acetaldehyde and 60 mm Hg Og-Ar Mixture at 25°C. I a t jg = 100 a r b i t r a r y u n i t s . A l l data are i n units of moles x 1 0 5 . P= Peracetic Aoid, D ^ D i a c e t y l Peroxide. Time ,-"ol2 ' EV2 «24 0 3 6 (z) W 018fa P D P/D sec. (x) <y> (x+y+z) 0 2 1 . 7 1 0.123 1.220 23.05 5.55 0 0 -90 19557 0.230 1.053 20.85 5.60 1.26 1 . 15 1.695 180 15.79 0 .331 0.764 16.84 5.52 2.86 2. 88 0.993 240 13.10 0.392 0.579 14.07 5 .51 4 .16 3 . 82 1.088 300 12.93 0.407 0.568 13.90 5 . 55 4 .78 4 . 37 1.094 360 11 .62 0.409 0 .505 12.53 5.66 5 .58 . 5. 58 1.000 mean value: 1.054± 0.052 42 TABLE II Analysis of Products During the Photochemical Oxidation of 150 mm Hg Acetaldehyde and 60 mm Hg Og-Ar Mixture at 25°C. 1^3= 3.4j3 a r b i t r a r y u n i t s . A l l data are i n units of moles x 10 5. P= Peracetic Acid, D - Diacetyl Peroxide. Time sec. 032 u2 (x) (y) °!6 (z) W (x+y+z) 018fo P D P/D 0 21.71 0.123 1.220 23.05 5.55 0 00 60 21.12 0.154 1.180 22.44 5.60 0.313 0.161 1.96 120 20.31 0.184 1.110 21.60 5.57 0.882 0.460 1.92 ' 180 19.65 0. 200' 1.048 20.90 5.49 1.345 0.702 1.92 300 17.68 0.241 0.926 18.85 5.55 2.50 1.473 1.70 420 15.71 0.28& 0.805 16.80 5.60 3.64 2.10 1.73 600 12.87 0.312 0.617 13.80 5.60 6.26 3.16 1.98 mean value,:: Lul.87 * 0.12 Prom these tables i t i s evident that oxygen -34 appears during the course of the reaction. Also, i n the last column of each table the r a t i o (Peracetic Acid)/(Diacetyl Peroxide) i s given, and i t i s seen that t h i s r a t i o i s constant for a given illumination. These results w i l l be interpreted i n the following section. 43 4. DISCUSSION It i a convenient to write down the complete mechanism for the photochemical oxidation of acetaldehyde as i t has been established by previous work (14,16). This mechanism i s the following: CHg'CHO 4- hv > CEZ' + CHO' l a CHgCHO f hv :—> (CHgCHO)* lb » R * l e CHO' f 0 2 j R. +- CHgCHO > RH 4- CHgCO' -\ (CHgCHO)^ 4- 0 2 ^CHgCO* f HOg* CHgCO* +- Og ^CH 3C0g» 2 CHgCOg' 4-CHgCH0 >CHgCOgH 4- CHgCO' 3 2CHgC0« ? (CHgCO)g 4 CHgCO* 4-CHgCOg' ^ (CHgCO) 20g 5 2(CHgCOg) _ >(CHgC0)g0 2 4- 0 2 6 It i s believed that reaction (2) i s fast and that at the oxygen pressures used i n the experiments, reaction (6) i s the only important chain terminating step. Therefore, i n deriving kinetic expressions for the production of the pe-roxides in the photochemical oxldarion of acetaldehyde, only reaction (2), (3) and (6) should be considered, along with the i n i t i a t i o n step. Application of the stationary state hypothesis to thi s scheme shows that (CHgCOg.) - t<F2Iabs/k6)* 44 The rate of formation of peracetio acid, i a then: d(CH 3C0 3H)/dt - k 3(CH 3C0 3')(CH 3CH0) - K 3 ^ a b s > *< CH 3CH0) /k 6* (A) The rate of formation of d i a c e t y l peroxide i a : d((CH 3C0 ) 2 0 2)/dt - k 6 ( C H 3 C 0 3 ' ) 2 =92lahs <B> Compariaon of (A) and (B) ahowa that the r a t i o (Peracetic Acid)/(Diacetyl Peroxide) i at a given time i a proportional to the r e c i p r o c a l of labs 8* An inapection of the mean valuea of thia r a t i o for the two li g h t i n t e n s i t i e s studied i n t h i s work shows that this re-l a t i o n i s obeyed within experimental error. In fact, the values of the r a t i o are 1.054 ± 0.052 and 1.87 t 0.12 for the l i g h t i n t e n s i t i e s 100 and 34.6 respectively: (1.87 ± 0.120)/(L. 054 ± 0.052) «• 1 . 7 8 * 0 . 2 0 ; (100)/34.6)®= 1 Since peracetic acid i s the chain propagation product and d i a c e t y l peroxide the chain termination product, t h e i r r a t i o i s equal to the chain length. The values 1.054 and 1.87 for the chain length may appear s u r p r i s i n g l y low. It should be noted, however, that chain lengths as chow as 2.2 have been observed ( 4 0 ) , i n experiments with I a l 3 s equal to 45 6.21$ of the output of a B.T.H. lamp at 3130 A. Since the AH6 lamp used i n the present work has a greater output, and 10% to 30$ of i t was absorbed by the reaction mixture, i t i s clear that the chain length would he shorter. It has been stated e a r l i e r (Introduction) that the r a d i -c a l - r a d i c a l termination step (reaction 6) can take place through two possible mechanisms: (a) when the two peraeetyl rad i c a l s interact, the oxygen molecule formed i s produced by the elimination of an oxygen atom from each r a d i c a l ; (b()) the oxygen molecule comes exclusively from one of the interac t i n g r a d i c a l s by a r a d i c a l displacement reaction which was previously suggested (18): CH3-C CH ,-0r 3 : v0-0 3 N 0 P I 4- 0 2 2CH3C02- » 0 . 0 CH 2-C( CH 3-0=0 N 0 When the oxygen used i n the photooxidation of acetal-dehyde i s a mixture of 0 1 6O l s and O 1 8 ©- 1 - 8 , a fr a c t i o n of the peraeetyl r a d i c a l s w i l l be unlabelled and another f r a c t i o n l a b e l l e d with oxygen-18. When a la b e l l e d and an unlabelled r a d i c a l interact according to reaction (6), Mechanism (6a) would allow for an oxygen molecule of mass 34 to be formed, while mechanism (6b) would allow only for expulsion of an oxygen molecule of ma&s 32 or 36, according to the following scheme; 46 A o 0 1 6 0 1 8 + 0 1 6 0 1 8 ? AcO^O1.6- • • >0 1 80 1 8Ac | Mechanism 6a A c 0 1 6 0 1 6 + A c 0 1 8 0 1 8 | Mechanism 6b AcO^O^.-.AcO^O 1 8 or AcO 1 8- Ji.e0 1 60 1 6 1 Ac0 1 60 1 6Ac + 0 1 8 0 1 8 or A c 0 1 8 0 1 8 4- 0 1 6 0 l S Therefore, any appearance of oxygen-34 i n the reaction mixture would be an evidence i n favour of mechanism (6a). This reasoning assumes that no contamination of a mixture or oxygen-32 and -36 with oxygen-34 takes place automatically or i n presence of decomposing peroxides. Traylor and B a r t l e t t (41) have shown that no such e q u i l i b r a t i o n takes place: (ifl) on long standing, ( i i ) during the photooxidation of rubrene, ( i i i ) from thermal decomposition of rubrene pero-xide, or (iv) during the decomposition of unlabelled t-but y l -hydroperoxide. From tables I and II, i t i s seen that oxygen-34 i s produced during the reaction. Its appearance i s an evidence that reaction (6) takes place by the mechanism (6a). The detailedd kinetic analysis of the r e s u l t s given below, i s i n support of t h i s view. According to the mechanism given by equations 2, 3 and 6f? 47 for- each mole of peracetic acid produced, the equivalent of My moles of oxygen-34 are consumed. My i s the mole f r a c t i o n of oxygen-24. For each mole of d i a c e t y l peroxide produced via mechanism (6a), EMy moles of oxygen-34 are consumed and fy moles of oxygen-34 are generated. "f y i s the p r o b a b i l i t y that a terminating c a l l i s i o n w i l l be between a la b e l l e d and an unlabelled peraeetyl r a d i c a l . It can be written y r y(P,D) (7) where y i s the t o t a l amount, i n moles, of oxygen-34; P i s the amount of peracetic: acid; and D i s the amount of d i a c e t y l peroxide produced i n the reaction. D i f f e r e n t i a t i o n of equation (7) yi e l d s 4y= (Qy/Qp)DdP (Qy/9D)pdD (8) But (Qy/^P)D - -My and (Qy/%>)p - -2My f f and substitution of these i n equation (8) leads to: dy - -MydP + ( f y - 2My)dD (9) Under the experimental conditions used, P/D i s equal to the chain length, C, which i s constant; therefore i t can be written dP — GdD. Substitution i n equation (9$ y i e l d s : dy = jfy - (o f 2)My]dD (10) My i s , however, equal to y/W, where W i s the t o t a l oxygen 48 concentration.. This concentration i s equal to the i n i t i a l oxygen concentration, WQ, minus the oxygen spent. Since one mole of oxygen is spent per uaole of peroxide or peraeid formed, W - \V0 - (P 4- D) and the following equation, i n which the substitution P CD has been made, can be derived for M ^ Z ( v n v W f t - (C + 1)D Substitution of thi s expression for My into equation (10) yield s the following d i f f e r e n t i a l equation i n y and D. dy + ( ( 6 g ) y * f U D - o (IS) lW0 - (C ¥• 1)D y J This can be r e a d i l y integrated to the following expression, i n which the substitutution M =. y/W has been made. f y ' M y r fy ^ V o j ( w/ wo> d 3> The symbols with subscript 0 denote i n i t i a l concentrations. On taking logarithms, this equation becomes: log ( f y - My) c log [ f y - (My)0] + [l/(c +3)]log(W/W0) (14) A similar expression can be derived r e l a t i n g Mz, the mole f r a c t i o n of oxygen-36, to chain length C: log(M z - f z ) = log j(M 2) 0- f z ] t [l/fc + ])|log(W/Wo) ( 1 5 ) Here f z i s the p r o b a b i l i t y that a terminating c o l l i s i o n w i l l 49 take place between two labelled peracetic radicals. The probabilities f and f z can be related to the atom-fraction of 0 1 8 in the total oxygen. If this at.om-fraction is o(, i t is clear that the mole-fraction of labelled pera-cetyl radicals w i l l also be 0( .while the mole fraction of unlabelled radicals w i l l be 1 - o( . Then the probability that two labelled radicals interact is * z = -of and the probabilij?y that one labelled and one unlabelled radical interact is f y = 2o<( 1 - & ). The value of <x calculated from the composition of the oxygen-argon mixture, taking into account the natural abundance $0,204$) of 0 1 8 in ordinary oxygen, is 0.0555. This value is verified by the experimental data, as i t can be seen in the sixth column of Tables I and II. The validity of the mechanism, postulated for the photo-chemical oxidation of acetaldehyde in general, and for the termination reaction in particular, can be tested now by applying equations (14) and (15) to the experimental results. The values for the various parameters in these equations are outlined in Tables III and IV, as calculated from the expe-rimental data given in Tables I and II* 50 TABLE III Variation of Oxygen Isotopes" During Oxidation of Acetaldehyde Intensity of illumination 100 units ( a r b i t r a r y ) . Time (min.) My x 10 2 w/w 0 M z x 10 2 M 2-f z 0 0.533 0.0995 1.000 5.29 0.0498 90 1.10 0.0938 0.904 5.05 0.0474 180 1.97 0.0851 0.730 4.54 0.0423 240 2.79 0.0769 0.610 4.12 0.0381 300 2.93 0.0755 0.602 4.09 0.0378 360 3.26 0.0722 0.543 4.03 0.0372 TABLE IV Variat i o n of Oxygen Isotopes During Oxidation of Acetaldehyde Intensity of ill u m i n a t i o n 34.6 units (arbitrary) Time (min.) My x 10 2 f y - My W/WQ M 2 x 10 2 M g - f 2 0 0.533 0.0995 1.000 5.29 0.0498 60 0.686 0.0979 0.975 5.26 0.0495 120 0.852 0.0963 0.938 5.14 0.0483 180 0.957 0.0952 0.907 5.01 0.0470 300 1.28 0.0920 0.817 4.91 0.0460 51 520 1.6.9 0 . 0 8 7 9 0 . 7 2 9 4 . 7 9 0 . 0 4 4 8 600 2 . 2 6 0 . 0 8 2 2 0 . 5 9 9 4 . 4 7 0 . 0 4 1 6 Figures 7 and. 8 show that both equations (14) and (15) are s a t i s f i e d "by the experimental r e s u l t s . Moreover, the chain lengths evaluated from these figures are i n agreement with those calculated from the r a t i o of the peroxides, as i t can he seen from Table V, where a concise account of the re s u l t s i s given. TABLE V Kinetic Analysis of Data i n Tables I and II « = 0 . 0 5 5 5 f = 0 . 1 0 4 8 f r 0 . 0 0 3 1 y C 1 0 0 f r o m aquation 1 4 figure 7 0.9.93 C]_oo from equation 15 figure 7 0 . 9 3 5 C 1 0 0 from P/D 1 . 0 5 4 + 0 . 0 5 2 C 3 4 6 f r o m s t a t i o n 14 figure 8 1 . 8 5 C34.6 from equation 15 figure 8 1 . 7 0 C 3 4 . 6 from P/D 1 . 8 7 + 0 . 1 2 FIGURE 7 VARIATION OF OXYGEN ISOTOPES fh 'IABS = loo. FIGURE 8 VARIATION OF OXYGEN ISOTOPES AT L=34.6 54 The aelf-consistenee of r e s u l t s obtained by using two d i f f e r e n t a n a l y t i c a l methods leaves no doubt about the correct-ness of the meciianism put forward. Since i t has been shown conclusively that the recomb-ination of peroxyacetic r a d i c a l s takes place according to equation (6a) with the elimination of one oxygen atom from each reacting peroxyacetyi r a d i c a l , i t remains, now, to suggest a detailed machanism for t h i s process. A s a t i s f a c t o r y reaction scheme i s that shown below, which involves a tran-s i t i o n state, where a planar four-centered oxygen atomic system i s the site of ohemical r e a c t i v i t y . O 1 6 0 1 8 O* 1 6- - o 1 8 CH 3-C - 0 l 6f 018-C-CH,, * 6 3 0 6 0 C H g - g - O ^ O 1 8 -C-CH 3 + o 1 6 o 1 8 5 5 Traylor and B a r t l e t t (41) used i s o t o p i c a l l y enriched oxygen to elucidate the mechanism of the chain termination step i n the autooxidation of oumene i n the l i q u i d phase and found that i n th i s case too the oxygen molecule i s formed by oxygen atoms coming from two d i f f e r e n t peroxy-radicals. It seems, therefore, that this mode of r a d i c a l - r a d i c a l termi-nation i s quite general i n reactions involving peroxy r a d i c a l . CHAPTER IV MECHANISM OF THE PHOTOLYSIS OF GASEOUS CROTONALDEHYDE AT 30°C AND 3450-4000 §. 56 IT. MECHAN ISM OF THE PHOTOLYSIS OF GASEOUS CROTONALDEHYDE at 30°C and 2450-4000 A 0 . 1. APPARATUS The apparatus used i n the photoohemieal oxidation of acetaldehyde was used i n the photolysis of crotonaldehyde too, with the following a l t e r a t i o n s . (i) The AH6 high pressure mercury arc was substituted "by a BTH lamp. The output of t h i s lamp (250 Watts) was smaller than that of the one previously used, but i t had the ad-vantage that i t gave a constant il l u m i n a t i o n for extended periods of time. ( i i ) The li g h t f i l t e r solutions were removed from the o p t i c a l system and a quartz f i l t e r permitting passage of l i g h t from 2450-4000 A 0 was used instead. ( i i i ) It was found that the l i g h t f a l l i n g on the photo-c e l l under these conditions was too strong to be measured by the photometer unit. F i r s t i t was attempted to extend the range of the unit by adding two more r e s i s t o r s to the r e s i s t o r chain &I„A> thus creating two more positions for the selector switch S^. It was observed, however, that the photocell did not respond l i n e a r l y to these high inten-s i t i e s , so i t was sought to reduce the l i g h t f a l l i n g on i t by placing neutral f i l t e r s i n front of i t . This technique gave better r e s u l t s , ana i n this way i t was possible to measure the i n t e n s i t y of the illumination throughout t h i s 57 part of the work. (iv) The thermoconductivity detector of the gas chroma atography apparatus was supplemented hy a Perkin-Elmer flame io n i z a t i o n detector. Using t h i s detector, the s e n s i t i v i t y of the apparatus increased hy almost 1000 times and i t was possible to detect and measure products i n trace quantities. Another advantage of the new detector was that i t could be operated at various s e n s i t i v i t i e s during the same analysis. The greatest s e n s i t i v i t y used was one quarter of the max^ imum s e n s i t i v i t y of the apparatus. Use of the maximum sen-s i t i v i t y was impractical, because trace impurities i n the system were i n t e r f e r i n g seriously with the analysis. There-fore the effective s e n s i t i v i t y of the apparatus was about E50 times greater than when the thermoconductivity detector was used. This l a t t e r detector was kept on the system and i t was used for analysis of inorganic gases, for which the flame i o n i z a t i o n detector does not give a signal. 2. EXPERIMENTAL Determination of the Absorption Curve for Orotonaldehyde  at a Wavelength 2450-4000 A°. The percentage absorption of gaseous orotonaldehyde was measured at 30°C for pressures ranging from 3 to 30 mm Hg i n the reaction vessel. The r e s u l t s are plotted i n Figure 9. This plot was checked at inte r v a l s during the work and found to be v a l i d . 58 FIQURE 9, ABSORPTION CURVE foa 0w)TONftLDEwyDE 59 The procedure used was the following. The lamp was switched on and allowed to warm for about 40 minutes i n order to s e t t l e down. When the v a r i a t i o n i n i n t e n s i t y was less than 0.5$ a measurement of the in t e n s i t y was taken with the reaction vessel completely evacuated. Then 30 mm Hg of crotonaldehyde were l e t into the vessel and the i n -tensity measured. Crotonaldehyde was pumped out to the next lower pressure required and again the in t e n s i t y was measured. This process was repeated u n t i l the c e l l was evacuated. The in t e n s i t y was measured i n order to ensure that i t was the same as at the beginning. The in t e n s i t y used throughout the experiment was s u f f i c i e n t l y low that no appreciable photo-l y s i s of crotonaldehyde occured. Determination of Expected Reaction Products. No products of photolysis of crotonaldehyde at low temperatures had been detected by e a r l i e r workers (23), therefore i t wss d i f f i c u l t to decide what to measure. Since propylene and CO had been detected i n the Hg-photosensitized decomposition (29) and they were also products of the ther-mal decomposition (24,25) i t was decided to investigate their formation during the photolysis. Using the flame ion-i z a t i o n detector i n the chromatography system i t was poss-i b l e to detect propylene i n amounts as low as 5 ,10"" L 0 moles -9 and measure i t with accuracy when 5*10 moles were present. For the estimation of CO, the thermoconductivity detector 60 had to he used, which required amounts of the order of 10 moles to give accurate r e s u l t s . But-2-ene, methane and hydrogen had "been found to be products of the h'igh temperature photolysis of crotonal&ahyde (25,28), therefore they had to be considered as possible products of the reaction, along with propylene and CO. It was found that two chromatography columns were needed to separate a l l these possible products. One was a 10 foot long HHP& column, (for Hexa-Methyl-Phospor-Amide), the other a 9 foot long molecular sieves column. The f i r s t column, when operated at 0°C and 8 p s i , could separate e f f i c i e n t l y COg and a l l the hydrocarbons from Cg to Gg. Even the c i s and trans but-2-enes could be separated from each other. The second column could separate Hg , 0 2 , Ng , CO, CH4 and C2H5 at 80°C and 1.5 p s i . The signal for Hg., however, was very weak when helium was the c a r r i e r gas, because the thermo-conductivities of the two gases are very close. For that reason Ng was used as c a r r i e r gas whenever Hg detection was attempted. None of these columns permitted orotonaldehyde, or any other polar compound, to pass. Therefore i t was necessary to f i n d another column which would be able to separate the f i r s t material from any polar products. A '5 foot long dinonyl phthalate column, operated at 45°C and 8 p s i , was found to r e t a i n orotonaldehyde for 15 minutes and separate i t e f f i c -61 i e n t l y from various other polar compounds of similar or smaller molecular weight. Also, i t could separate higher hydrocarbons i f they d i f f e r e d s u f f i c i e n t l y i n b o i l i n g point. Unfortunately, glyoxal, CHOCHO, which could be a possible product of the photolysis, f a i l e d to give a peak. I d e n t i f i c a t i o n of Reaction Products Before describing the experimental procedure followed when i t was already known what compounds to determine, i t is thought necessary to describe how these compounds were established as products of the reaction. The necessity"of th i s way of exposure arises from the fact that some of these products had never been observed or postulated before and therefore they were quite unexpected. The following preliminary experiments were performed, a l l of them with 20 mm Hg crotonaldehyde, at 30°C, and using the f u l l i n t e n s i t y of illumination that the B.T.H. lamp could give at 2450-4000 A 0. Crotonaldehyde was photolyzed for 30 minutes. The conden-sable portion of the reaction mixture was frozen with l i q u i d nitrogen into the gas chromatography sampler and the gases toeplered into the gas burette. The condensable mat-e r i a l was analyzed using the HMPA oolumn at 0°C and 8 psi and found to contain propylene. No other hydrocarbons were found up to Cg. The gases, 1 ml under pressure of 5-6 mm Hg, were analyzed by the molecular sieves column at 80°C and 1.5 62 p s i . Using the thermoconductivity detector, only CO was found. In a similar experiment the gases were analyzed with the same column as before, hut with the flame i o n i z a t i o n detector i n operation. Using one quarter of the maximum s e n s i t i v i t y of the apparatus, no methane or ethane was det-ected. In order to test for hydrogen i n the reaction pro-ducts, helium was substituted by nitrogen as a c a r r i e r gas. After making sure that no helium remained i n the system, the gases from a 30 minute photolysis were analyzed using the molecular sieves column and the thermoconductivity detector. The r e s u l t was negative, even when the analysis was repeated on gases froma a 1 hour photolysis. It was concluded that the only detectable products i n t the low temperature photolysis of orotonaldehyde from the domain of lower hydrocarbons and li g h t gases were propylene and CO. This conclusion was supplemented l a t e r , when the absence of COg from the reaction products was confirmed by using the HMPAoolumn and the thermoconductivity detector. In order to test for ether products, the condensable material from a 30 minute photolysis was passed through the dinonyl phthalate column at 45°C and 8 p s i . Using the flame io n i z a t i o n detector, three peaks appeared on the chromato-r gram, apart from the peak of orotonaldehyde, at 15 minutes: a large peak at 7 minutes and two smaller ones at 4.5 and 1.5 minutes. The peak at 1.5 minutes was assumed to belong to propylene, because th i s was the only lower hydrocarbon 63 detected e a r l i e r , and only lower hydrocarbons came so fast out of the dinonyl phthalate column. It was more d i f f i c u l t to i d e n t i f y the peaks at 4.5 and 7 min, because quite a few compounds were found to have these retention times. In order to make separation more e f f i c i e n t , the operating temperature of the column was lowered to 0°C. Under these conditions the two unknown products gave peaks at 16 and 39 min. Tests with various compounds revealed that 16 min was the retention time for hydrocarbons with s i x carbon atoms, while the only e a s i l y available substance with retention time 39 min was found to be butyraldehyde. It was decided to use the HMPA column i n order to gather some more evidence about the i d e n t i t y of the unknown at 16 min. It was found that t h i s column could be extended to sepa-rate Cg hydrocarbons i f operated at 20°C. At th i s tempera-ture i t was possible to observe the peaks of two hydrocarbons i n the chromatogram of the condensible products, one belonging to propylene. The new peak appeared at 59 min, and i t was proved to belong to the same product which appeared at 16 min i n the dinonyl phthalate column by trapping the product after the column and checking the retention time i n the other column. 1,5-hexadiene was found to have retention time 59 min i n the HMPA column at 20°Cj i t was decided that the unknown product i s this hydrocarbon, a f t e r the following a d d i t i o n a l evidence was gathered: (i) The p o s s i b i l i t y that the unknown was a saturated hydro-carbon was excluded because i t s smell, tested at the ex i t of 64 the chromatography apparatus, was the c h a r a c t e r i s t i c smell of an unsaturated hydrocarbon. ( i i ) Olefins with 6 carbon atoms were tested and found to have retention times shorter than 59 minutes. ( i i i ) Of the three dienes with the same number of carbon atoms (not considering dienes of the allene type), only 1,5-hexadiene was available, and i t was assumed to be the unknown product. 2,4-hexadiene was excluded as a p o s s i b i l i t y after anaamount of 1,5-hexadiene was passed over allumina at 350°C and the products subjected to gas chromatographic analysis: two peaks appeared i n the chromatogram when the dinonyl phthalate column was used at 0°C, one at 16 minutes, the other at 32 minutes. It i s known (42,43) that when 1,5-hexadiene i s subjected to the treatment described above, i t isomerizes to 2,4-hexadiene. It was therefore assumed that the peak at 32°C belonged to this conjugated diene. This assumption was further j u s t i f i e d by. the fact that when the subrange* was trapped at the e x i t of the chromatography system and the u l t r a v i o l e t spectrum taken, i t showed a very strong absorption at 2300 A, i n agreement with the spectrum of 2,4-hexadiene (44). A f i n a l proof that the un-known product of the photolysis of crotonaldehyde i s not 2,4-heaadiene was provided when a photolysis was run over-night and the unknown co l l e c t e d i n s u f f i c i e n t amount to give an u l t r a - v i o l e t spectrum: i t showed a very weak absorption at 2100 A, a fact which excludes the p o s s i b i l i t y of a 65 conjugated diene but not of 1,5-hexadiene (45). (iv) The p o s s i b i l i t y of the unknown being 1,4-hexadiene was excluded on t h e o r e t i c a l grounds, explained i n the last sec-t i o n (Discussion) of the present chapter. So was the possi-b i l i t y of dienes of the allene type. The unknown appearing at 39 min. i n the dinonyl phtha-late column at 0°C, was thought for a short time to be butyraldehyde, because t h i s aldehyde had the same retention time. In order to gather more,information, orotonaldehyde was photolyzed for two hours and the unknown at 39 min. trapped at the exit of the chromatography apparatus and subjected to i n f r a red spectroscopic analysis, The spectrum did not quite agree with that of butyraldehyde, although i t showed a strong absorption at 1735 cm"-*-, the same wavenumber where the carbonyl of butyraldehyde absorbs (Figure 10) It was concluded that the unknown was an aldehyde, but not bu-tyraldehyde. By the same time i t b/Sdrbeen observed, that i f orotonaldehyde was illuminated for a short period of time, the only detectable product was the unknown aldehyde appea-ri n g at 39 min. Propylene and b i a l l y l were produced i n mea-surable amounts i n reaction times longer than 3 min and CO, due to the i n f e r i o r s e n s i t i v i t y of the thermoconductivity detector, in times longer that 15 min. These facts showed that the unknown aldehyde must derive from erotonal&ehyde through a process not involving any fragmentation. Such a process could be only an isomerization reaction. 67 This assumption was further substantiated from the observation that the unlmown aldehyde gave, among other pro-ducts, orotonaldehyde, when illuminated or on long standing at room temperature. There are three conceivable ways i n which orotonalde-hyde can isomerize to another aldehyde: (i) By changing geometric configuration i . e . by under-going a eis-trans isomerization. ( i i ) By s h i f t i n g the double bond to pos i t i o n 3 ( i i i ) By rearranging to 1-methyl-acrolein. Ordinary orotonaldehyde has the trans-configuration ( 4 6 ) . The c i s form has never been isol a t e d and a l l attempts to prepare i t resulted i n ordinary orotonaldehyde (46, 47). This fact made geometric Isomerization an implausible assum-ption, because the unknown aldehyde was a reasonably stable compound. Also, i f i t had a conjugated system of double bonds, the carbonyl absorption i n the infrared spectrum would be displaced by 20-40 cm"-1-, as the case i s with the spectrum of (trans)-orotonaldehyde. This lack of displace-ment excluded the p o s s i b i l i t y for the unknown to be either cis-crotonaldehyde or 1-methyl-acrolein, leaving as the only p o s s i b i l i t y the product of doule bond s h i f t , namely 3-butene-l - a l . This aldehyde was prepared and i t s i n f r a red spectrum compared with that of the unknown aldehyde (Figure lID).The two spectra were i d e n t i c a l . It was concluded that the 68 "unknown aldehyde was 3-butene-1-a 1. Polymer Formation Another fact, which became obvious while crotonaldehyde was illuminated i n order to^ i d e n t i f y the y.arious products of photolysis, was that a substance was accumulating slowly on the walls of the reaction vessel. Evacuation of the c a l l for several hours at 30°C did not remove thi s substance. Heating to 200°C under vacuum simply caused the substance to carbonize pa r t l y , thus hindering the passage of l i g h t . Only heating with a gas-oxygen flame to white glow complete-l y cleaned the c e l l . Blacet and Roof (23) have observed that when crotonaldehyde i s illuminated at 3660iLat room tempera-ture, polymerization occurs with a quantum y i e l d of 0.02 based on the number of molecules, disappearing per quantum absorbed. Since the illumination used i n the present work comprised the wavelength of 3660 i , i t i s reasonable to assume that the substance accumulated on the walls of the reaction vessel was a polymer. P&lymer formation was creating some problems of expe-rimental nature i n an eventual kinetic study of the photo-l y s i s of crotonaldehyde. Gleaning the c e l l completely before a reaction would require dismantling the furnace, cutting the c e l l , biking i t to white glow and then assembling the whole system back i n place. It was thought that a l l t h i s process could be avoided i f the polymer layer on the walls of the reaction vessel could come to a "stationary state"; 69 i f , i n other words, the c e l l could a t t a i n the condition which i s often referred to as "seasoning". Polymer accumu-l a t i o n could he followed hy recording the i n t e n s i t y of the l i g h t f a l l i n g on the photocell when the reaction vessel was evacuated. It was found that when the c e l l was quite clean ( i . e . subjected to flaming), the illumination passing through i t was appreciably decreasing after each reaction, even when the c e l l was kept uder vacuum for several hours. This decrease of the i n t e n s i t y of passing l i g h t was taken as a measure of polymer accumulation. When, however, a c e r t a i n number of photolyses had been conducted, the c e l l could be e a s i l y made to a t t a i n the same condition as before the experiment by evacuating, i t for one hour, as judged by the fact that the i n t e n s i t y of illumination reaching the photocell was the same as before the experiment. The "seasoned" c e l l was absor-bing about 60$ of the i l l u m i n a t i o n f a l l i n g upon i t . The r e s u l t s recorded i n the next section were obtained with the c e l l t i n this condition. No detailed kinetic study of the polymer formation was undertaken* Estimation of i t s extent was attempted by mea-suring the amount of orotonaldehyde and i t s photolysis pro-ducts i n the reaction mixture and a t t r i b u t i n g the d e f i c i t of the material balance to polymer formation, but the r e s u l t s were not reproducible, due to the large experimental error inherent i n the method. 70 Determination of Reaction Products It has "been seen i n a previous section how 3-butene-l-al was establfeshed as the primary product of the photolysis of crotonaldehyde at S0°C, accompanied later by propylene, 1,5-hexadiene and CO. It was decided to follow the rate of formation of a l l four products; also, the rate of disappea-rance of crotonaldehyde, for experiments i n which s u f f i c i e n t amount of i t would be spent. This could be achieved rather e a s i l y , because the reaction mixture could be separated into two portions; one non-condensable at l i q u i d nitrogen temperature and containing CO only, the other condensable and containing enerything else. CO was determined gas chromatograghically using the molecular sieves column. Measuring the volume of the gases i n the gas burette 1 was used as an additi o n a l check. The remaining compounds were determined by using the dinonyl phthalate columnaat 45°C. It w i l l be r e c a l l e d that under these conditions the retention times of propylene, 1,5-hexadiene, 3-butene-1-al and crotonaldehyde are 1.5, 4.5 , 7 , and 15 minutes i n the order given. The use of t h i s compact method of analysis was possible only i f i t was made sure that theses were the only substances i n the mixture. This was ensured by making addit i o n a l checks of the i d e n t i t y of the products every time that some a l t e r a t i o n was made i n the conditions of photolysis. The two columns were calibrated q u a n t i t a t i v e l y by using measured samples of pure compounds. It was found 7 1 that the areas of the peaks are proportional to the amount of the substance injected into the apparatus. This l i n e a r i t y was checked for amounts up to 10"^ mole. The r e s u l t s are outlined i n Table VI." TABLE VI Cal i b r a t i o n of the chromatography system for the components of the reaction mixture i n the photo-l y s i s of orotonaldehyde. The areas of the peaks are calculated for the maximum s e n s i t i v i t y of the system. Compound moles/(area unit) crotonaldehyde 3-butene-1-al propylene 1,5-hexadiene CO Actinometry The photometer unit was calibrated by using the potassium fe r r i o x a l a t e actinometer. The procedure followed was essen-t i a l l y the one described by Hatohard and Parker (39). 1. C a l i b r a t i o n graph for ferrous ion. The following reagents were f i r s t prepared. 1.67 x 10" •10 1.3S x 10 -10 1.68 x 10 6.40 x 10 -10 -11 1.32 x 10 -7 72 (a ) A solution containing 1.0 x 10" b mole/ml of Fe + + i n 6.1 N H 2S6 4. This solution was freshly prepared hy d i l u t i o n of standardized 0.1 FeS6 4 i n 0.1 N HgS0 4. (h$ 0.1$ aqueous solution of 1:10 phenanthroline. (o) A buffer solution prepared hy adding 360 ml of I N H SO, to 600 ml of 1 N sodium acetate and d i l u t i n g to 1 l i t r e . In a series of 50 ml volumetric flasks the following volumes of solution (a) were added: 0 , 100,, 2.0 , 3.0 , 4.0 , 5.0 ml Then 0.1 N HgSO^ . was added to make the t o t a l volume 10 ml. After adding 5 ml of solution (b) and 5 ml of solution (c), the volume was made up to the mark with d i s t i l l e d water and the flasks were allowed to stand for % hour. The o p t i c a l density r e l a t i v e to that of d i s t i l l e d water was measured i n a 1 cm c e l l at 510 m^ . The r e s u l t s appear i n Table VII and a graph i n Figure 11. TABLE VII Optical Density of Ferrous Idn Solutions. Moles Fe + f x 106/50 ml Optical Density 0 0.001 1.0 0.231 2.0 0.422 3.0 0.644 4.0 0.906 5.0 1.090 73 FIGURE 11. CALIBRATION GRAPH FOR FERROUS IONS 74 From the graph i t i s seen that the o p t i c a l density varies l i n e a r l y with the concentration of the ferrous ion; to o p t i c a l density "1" there corresponds a concentration of 4.5 x 10 moles Fe per 50 ml. 2. Cal i b r a t i o n of the photometer unit. The lamp was switched on and allowed to warm for about 40 minutes. When the lamp output became constant, a reading was taken with the "seasoned" reaction c e l l evacuated. Then the c e l l was cut and taken out of the l i g h t path and another reading taken. In this way i t could be calculated what per-centage of illumination was absorbed by the vessel: i t was 61.5$ . The vessel was put back on the l i g h t beam and a 1 cm quartz c e l l containing approximately 10 ml of a 0.006 M solution of potassium f e r r i o x a l a t e i n O . L N H g S C ^ was placed behind i t and illuminated for a measured time. After photo-l y s i s , the contents of the c e l l were transferred quantitat-i v e l y into a dark-coloured 50 ml volumetric fl a s k , 5 ml of solution (b) and 5 ml of solution (e) added and the volume was made up to the mark with d i s t i l l e d water. After allowing the f l a s k to stand for •§- hour i n the dark, the o p t i c a l density of the photolyte was measured r e l a t i v e to that of d i s t i l l e d , water i n a 1 cm d e l l at 510 m|*. Three measurements were made which appear i n Table V I I I . A solution of potassium fe r r i o x a l a t e , which was not illuminated,showed, under the same conditions, n e g l i g i b l e o p t i c a l density, thjis confirming the purity of the reagents. 75 TABLE T i l l The i n t e n s i t i e s of il l u m i n a t i o n were measured with the "seasoned" reaction vessel on the l i g h t path. Intensity of Relative F e f f Fe f +/sec x f l i l l u m i n a t i o n Time o p t i c a l moles•sec" 1 on the photodell (sec.) density moles x l O 6 CI'1 x l O 1 1 1410 30 0.472 2.12 5.01 1410 50 0.767 3.45 4.89 600 60 0.387 1.74 4.84 Mean value: 4.9110.10 The quantum e f f i c i e n c y for the process at 3000-4000 A 0 given by Hatchard and Parker (39) i s 1.23. The se l e c t i o n f i l t e r i n the work described here permitted passage of li g h t from 2450-4000 A 0. But the BTH lamp i s known to produce a very small percentage of i t s output below 3000 A 0, therefore the above factor can be used with a f a i r degree of approx-imation. The number of quanta per second per ohm reaching the 13 photocell are thus calculated to be 2.40 '(£0.05) x 10 In order to calculate the number of quanta passing through the reaction vessel per second per ohm, i t i s necessary to take into account the fact that the walls of the vessel were covered with a layer of polymer which absorbed 61.5$ of the t o t a l illumination. Assuming homogeneous d i s t r i b u t i o n of the polymer on the walls, i t can be calculated that each 76 o p t i c a l surface of the vessel was absorbing 38$ of the i l l -umination reaching i t . Therefore the number of quanta reaching the photocell was only 62$ of the number of quanta passing the space between the two o p t i c a l surfaces of the reaction vessel. Therefore, quanta per second per ohm i n the vessel - 2.40(±0.05) x 1 0 l g - 3.87( t O .08) x 1 0 1 2 This number of quanta i s absorbed per second and ohm, by any gas i n the reaction vessel, which has an illuminated volume of 73 ml. For convenience, a l l absorbed i n t e n s i t i e s , I a D a » quoted i n this chapter, w i l l have the units einstein* V^-» sec""-*-. 3. RESULTS Investigation of the Isomerization Reaction. It was stated e a r l i e r that the only detectable product of the photolysis of orotonaldehyde i n experiments of small duration was 3-butene-l-al. This fact showed that the prim-ary process taking place when orotonaldehyde was illuminated at 30°C and 245O-4Q00 A 0 was isorcerization. A detailed study of t h i s process was undertaken by measuring the rate of formation of the isomer under varying i n t e n s i t y of il l u m i n -ation, orotonaldehyde pressure and temperature. The re s u l t s are recorded i n Tables IX, X and XI. 77 TABLE IX Dependence of Isomerizat&on of Crotonaldehyde on I a l 3 s A l l experiments were conducted at 30°C with EO mm Hg of crotonaldehyde. Total *aba • e i n * ! - 1 ' s e c " 1 time sec. 3-hut§nerils.al-: f o r m e d o r r r a d m o l e ? 1*1 • 10 7 - ] absorbed radiatio n :.§£n«l-l.l06 Quantum y i e l d mole-ein" 1*10 2 31.1 50 10.1 15.5 9.8 31.1 80 15.7 S4.9 9.1 S1.7 60 8.6 13.0 9.5 El.7 ISO 17.0 S6.0 9.0 19. S 60 7.5 11.5 9.3 19.E ISO 14.5 S3.0 9.8 IS.4 100 8.4 IS.4 9.4 IS.4 S40 19.3 S9.8 9.9 5.0 S40 7.9 1S.0 9.3 3.4 300 6.4 10. S 9.7 me an value: 9.5 ±. 0.5 From the last column of Table IX i t i s seen that the quantum yie&d of isomerization remains constant over a ten-fold increase of i n t e n s i t y of the absorbed il l u m i n a t i o n . This means that isomerization ia d i r e c t l y proportional to 1 ^ 3 -In the experiments with variable crotonaldehyde pressure (Table X) the i n i t i a l i n t e n s i t y of il l u m i n a t i o n was kept constant. The absorbed i n t e n s i t y , however, waa varying, 78 because of di f f e r e n t percentage of absorption by the d i f f e r e n t aldehyde pressures studied. The absorbed i n t e n s i t y was taken by using the absorption curve for orotonaldehyde (Figure 10). TABLE X Dependence of Isomerization on the pressure of Orotonaldehyde A l l experiments were conducted at 30°C with I j _ n 66.6 #10~ 8 ein*l"i»sec-1 and for 1 minute. Total Quantum Croton- •'•aba 3-butene-l-al absorbed y i e l d aldehyde eta* 1-1.sec" 1 formed r a d i a t i o n mole»ein~ mm Hg x 1Q8 mole* 1-1* 10 7 ein.1-1'10 6 x 10 2 5.1 6.0 3.28 3.60 9.1 8.E 9,6 5.40 5.76 9.4 16.0 11.6 6.49 6.96 9.3 15.6 17.4 9.89 10.4 9.5 30.0 21.7 12.8 13.0 9.5 34.0 25.3 15.1 15.2 9.6 26.8 27.6 16.5 16.5 10.0 30.0 30.0 16.9 18.0 9.4 mean value : 9.5+0.5 The Constance of the quantum y i e l d over a s i x - f o l d v a r i a t i o n of aldehyde pressure i n the la s t column of Table X demon-strates the fact that isomerization i s indepenent i of the concentration of orotonaldehyde. 79 In or&elr to study the tt&mperature dependence, 10 mm Hg of crotonaldehyde were photolyzed at temperatures ranging from 20° to 35°C. Higher temperatures were avoided, because 3-butene-1-al underwent a thermal isomerization to crotonal-dehyde. F^ -om the data on Table SI i t i s clear that the rate of formation of 3-birfcene-l-al i s independent of temperature. TABIB SI Dependence of Isomerlzation on Temperature. A l l experiments were conducted with I at, ar21.7 ein»l~l«sec~l and 10 mm Hg of crotonaldehyde. Time 1 minute. Temperature 3-butene-1-al °G mole.l" 1 10 7 20.0 6.81 23.1 6.53 27.8 6.39 30.0 6.49 32.3 6.60 35.0 6.28 From an inspection of the three Tables i t becomes evident that isomerization of crotonaldehyde to 3-butene-1-al i s a purely photochemioal reaction. The quantum y i e l d of isomeri-zatlon i s 0.095 10.005. 80 Investigation of the T o t a l Reaction. Dependence on Time. When orotonaldehyde was illuminated for a longer period of time, i t was possible to determine the rate of formation of the three other produots, i . e . propylene, 1,5-hexadiene and GO. The two hydrocarbons could be determined when about 10" 4 e i n . l " 1 of l i g h t energy had been absorbed, while CO became measurable only after 5 x 10"* e i n . l ' i . From preliminary experiments i t became obvious that while the formation of 3-butene-l-al varies l i n e a r l y with time, at least for experiments of short duration, that of propylene, 1,5-hexadiene and CO seems to bear some exponen-t i a l r e l a t i o n with time. A detailed investigation of the time dependence of a l l the products was undertaken. The r e s u l t s appear on Table X I I and a graph i n Figure IE. From an inspection of both Table XII and Figure IE, i t i s evident that 3-butene-l-al increases, at the beginning, l i n e a r l y with time, then the rate of formation decreases and even becomes negative a f t e r the isomer reaches a maximum at about ISO minutes. This i s an evidence that 3-butene-l-al i s consumed during the reaction. The three other products c l e a r l y increase exponentially with time. The degree of dependence i s seen to be two, from a logarithmic plot i n Figure 13. It should also be noticed that propylene and CO are produced i n approximately equimolecular amounts. 81 TIME , MINUTES FIGURE. 12. PHOTOLYSIS O F CROTONALDEHYDE ; PRODUCTS V. Tint 82 l.o 2.0 L0& [ T W A E ] FIGURE 13. PHOTOLYSIS OF CROTONALDEHYDE T I M E DEPENDENCE 8 2 83 TABLE XII Analysis of Products During the Photolysis of Crotonaldehyde at 30°C and I l n=.66.6 x 10" 8 ein. I - 1 . sec" 1. A l l data are i n units of mole.!" 1 x 10 7. Time min. Crotonal-dehyde. 3-hutene-1-al Propy-lene 1,5-he-xadiene CO 1 10700C' 18.1 - - -3 106000 57.8 - - -5 10600 96.1 1.93 -8 10500 143 4.47 1.41 -12 10350 195 9.70 3.35 -20 10200 315 29.7 7.78 -30 10100 444 70.1 17.4 52.! 40 9800 520 119 30.4 123 50 9700 604 184 46.2 123 65 9500 687 316 75.9 352 80 9030 722 450 114 -100 8420 800 608 151 550 120 7820 758 849 186 783 136 8220 825 1066 217 1000 156 7720 789 1293 259 u s e 170 6390 645 1555 301 1510 180 6040 660 1646 297 1580 200 5460 600 1931 337 1720 84 Dependence on Intensity of Absorbed Illumination. 20 mm Hg crotonmldehyde were illuminated for 30 minutes under variable intensity of absorbed illumination. The amount of absorbed radiation waa enough for propylene and 1,5-hexa-diene to be produced in measurable amounts, but? CO production was not followed. It is probable that i t s rate of appearance was the same as that of propylene. The results appear in Table XIII. TABLE XIII Dependence of Products on Intensity of Absorbed Illumination. A l l experiments were conducted at 30°C with SO mm Hg orotonal-dehyde and for 30 minutes. Data are in units of m o l e . 1 0 7 . Iabs , eth, 1-1. sec x 10 8 3-butene-l-al Propylene 1,5-hexadiene 4.47 84 1.-21 -6.E8 ISO 3.00 1.01 10.9 181 4.80 1.32 10.9 175 4.86 1.35 17.1 288 25.6 6.77 El. 8 373 34.6 8.96 31.1 444 70.1 17.4 From an inspection of the data in Table XIII, i t is clear that propylene and 1,5-hexadiene increase exponentially with 85 LOC*.olabs. FIGURE l*f. PHOTOLYSIS OF CROTONALDEHYDE ; DEPENDENCE O N 1 ^ 86 the i n t e n s i t y of absorbed illumination. A logarithmic plot i n 2 Figure 14 shows that they increase proportionally to Iab s . Dependence on Pressure of Orotonaldehyde. A set of experiments with constant amount of absorbed ra d i a t i o n and variable orotonaldehyde pressure was carried out i n order to determine the dependence of reaction products on the l a t t e r . The t o t a l amount of absorbed r a d i a t i o n was kept constant by adjusting the duration of each experiment i n such a way, that the product of time to I abs w a s always the same. This method i s quite legitimate, since i t has been established that the products depend on I f"b a and time with the same degree. TABLS XIV Dependence of Products on Pressure of Orotonaldehyde, In a l l the experiments the t o t a l absorbed radiati o n was 22.4 x 10""5 e i n . l " 1 . A l l data are i n units of mole.l" 1*!© Orotonaldehyde 3-butene-l-al Propylene 1,5-hexadiene mm Hg  5.0 1|1 64.7 12.9 7.1 153 42.7 9.78 10.0 160 32.4 7.05 12.9 176 21.9 5.37 17.1 196 15.5 4.09 20.0 195 9.70 3.35 87 FIQURE 15. PHOTOLYSIS OF CROTONALDEHYDE DEPENDENCE O N CROTONALDEHYDE PRESSURE 88 21.9 25.6 26.2 27.8 19.6 199 200 205 9.64 9.23 7.82 7.40 3.22 2.80 2.57 2.53 The data on Table XIV show that the quantum y i e l d s of propylene had 1,5-hexadiene decrease as the pressure of croto-naldehyde increases.A;*.Iogarithmic plot i n Figure 15 shows that 1,5-hexadiene formation i s Ihnrersely proportional to the cro-tonaldehyde pressure. For propylene the r e l a t i o n i s somewhat more complex; the slope of the curve i s -1.3 ( f u l l l i ne) to -1.5 (dotted l i n e ) . A possible explanation w i l l be discussed la t e r i n t h i s chapter. Recapitulation of Results. It N i s convenient to outline the r e s u l t s obtained i n the photolysis of crotonaldehyde at 30°C and 2450 -4000 £. These are the following. 1. - For experiments of short duration the only important process taking place i s isomerization to 3-butene-1-al. The experimental law was observed d(3-butene-1-al)/dt z ^ x I a t t a where 0.095 £ 0.005 i s the quantum y i e l d of isomeri-zation. 2. - When the photolysis i s allowed to proceed further, propy-lene, 1,5-hexadiene and CO are produced along with 3-butene-89 1 - a l . From the f a c t that these products i n c r e a s e with the square of time, while 3 - b u t e n e - l - a l s t a r t s d e c r e a s i n g a f t e r r e a c h i n g a maximum, i t i s evidenced t h a t they are formed through decomposition o f 3-butene- l r-al. The e x p e r i m e n t a l laws were observed: I a t a 2 - t 2 d(1,5-hexadiene)/dt< (orotonaldehyde) and d(propyleneJ/dt ~ ( o r o t o n a l d e h y d e ) 1 * 3 - 1 , 5 No d e t a i l e d law was found f o r CO, because o f d i f f i c u l t y i n determining i t , but i t was observed t h a t , whenever mea-sured, i t was found to be at approximately the same amount as propylene. 4. DISCUSSION It i s q u i t e c l e a r from the experimental r e s u l t s that the primary process t a k i n g p l a c e when orotonaldehyde i s i l l u m i n a t e d at 30°C and a wavelength o f 2450-4000 £" i s i s o -m e r i z a t i o n to 3 - b u t e n e - l - a l . The quantum y i e l d (0.095) of the process does not lead to any d e f i n i t e c o n c l u s i o n .about the a c t i v a t i o n energy of i s o m e r i z a t i o n , because o f the wide range of wavelength used . It c o u l d be that o n l y r a d i a t i o n of s h o r t e r wavelength, e.g. ^3130, i s e f f e c t i v e . T h i s view i s supported by the f a c t that photodecomposition of c r o t o n a l dehyde at 150°C increases r a p i d l y with decrease i n wavelength (25). I f t h i s i s the case, the value 0.095, found f o r the 90 quantum y i e l d o f i s o m e r i z a t i o n , i s t o o low, g i v e n t h a t not a l l t he r a d i a t i o n absorbed was e f f e c t i v e . T h e r e f o r e t h i s v a l u e can be c o n s i d e r e d o n l y as a lower l i m i t . I t i s v e r y p r o b a b l e t h a t the upper l i m i t o f the quantum y i e l d o f i s o m e r i z a t i o n cannot be g r e a t e r than one. The ex-p e r i m e n t a l data show c l e a r l y t h a t t h i s r e a c t i o n i s a prim -a r y p r o c e s s depending l i n e a r l y on the amount o f absorbed r a d i a t i o n . The p r o c e s s can be s i m p l y d e s c r i b e d by the e q u a t i o n CH3CH:CHCH0 f hv T-Iafrs ^ QH2;CHGH2CH0 ( l ) I f i t i s assumed t h a t e v e r y quantum o f r a d i a t i o n absorbed by cr o t o n a l d e h y d e i s e f f e c t i v e , the quantum y i e l d f o r ( l ) w i l l be one. The f o l l o w i n g p o s s i b i l i t i e s e x i s t f o r a d e t a i l e d mechanism o f (1). When c r o t o n a l d e h y d e absorbs one quantum o f r a d i a t i o n e nergy, i t i s r a i s e d t o some e x c i t e d s t a t e . S i n c e the r a d i a t -i o n used i s o f a wide w a v e l e n g t h range, i t i s p r o b a b l e t h a t the e x c i t e d s t a t e s w i l l be more th a n one. Under the exper-i m e n t a l c o n d i t i o n s s t u d i e d , most o f the e x c i t e d m o l e c u l e s f a l l back t o t h e i r ground s t a t e e i t h e r by e m i t t i n g f l u o r -escence or by c o l l i s i o n w i t h o t h e r c r o t o n a l d e h y d e m o l e c u l e s . U n i m o l e c u l a r d e c o m p o s i t i o n may be s a f e l y e x c l u d e d as a pess-i b i l i t y , because no low m o l e c u l a r fragments were d e t e c t e d i n the f i r s t minutes of the r e a c t i o n . A p p r o x i m a t e l y one out of t e n e x c i t e d m o l e c u l e s was c o n v e r t e d t o 3 - b u t e n e - l - a l . T h i s f a c t i m p l i e s t h a t a f r a c t i o n o f the e x c i t e d m o l e c u l e s , r a n g i n g from 0.095-to 1, must have a s t r u c t u r e c l o s e t o t h a t of 3 - b u t e n e - l - a l . Since a hydrogen atom i s t r a n s f e r r e d from 91 p o s i t i o n (4) to p o s i t i o n . ( 2 ) i n the i s o m e r i z a t i o n process, i t can he suggested that the s t r u c t u r e o f an e f f e c t i v e a c t -i v a t e d molecule w i l l he one with a hydrogen atom shared e q u a l l y "between carbon atoms (2) and (4). Then the mechanism of the i s o m e r i z a t i o n r e a c t i o n (1) would be the f o l l o w i n g : t c 1 C=rC —CHO*- hv I I H H H H 0—C—CHO I I H H H H Y H 0—0—OHO I I H H Once 3-butene-l-tal has been formed, i t can undergo p h o t o l y s i s by absorbing a p o r t i o n o f the i l l u m i n a t i o n pas-s i n g through the r e a c t i o n v e s s e l . Thermal decomposition under the experimental c o n d i t i o n s used i s not probable, because the o n l y thermal r e a c t i o n observed i n samples of 3 - b u t e n e - l - a l was slow i s o m e r i z a t i o n tocerotonaldehyde. The presence of 1,5-hexadiene i n the r e a c t i o n products shows that a l l y l r a d i c a l s are present i n the r e a c t i o n c e l l . T h i s -i m p l i e s t h a t 3 - b u t e n e ^ l - a l must undergo, at l e a s t p a r t l y , the r a d i c a l decomposition CH 2:CHCH 2CH0 4- hv ^afc > CH2:CHCHg* + CHO* (2) Then the presence o f 1,5-hexadiene c o u l d be e a s i l y accounted for by a recombination r e a c t i o n o f a l l y l r a d i c a l s : 2CH2:CHCH2. CH 2: CHCH 2 g| 2CH: CH 2 (3) I t . i s important to i n v e s t i g a t e i n t o which other r e a c t i o n s , apart from recombination, a l l y l r a d i c a l s can e n t e r . We w i l l c o n s i d e r fforat the p o s s i b i l i t y o f a c h a i n mechanism, 92 sim i l a r to the one proposed, by Rice and Herzfeld (48) for the pyrolysis of hydrocarbons. Such a mechanism would involve abstraction of a hydrogen by the a l l y l r a d i c a l to produoe propylene and a new r a d i c a l , which might be able to con-tinue the chain. Since crotonaldehyde i s the compound with the highest concentration i n the reaction mixture, i t would Have the highest p r o b a b i l i t y to be attacked by r a d i c a l s . Of a l l the hydrogen atoms i n the crotonaldehyde molecule the aldehydio hydrogen i s the one with the smallest bond diss-ociation energy (49). Therefore, i f a l l y l r a d i c a l s attack, th i s hydrogen w i l l be abstracted: CH2:CH2CH2' + CH 3«CH:CHCH0 »CH 2:CHCH 3 + CH 3«CH:CHC0» (4) Then the chain could be continued by the crotonoyl r a d i c a l s d i s s o c i a t i n g to CO and propenyl r a d i c a l s , which could either abstract a hydrogen from crotonaldehyde, to form propylene and a crotonoyl r a d i c a l , or isomerize to a l l y l r a d i c a l s , or dimerize to 2,4-hexadiene. CH3CH:CHC0- CM C H i C E C " > CH3CH:CH' -»• CO (5) CHgCH:GH» + CH3CH:CHCH0 > CHgCH-.CHg-r CH 2CH:CHC0« ( 6 ) GH 2CH:CH« > CH 2:GHCH 2« (7) 2CH3CH:CH ->CH3CH:CHCH:CHCHg (8) The fact that no 2,4-hexadiene was detected i n the reac-t i o n products shows that propenyl r a d i c a l s are not produced in the reaction, unless i t i s assumed that the recombination reaction (8) i s too slow compared to (6) and (7). This l a s t reaction can be very f a s t , because the a l l y l r a d i c a l s formed 93 in i'h are s t a b i l i z e d by resonance energy estimated at about 25 kcal/mole by Szwarc et a l (50). Then the chain would be propagated by reaction ( 4 ) , (5), (6) and (7), and termina-ted by (3) only. I f propylene were produced by the r a d i c a l chain reaction described above, i t s r a t i o to 1,5-hexadiene would increase with increasing aldehyde concentration and decrease with decreasing I ^ g * Inspection of the data i n Tables XIII and XIV and plots of the r a t i o (propylene)/(I,5-hexaidene) against *abs ( F i g u r e 16) a n < i orotonaldehyde pressure (Figure 17) shovv's that this i s not the case. In fact the opposite ten-dency i s observed, i . e . the r a t i o (propylene)/(biallyl) decreases with increasing aldehyde concentration and increases with decreasing I a b a * These facts force one to reconsider the p o s s i b i l i t y of the mechanism represented by reactions ( 4 ) , (5), (6) and (7). A reaction similar to ( 4 ) , studied by B i r r e l and Trotman-Dickenson ( 4 9 ) i s the reaction of methyl r a d i c a l s with various aldehydes, including orotonaldehyde. They ob-served that the act i v a t i o n energy for methane formation i s almost the same for all:--.aldehyd.es studied and they concluded that the methyl r a d i c a l s react by abstracting the aldehydic hydrogen. For the reaction with orotonaldehyde CH 3 + CH3CH:0HCHO > CH 4 + CH3CH:CHCH0 ( 4 ' ) they found log A « 16.3 (i n units of mole'1.1.sec" 1) and E=10.9 kcal/mole. Given that the a l l y l r a d i c a l i s s t a b i l i z e d 94 FIGURE 16. DEPENDENCE OF [propyknel/[hexadta«p] ON labs. 95 96 by resonance energy, while methyl i s not, i t i s reasonable to assume that reaction (4) would have an a c t i v a t i o n energy greater than 10.9 kcal/mole. I f i t i s further assumed that the pre-exponential factor for (4) i s of the same order of magnitude as for ( 4 1 ) , i t can be e a s i l y seen that t h i s reac-t i o n would be very slow at 30°C. In fact, i t can be c a l c u l -ated that at t h i s temperature K4<2.5 x 10 8 mole" 1. L sec" 1 The s t a b i l i t y of the a l l y l r a d i c a l , r e s u l t i n g i n prepon-derance of the r a d i c a l recombination reaction to hydrogen abstraction from hydrogen donors, has been demonstrated b y i t s i n a b i l i t y to react with toluene (50), and by the fact that the main product of the pyrolysis of a l l y l chloride at 540-550°C i s 1,5-hexadiene, propylene being formed at higher temperatures only (51). The mechanism suggested i n the l a t t e r case i s s p l i t t i n g of a l l y l chloride to a l l y l r a d i c a l s and chlorine atoms, followed by recombination of the former to form 1,5-hexadiene. At higher temperature, hydrogen abstrac-t i o n becomes important, and also 1,5-hexadiene starts de-composing to a l l y l r a d i c a l s i n a reversal of the r a d i c a l re-combination process..The hydrogen atoms involved i n t h i s case are not as mobile as the aldehydic hydrogens involved i n reaction (4), but the fact remains that even at 540°C, rad-i c a l recombination takes preference over the alternative of hydrogen abstraction. It should be very much more so at room temperature. 97 Since i t has been shown that reaction (4) cannot take place under the experimental conditions studied, the whole chain mechanism needs no further consideration. Some authors (28,5£) suggest that the most important rsact i o n of methyl r a d i c a l s with orotonaldehyde at 120-350°C i s the r a d i c a l displacement CH3« -r- CH3CH:CHCH0 CH3CH: CHCH3 + HC0« A similar reaction with a l l y l i n place of methyl r a d i c a l would produce the assymetric diene, 1,4-hexadiene: 0H2:CHCH2' + CH3CH: CHCHO »> CH3CH: CHCH2CH: CH 2 4- HCO* (9) Considering that reaction (9) would, have approximately the same a c t i v a t i o n energy as reaction (4), i t can he safely excluded as a p o s s i b i l i t y . The only other reaction into which a l l y l r a d i c a l s can enter i s abstraction of hydrogen from formyl r a d i c a l s : CH2:CHCH2 + HCO' * CH2:CHCH3 +• CO (10) Reaction (10) should not have any a c t i v a t i o n energy, there-fore i t s importance would depend on the concentration of the radica l s involved. In t h i s stage i t i s necessary to invest-igate what other p o s s i b i l i t i e s are offered for the formyl r a d i c a l . It i s generally recognized (53) that the formyl r a d i c a l i s stable at room temperature. In fact, the decomposition process HCO* » H " + C 0 (11) has an acti v a t i o n energy estimated from 14 to 30 kcal/mole 98 by various authors (54,55,56,57). The fact that no hydrogen was detected among the reaction products confirms the stab-i l i t y of th i s r a d i c a l . Abstraction of hydrogen from crotonaldehyde by the formyl radical.should meet with the same unfavourable act-i v a t i o n energy as abstraction of hydrogen by. the a l l y l r a d i c a l . HCO• CH3CH:CHCH0 ?HCHO + CHgCH:CHCO' (12) In f a c t , the bond d i s s o c i a t i o n energy of the hydrogen i n formaldehyde, 79,3 kcal/mole ( 5 8 ) , i s too close to the bond di s s o c i a t i o n energy of the a l l y l i e hydrogen i n propylene, 76.5 (59), to assume that reaction (11) would be possible at room temperature while reaction (4) i s completely excluded. The fact that no formaldehyde was detected among the reaction products i s i n agreement with t h i s view. There i s at least one reaction into which formyl rad-i c a l s can enter, and thi s i s recombination to glyoxal 2CH0 > 0H0CH0 (13) Reaction (13) i s quite plausible, although the presence of glyoxal among the reaction products could not be v e r i f i e d by the a n a l y t i c a l tools used. If the formyl and a l l y l r a d i c a l s are eliminated only bgr r a d i c a l recombination reactions, then reaction (10) i s of considerable importance, and i t can account for at least part of propylene and CO found i n the reaction prodxicts. The assumption, however, that a l l the propylene and CO are produced 99 by means of reaction (10) comes to complete disagreement with the observation of Harrison and Lossing (29) that these two products of the Hg-fahotosensitized decomposition of crotonaldehyde arise mainly through a molecular reaction. It is reasonable to assume that such a reaction can take place also i n the photolysis of orotonaldehyde (indeed, of 3-butene-l - a l ) concurrently with (10) CH2:CHCH2CH0 + hv —>CH2:CHCHg + CO (14) From the above discussion i t follows that the experiment-a l data observed i n the photolysis of crotonaldehyde should be interpreted by means of a mechanism comprising reactions (1), (2), (3), (10), (13) and (14); CH2CH:CHCH0 +• hv l l ^ i > CHg: CHCHgCHO (l) CH2:CHCH2CH0 •»• h v — 9 ' r < L b ' ">0H2.: CHCHg- +• HCO- (2) 2CH2:CHCH2' > CH 2: CHCHgCHgCH: CH 2 (3) CH 2: CHCHg. + HCO > CH g: CHCH3 _ CO (10) 2HC0 > CHOCHO (13) CH2:CHCH2CH0 + hv >CH2; CHCHg +• CO (14) According to this mechanism, crotonaldehyde participates i n the reaction only i n so far as i t isomerizes to 3-butene-l-al, the secondary products a r i s i n g only by decomposition of the l a t t e r . This view can be tested by p l o t t i n g the sum of 3-butene-l-al and i t s fragments against time. Since each 3-butene-l-al moledule y i e l d s either one propyleneoor 1,5-100 hexadiene molecule, the sum should-he (3-butene-l-al) •*- (pro-pylene) +• §-( 1,5-hexadiene). Such a plot appears i n Figure 18. The data are taken from Table XII. From Figure 18, i t can be seen that this sum varies l i n e a r l y with time, as expected. Moreover, the quantum y i e l d of the sum i s equal to the quantum y i e l d of isomerization. In order to derive a kinetic expression for the products of the advanced reaction, i t i s necessary to estimate the i n t e n s i t y of illumination, Iatg. absorbed by 3-butene-l-al. In the reaction mixture, both 3-butene-l-al and orotonaldehyde are present. The other components need not be considered i n t h i s context, because they do not absorb at the wavelength used. Application of Beer's law to both aldehydes gives the expression: I f l b _ 1 - exp[-M: (RH) a 3 1 J (15) labs ~ ' t - exp[-e'.l.(R'HJ where (RH) and (R'H) are the concentrations of orotonaldehyde and 3-butene-l-al respectively and 6, £' their absorption c o e f f i c i e n t s ; 1 i s the length of the reaction vessel. The exponents i n (IS) are much smaller than one, therefore the approximation l n ( l + x ) = £ x can be applied. This y i e l d s the following expression for I aosS I ' v T v £ ! . ( R 'H) •'•abs = •'•abs — (15) £•(RH) In order to obtain an expression for (R'H)/(RH), account 101 FIGURE IS VARIATION Or COMBINED PRODUCTS WITH T I M E 102 should he taken of the faots that (a) orotonaldehyde ceo oonoentration diminishes during photolysis r e s u l t i n g i n de-crease of o v e r a l l absorbed intensity, and (b) the 3-butene-1-aJfc formed photolyzes according to reactions (2) and (14). The rate of disappearance of crotoaaldehyde i s -d(RH)/dt = <f I a b a , aos where *abs = S'^iri I i n i s the i n t e n s i t y of illumination with empty reaction vessel. Integration of th i s expression from t = 0 and (RH) = (RH) Q to t = t and (RH) - (RH) gives the following equation for the concentration of orotonaldehyde at any time: (SH) = (RH)oexp(-1>I i f ie-l-t) (18) The concentration of 3-butene-l-al varies as follows: d(R'H)/dt = <p;.r-^l-iRH) - ( 9 . + ? J I i n F ' l ( R , H ) (19) Substitution of (18) into (19) yiel d s the following d i f f e -r e n t i a l equation: y' +• by * a»exp(-cx) (20) where % = (R'H) x = t a = <P-I lnrl-(RH)0 b (*, + cpx)linf-l The general solution of (20) i s : 1 0 3 y = (a/ (b-c) |exp(-cx) +- c«exp(^-bx) Imposing the boundary condition y = 0 when x » 0 , c i s found to be c = -a/(b-c), and the solution of (SO): y = [a/(b - c) ) [exp(-ex) - exp(-bx)j ^•(RH), ^ l ^ ^ x p ^ I i ^ ^ - expl-^^l^ltjj ( 2 1 ) or (R'H) = Correlation of thi s expression with (18) y i e l d s : (R'H)/(RH) ^  1 - exp | (<P , ^ l )E ' ^ . f ] ( 2 2 ) Two approximate solutions for (R'H)/(RH) can be derived from ( 2 2 ) : (i) When photolysis has proceeded for a short time, the approximation l n ( l t x ) z i x can be applied for the quantity i n parentheses, y i e l d i n g the expression: (R'H)/(RH) £ (RH), ( 2 3 ) where the substitution I ^ g = I i r £gl-(RH) 0 has been made and (RH) put equal to (RH) . ( i i ) When the reaction time i s long, the exponential term becomes i n s i g n i f i c a n t with respect to one and the r a t i o of the aldehydes tends to a constant value: (R'H)/(RH) - 3±. ( 2 4 ) This long time l i m i t i s approached i n the set of experiments where the time dependence of the products was studied. A graph of the r a t i o (RH)f(R'H), calculated from the data on Table XII FIQURE: 19. DEPENDENCE OF [crobnal<lckyck]A3-butc«€-l-fltl] ON T»/AE 105 appears i n Figure 19. It i s seen that ifehe r a t i o approaches the constant value 9.10 . Since fo^fe'- 1 - 9.10 , and <+?- 0.095 t 0.005 , i t follows that (S*^ ) - 10.10 x (0.095 + o.oo5)(e/e;) - d.05 + o.o5)ce/e) (25 ) When the absorption curve of 3-butene-l-al was taken, (chapter V), i t was found that i t s absorption c o e f f i c i e n t i s very close to that of crotonakdehyde. Therefore the r a t i o l/E'can be taken as equal to one. Then the sum of the quantum yield s - 1.05 t 0.05 This estimate w i l l be checked l a t e r by calculations from other sources. . Substitution of the r a t i o (R'H)/(RH) from (23) into (16) yields the following expression for the l i g h t i n t e n s i t y ab-sorbed by 3-butene-l-al at the beginning of the photolysis: i labs r (Iabsf-tJ/tRHJo where E/t i s taken as equal to one. It i s seen that I' abs increases proportionally to the time. This r e s u l t was expected because 3-butene-I-al increases hy the same way. The increase 1 0 6 o f l a b s m a k e s i t i m p o s s i b l e t o c a l c u l a t e s t a t i o n a r y s t a t e c o n c u n t r a t i o n s f o r t h e s p e c i e s C H 2 : C H C H 2 # a n d GEO*/ I n f a c t , t h e f o l l o w i n g d i f f e r e n t i a l e q u a t i o n s h a v e t o be s o l v e d : d ( e H 2 : C H C H 2 . ) / d t = ( I a b a 2 . ^ t ) / ( R H ) d - 2 k g ( C H g : C H C H g • ) ^ » k 1 0 ( C H 2 : C H C H 2 . ) ( C H 0 « ) ( 2 7 ) d(OHO;)/dt - ( I a b a 2 . q > . t ) / ( R H ) 0 - 2 k 1 3 ( C H 0 - ) 2 -k 1 0 ( C H 2 : C H C H 2 « ) ( C H O * ) ( 2 8 ) I f i t i s a s s u m e d t h a t d ( C H 2 : C H C H 2 « ) / d t - d ( C H 0 . ) / d t , i t f o l l o w s t h a t ( C H O . ) r ( k 3 / k 1 2 ) £ ( C H 2 : C H C H 2 . ) a n d t h e p r e v i o u s e q u a t i o n s s i m p l i f y t o r e l a t i o n s o f t h e f o r m y f +- a y 2 ~ b t ( 2 9 ) w h i c h s t i l l h a s no a c c u r a t e s o l u t i o n . I f t h e a p p r o x i m a t i o n i s i n t r o d u c e d - ' ; y 1 =. 0 ( w h i c h i s e q u i v a l e n t t o a s t a t i o n a r y s t a t e h y p o t h e s i s ) , t h e s o l u t i o n o f ( 2 9 ) i s y - - | b t ) / a ] * ( 3 0 ) a n d . y » - . i ( b / (at) ) * T h e n y ' / y - l / ( 2 t ) I f t > 6 0 s e c o n d s , a s t h e c a s e i s w h e n t h e s e c o n d a r y p r o d u c t s 107 of the photolysis are measured, i t can "be seen that Me error introduced by the approximation i s less than Vfo, Substitution of the relevant variables i n (30) y i e l d s the following expressions for the concentrations of species CH2:CHCHg- and CHO* at time t : (CH 2: CHCHg*) r (CHO* ) Iabs^ 1 3 A(RH), q>q> t L a b a ^ B (RH) Vi (31) (32) x where A - 2k3 + ^3/^13) 8 B - 2fc l 3^ k i 0 ( k i 3 / f c 3 ^ Now the rates of formation of 1,5-hexadiene and propylene can fce e a s i l y calculated: d / l , 5-hexadiene )/dt =. k.r, ! A(RH)ft (33) ^ b s ^ * ^ b i T ^ t d( propylene )/dt ^ k 1 0 x \ (34) 1 0 ( A B ) t ( R H ) 0 (RH) 0 The integrated forms of (33) and (34) are: (1,5-hexadiene) ' d a b s ' * )2 A(RH), (propylene) -2(AB)^ 2 a b a ' t )2 (RH)rt (35) $36) 108 According to these expressions, the concentration of hoth hydrocarbons should be proportional to (Iabs**^ 2/^ R H^o« This r e l a t i o n i s obeyed quitewell by 15-hexaftiene. Propylene concentration, however, was found to vary with d a b s * * ) 2 / ( R H ) 0 1 , 3 " " 1 , 5 .Suoh a dependence on (RH) 0 could be j u s t i f i e d i f reaction (14) takes place through an activated complex, which i s long-lived enough to undergo c o l l i s i o n a l deactivation with orotonaldehyde molecules. In t h i s case, reaction (14)' would be substituted by the following steps: CH2: CHCHgCHO + hy > (CHg: CHCHgCHO, ^ 1 » a b a (14a) (CHg: CHCHgCHO? +- M >CHg:CHCHgCHO) + M , (14b) (CHg: CHCHgCHO )* »CHgCHCH 3 +- CO , k r (14c) This reaction scheme yi e l d s the following expression for the rate of formation of propylene: d(propylene)/dt - k 1 0 T f k r (37) (AB)s(RH) 0 tkdM4- k r) (RH) 0 -Integration and substitution of M aby..(RH)0 gives the equation (propylene) 2(AB)*(RH) Q ^(RH)^ f k r(RH) 0 2 which contains the term (RH) 0 i n the denominator. Inclusion of the deactivation step (14b) i s further j u s t i f i e d by the fact that the r a t i o (propylene)/(l,5-he&adiene) decreases as the orotonaldehyde pressure increases. This can be seen i n Figure 17. The fact that the same r a t i o increases as the re-action proceeds further, as i t can be seen i n Figures 16 and 20, can be attributed p a r t l y to the gradual decrease of 109 110 crotonaldehyde pressure i n the course of the reaction. This factor, however, i s not big enough to'account for a l l the i n -crease observed. An additional reaction may be taking place, which involves formation of propylene, e.g. by d i s s o c i a t i o n of 1,5-irhexadiene. This l a t t e r hydrocarbon was found to be quite stable, when illuminated alone at 2450-4000 S , and t h i s r e s u l t was expected because i t does not absorb at this wave-length. It could be, however, that i t s decomposition i s sensitized by the other compouds i n the reaction mixture. No further guesses can be made on the subject, because of lack of s u f f i c i e n t data. CHAPTER V. MECHANISM OF THE PHOTOLYSIS OF GASEOUS 3-BUTENE-1-AL AT 30°C AND 2450-4000 £ V. PHOTOLYSIS OF 3-BUTENE-1-AL AT 2450-4000 £ AND 30°G. 1. EXPERIMENTAL The same experimental technique was used as i n the photolysis of crotonaldehyde. The reaction c e l l wag used without removing the layer of polymer deposited 6n i t s sur-face during previous experiments, fine hour evacuation, before each experiment, was found an adequate precaution for the re-sults to be reproduceable. The I abs f ° r each reaction was calculated, as i n the case of crotonaldehyde, by measuring the int e n s i t y of i l l -umination with the c e l l evacuated, tin » a n& then evaluating *abs f ° r a giva* 1 pressure by using the absorption curve for 3-butene-l-al, Figure 21. 2. RESULTS Preliminary experiments showed that propylene, 1,5-hexadiene and CO were produced during the photolysis of 3-butene-l-al with a quantum y i e l d several times higher than the one observed i n the crotonaldehyde isomerization. Small amounts of crotonaldehyde were detected i n each experiment, but the r e s u l t s were not reproductible because 3-butene-l-al i n the storage trap contained about ifo crotonaldehyde and also additional amounts were produced during the reaction and the subsequent chromatographic analysis. Therefore no r e s u l t s 112 FIGURE 21 . ABSORPTION CURVE FOR 3-^'UTCNC-I-AL 113 on orotonaldehyde. production are recorded i n the present work. It can only he noted that the amount of orotonaldehyde obser-ved i n a reaction was never larger than 10$ of the amount of the other products. Dependence on Time 20 mm Hg of' 8-rbutane-1-al were photolyzed for various time inter v a l s using the same lig h t i n t e n s i t y . The res u l t s appear on Table XV. The quantum yields of propylene and 1,5-hexadiene are calculated for the f i r s t 10 minutes of the reaction. The quantum y i e l d of Cffi can be taken as equal to that of propylene. From a graph on Figure 22 i t i s seen that the rate of formation of the products i s constant for a f a i r l y long period of time. The reaction time was extended up to 135 minutes i n order to study i t s influence on the r a t i o propylene/l,5-hexadiene. From Figure 23 i t i s seen that this r a t i o increases with time. TABLE XV Analysis of Products During the Photolysis of 20 mm Hg 3-butene-l-al at 30°C. I i n i 66.6xl0~ 8 e i n ' l " 1 . s e c _ 1 A l l data are i n units of mole•l'^xlO 6 Time Propylene 1,5-hexadiene CO prop. hex. min. 2 16.5 2.99- , 13.0 0.71 0.128 3 26.9 5.02 25.1 0.77 o . m 5 40.0 .7.51 45.5 0.68 0.128 114 FIGURE 22. PRODUCTS O F PHOTOLYSIS OF 3-BUTGNG-I-AL 115 LEGEMTJ  Q CROTONALPEHVDf I I I 60 120 T l t t E > M I N U T E S FIGURE 7i. DEPENDENCE OF [\>ro\>y\tv\9]/[\5-hexoi<t\me] ON T I M E 116 7 61.8 IE.4 10 87.4 15.8 15 1SS SE.O SO 165 S8.0 S5 SOI 34.1 30 S3S 40.3 60 304 50.8 100 350 56.1 135 373 57.3 56.5 79.3 160 188 S18 34S Mean value: 0.73+0.05 0.137*0.014 0.75 0.75 0.151 0.135 Dependence on labs SO mm Hg of 3-butena-l-al were illuminated for 10 minutes with v a i i a b l e i n t e n s i t y of illumination. The r e s u l t s appear i n Table XVI. TABL5 XVI Dependence of Products on I ^ g . In a l l experiments EO mm Hg 3-butene-l-al were photolyzed for 10 minutes. A l l r e s u l t s are i n units of mole• 1 - I x l 0 7 . I abs ein»l" 1.sec" 1 x l O 8 Propylene 1,5-hex. CO prop. hex. 0.855 34.S 6.65 - 0.66 0.1S9 117 1.23 55.2 10.2 - 0.75 0.152 1.88 77.5 15.2 - 0.69 0.135 2.74 121 23.6 137 0.74 0.148 3.93 170 34.3 144 0.72 0.146 6.98 27 5 50.6 260 0.66 0.121 10.7 438 83.4 406 0.68 0.130 13.6 576 101 - 0.71 0.124 19.5 874 150 793 0.75 0.128 mean value : 0.7lt0.05 0.134t0.018 The quantum y i e l d of propylene, 0.71+0.05 and 1,5-hexadiene 0.134t0.018 oalculated i n Table XVI i a i n agree-ment with the quantum y i e l d s , 0.73+-0.05 and 0*137+0.014 reapectively, calculated i n Table XV. Dependence on Pressure of 3-butene-I-al. A set of experiments was carr i e d out i n which the 3-butene-l-al pressure was varied from 4-30 mm Hg. The t o t a l amount of absorbed r a d i a t i o n was kept constant by adjusting the duration of the experiment i n such a way that the prod-uct I ai3 Sx time was constant. From the data on Table XVII i t i s obvious that the quantum y i e l d of 1,5-hexadiene i s inde-pendent of the 3-butene-l-al pressure, while that of pro-pylene decreases as the pressure inoreases. This d r i f t i s shown i n Figure 24. 118 119 L E T T F MP  O FROrtVBUT£N£-l-AL PHOTOLYSIS Q » CROToN^LPEHyOE " 10.0 20.0 30.0 PRESSURE, mm H<j FIGURE 25. DEPENDENCE OF [propylewc]/[i,5-\iexadie-ne] ON FKESSURE ISO TABLE XVII Dependence on Pressure of 3-butene-l-al. Absorbed r a d i a t i o n E.34 x 10~ 5 e i n - 1 " 1 3-butene-l-al propylene 1,5-hex. ' mm Hg mole • 1-1x10-6 mole. l'^-xlO" 6 prop hex 4.0 18.2 3.06 0.78 0.131 6.9 18.3 3.50 0.78 0.150 10.1 17.5 3.30 0.75 0.141 13. S 18.0 3.02 0.77 0.129 15.7 17.8 3.18 0.76 0.136 SO.O 16.5 2.99 0.71 0.12E 23.6 17.1 3.06 0.73 0.131 S7.S 16.1 3.26 0.69 0.139 30.0 16.3 3.19 0.70 0.136 DISCUSSION From the r e s u l t s shown i n the previous section i t is clear that 3-butene-l-al, when illuminated at 30°C with l i g h t wavelength S450-4000 A0, undergoes decomposition to 1,5-hexadiene, propylene and CO. The two last produdts, i n so far that CO was determined, are formed i n almost equimoledular amounts. These results are i n agreement with the reaction mech-anism proposed for the photolysis of crotonaldehyde. Reactions 121 (2), (3), (10), (13) and (14a,b,c,), of the la s t chapter explain very well the experimental r e s u l t s . CH 2: CHCHgCHO f h» » CH2:CHCHg. +• CHO. ;2CH9:CHCH0. ^ CH 2: CHCHgCHgCH: CHg CH2:CHCHg. f CHC 2CH0. -> CH2:CHCH3 4- CO -J>CH0CH0 CH2: CHCHgCHO 5Jak > (CH 2: CHCHgCHO)* (CH2:CHCH2CH0)t4/ M ^ CHg:CHCHgCHO f M (CHg:CHCHgCHOV ->'CH2:CHCH3 + CO (2) (3) (10) (13) (14a) ( 1 4b ) (14c) The following kinetic expressions can be derived for propylene and 1,5-hexadiene: d (propylene) /dt •= k10*, kr^i (AB) ^  ka.(RH) 4- k r ^bs (1) d ( l , 5-hexadiene )/dt - ( ^ I a b s ^ A (2) where A - 2k 2 + k 1 0 ( k 3 / k 1 3 ^ and B = 2k 1 3 +-k1Q( k 1 3/kg )^" Theses expressions agree well with the experimental r e s u l t s . In chapter I? the sum of the quantum y i e l d s + ^ i) was estimated to be 1.0510.05. Now this sum can be calculated d i r e c t l y from the quantum yi e l d s of propylene and 1,5-hexad-iene. From the mechanism of the phto t l y s i s i t i s seen that the stoichiometry of the o v e r a l l reaction i s : (m+n) CHg: CHCH gCHO +• hv -3i3L* mCHg:CHCH3 + mCO f nCHg:CHCHgCHgCH:CHg (3) Therefore the sum ( ^ v ^ i s equal to the quantum y i e l d of 128 p r o p y l e n e p l u s t w i c e the quantum y i e l d o f 1,5-hexadien6. The l a t t e r can "be t a k e n , from T a b l e s XV and X V I , as e q u a l t o 6 .1310.02. The quantum y i e l d o f p r o p y l e n e has been seen t o i n c r e a s e w i t h d e c r e a s i n g p r e s s u r e o f 3 - b u t e n e - l - a l . The val u e at z e r o p r e s s u r e , o b t a i n e d by e x t r a p o l a t i o n o f the curve i n F i g u r e 24, i s O.l^iO.oS • T h e r e f o r e ^,+^=(0.7910.05)2(0.135 0.020) r 1.0610.09 T h i s v a l u e o f ( ^ J - ^ ) i s i n good agreement w i t h the one e s t i m a t e d i n c h a p t e r IV. I t i s p r o b a b l e t h a t the lower l i m i t o f t h i s v a l u e i s n e a r e r t o the t r u e v a l u e o f the quantum y i e l d o f the o v e r a l l r e a c t i o n , because v a l u e s h i g h e r t h e n 1 would i m p l y a c h a i n mechanism. The f a c t t h a t the quantum y i e l d ( ^  ) c a l c u l a t e d i n the experiments w i t h c r o t o n a l d e h y d e was v e r i f i e d , i s i n sup-p o r t o f the proposed medhanism f o r the p h o t o l y s i s o f c r o t -onaldehyde. The s i m i l a r i t y between the e x p e r i m e n t a l r e s u l t s o b t a i n e d i n the p h o t o l y s i s o f the two i s o m e r i c a l d e h y d e s i s f u r t h e r m a n i f e s t e d i n the f a c t t h a t the r a t i o ( p r o p y l e n e ) / (1,5-hexadiene*y v a r i e s i n a s i m i l a r way, as seen i n F i g u r e s 23 and 25. T h i s r a t i o appears t o be h i g h e r i n the case o f 3 - b u t e n e - l - a l , but e x t r a p o l a t i o n t o z e r o p r e s s u r e o f the c u r v e s i n F i g u r e 25 shows t h a t p r o b a b l y t h i s r a t i o i s the same i n the p h o t o l y s i s o f b o t h a l d e h y d e s , when no t h i r d body i n t e r f e r e s . 123 I f the quantum y i e l d of 3-butene-l-al decomposition i s taken as one, i t means that every quantum of l i g h t absorbed by 3-butene-4-al causes i t to decompose either to r a d i c a l s or to propylene and CO. It i s not possible to estimate the in d i v i d u a l values of T, and ^ I f i t i s assumed that a l l the a l l y l r a d i c a l s dimerize to 1,5-hexadiene, i . e . that re-action (11) i s unimportant, then ^ ~ 2<V hexad. - 6.27 t0.04 This i s a lower l i m i t for <p, , leaving an upper l i m i t f o r 9 z : <Pa4 0.79 ir 0.05 It cannot be considered that the r e s u l t s obtained i n the photolysis of crotonaldehyde and 3-butene-l-al are f i n a l . There are s t i l l some points of the reactions which need further study. One point i s the increase of the r a t i o (propylene)/(1,5-hexadiene) with time i n the photolysis of both aldehydes. Another point i s the thermal isomerization of 3-butene-l-al to crotonaldehyde, a reaction which can be investigated successfully only i f pure 3-butene-l-al i s prepared. 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