Jan., 1963
FOElnLkTION O F E T H Y L E N E I N
cule-molecule collisions a t about 5 X mm.; the corresponding figure for the packed vessel is about 5 X 10+ mm, These are approximately the pressures where the low pressure first-order regions begin in each case and this is consistent with the interpretation that collisions with the walls are some 3-5 times as efficient as collisions with other cyclopropane molecules. Furthermore, if we regard the walls as acting instead of gas molecules as far as energizatioii is concerned, the temperature coefficient of the reaction shows the expected behavior. The variation in temperature
PHOTOLYSIS OF ETHYL ACETATE
163
coefficient with pressure has been observed for azomethane5 and l,l-dimethylcyclopropane.6 In both cases, as the pressure is reduced, the activation energy falls by about 5 to 7 kcal. and the frequency factor Falls by a factor of lo2 to 104. Our activation energy is 8 f 2 kcal. below the high pressure value, and the frequency factor, although it is to some extent dependent on the surface area, is down by a factor of about l o a to 104. (6) C. Steel and A. F. Trotman-Dickenson, J . Chem. Soc., 976 (1959:l. (6) M. C. Flowers and H. It. Frey, ibid., 1167 (1962).
IXTRAMOLECULAR REARRAKGEMENTS. V. FORMATION OF ETHYLENE I N THE PHOTOLYSIS OF ETHYL ACETATE FROM 4 TO 500°K.1 BY P. AUSLOOSASD RICHARD E. REBBERT Sational 3ureau of Standards, Washington, D. C. Received J u l y 16, 1962 The formation of ethylene in the gas, liquid, and solid phase photolysis of CH,COOCZH, CHsCOOCDzCD211, and CHJCOOC2D5has been investigated a t various wave lengths and temperatures. The intramolecular isotope effect increases with a decrease in temperature in both the gas and liquid phase photolysis. In the liquid phase, an activation energy difference of 1.0 lrcal./mole was obtained for D and H-atom transfer in the photolysis of CHBCOOCD2CD2H. A variation in wave length influences the formation of ethylene in the gas and solid phases but does not affect the intra- and intermolecular ieiotope effects in the liquid phase. A comparison of the product yields in the mercury sensitized decomposition with those obtained in the direct photolysis, as well a8 a study of the effect of biacetyl on the ethylene yield, indicates that the formation of ethylene by an intramolecular rearrangement of the ester molecule excited to an upper triplet state cannot be ruled out.
Introduction It is now well established2 that esters containing one or more p-hydrogen atoms in the alkyl group may decompose photochemically by an intra,molecular rearrangement into an olefin and the corresponding acid. In the present work, the gas, liquid, and solid phase photochemical decompositions of CH3COOC2H6, CII3COOCD2CD2H, and CH3COOC2D6have been investigated in order to obta,in a better understanding of the intra- and intermolecular isotope effects on the primary process. The comparison of these results with those obtained on the photolysis of CH3COCH2CRDCD2R,3 and on the pyrolysis of isotopically labeled ethyl a ~ e t a t ewas , ~ expected to lead to interesting correlations. Experimental Apparatus.-The gas phase was photolyzed in a quartz cell (10 cm. in length, 5 cm. in diameter) enclosed in a heavy alunninum furnace provided with double quartz windows. The temperature was automatically controlled within half a degree. The liquid phase was irradiated in a quartz cell (0.05 cm. in depth, 2.5 cm. in diameter) provided with two outlets, one of which was sealed a f k r filling and the other closed by a break seal. The liquid cell was immersed in a Pyrex dewar flask with double quartz windows. Cold ethyl alcohol was the refrigerant for experiments (carriedout from 193 t o 273°K. The solid phase experiments were carried out in a stainless steel low-temperature dewar The ethyl acetate was deposited on a gold plated brass plate, in order t o ensure good thermal conduction. The flow rate for deposition of the sample was
~-
( I ) This research was supported by a grant from the U. S. Public Health Service, Department of Health, Education, a n d Welfare. (2) (a) I'. Ausloos, Can. J . Chem., 36, 383 (1968); (b) P. Ausloos, J . -4% Chem. Soc., 80, 1310 (1958); (0) M. H. J. Wijnen, ibid., Sa, 3034 (1960); (d) R. Borkowski and P. Ausloos, ibid., 83, 1053 (1961). (3) R. P. Borkoivski and P. Ausloos, J . Phys. Chem., 65, 2267 (1961). (4) (a) A. T. Blades and P. W. Gilderson, Can. J . Chem., 38, 1401 (1960); (b) A. T. Blades and P. W. Gilderson, i b i d . , 38, 1407 (1960). ( 5 ) Hofrnan Free Radical Research Dewar, D-1288.
about 2 cc. (STP)/min. I n most experiments about 30 cc. (STP) of gas was depoLitedon the brass plate. In the mercury photosensitized experiments a flat spiral lowpressure mercury arc waa used in conjunction with a Corning filter 7-54. A Hanovia SH-100 lamp was used in all other experiments. The incident wave length range was varied by ulsing Corning filters 9-54 and 7-54, transmitting radiation a t Rave lengths greater than 2150 and 2250 A,, respectively. Because ethyl acetate absorbs mostly below 2350 d., the effectiye wave length regions were roughly 2150-2350 A. and 2250-2350 A. Materials.-Ethyl-d5 acetate and ethyl-& acetate were obtained from Merck, Sharp & Dohme of Canada, Ltd. Mass spectrometric analysis showed that the three batches of ethyl-& acetate used in this work contained about 5% ethyl-& acetate, the remainder consisted of CHaCOOCDzCDzH. It was not gossible t o tell what fraction of the ethyl-& acetate was CH,COOCDzCDHz or CH~COOCHDCDZR;consequently, the results have not been corrected for the presence of this impurity. At any rate, this correction would only be a minor one and certainly would not affect any of the conclusions drawn in this paper. The ethyl-d5 acetate batch contained 2.5% ethyl-d., acetate. Chromatographic analysis indicated that after a trap-to-trap distillation a t --80°, the total chemical impurities amounted to not more than lye, ethanol being the major impurity. CH8COOC2H6was obtained from Eastman Kodak Co. After distillation the impurities did not exceed 0.1%. Analysis.-The analytical system consisted of a solid nitrogen trap, a modified Ward still, an automatic Toepler pump, antd a Toepler gas buret. The carbon monoxide-methane-hydrogen fraction was removed a t -210'. Ethylene and ethane subsequently were distilled off a t -180'. Propane and carbon dioxide were removed a t -150'. A11 fractions were analyzed mass spectrometrically using a Consolidated mass spectrometer Model 21-101. Standard samples of CzD4 and C2D3H obtained from Merck, Sharp & Dohme were run on the same mass spectrometer in order to calculate the isotopic distribution of the ethylenes.
Results In order to study the relative importance of a H atom t o a D atom transfer in the intramolecular rearrangements of the labeled ethyl acetates, ethylene
P. ACSLOOSAND RICHARD E. REBBERT
164
rather than acetic acid has been determined quantitatively. It may be noted that acetic acid has been observed as a product in the liquid phase experiments, but the fast exchange of CH3COOD to CH3COOH during the analytical procedure reduces the mefulness of a systematic analysis of this compound. I n order to inhibit radical reactions which may eventually lead to the formation of ethylene, oxygen was present in nearly all vapor phase photolysis experiments. Under these conditions, ethylene was the only hydrocarbon product formed in the gas-phase photolysis of ethyl acetate. I n the liquid phase experiments, no radical scavengers have been used, because ethylene always accounted for more than 95% of the hydrocarbon products. All vapor phase experiments were carried out a t conversions varying between 0.03 and 0.1%. The extent of decomposition in the liquid phase experiments was always less than 0.0270. (A) Vapor Phase Photolysis.-In order to obtain information on the relative importance of the olefin elimination process, CH&OOC2H6 has been photolyzed in the absence of oxygen a t 135" and a t a pressure of 5 cm. At this temperature the relative distribution of CO, Con,and C2H4 is Full arc: CO, 1.0; COe, 0.25; CzH4, 0.39
Hg(3Pl)sensitized: CO, 1.0; COz,0.15; CzH4,0.83 I n the mercury photosensitized decomposition a t 135", the formation of ethane, propane, and butane was completely inhibited by the addition of 1% oxygen, while the yield of ethylene was reduced by only G%. I n the mercury sensitized decomposition a t 30" carried out in absence of oxygen the ethane, propane, and butane yields were, respectively, 1.1,3.0, and 1.5% of the ethylene yield. The results6 given in Table I can be summarized as follows: A variation in the pressure of oxygen from 0.1 to 1 cm. has within experimental error no effect on the yield as well as on the distribution of the ethylenes. Absorption of shorter wave lengths by CH3COOC2D4Hor an increase in temperature reduces the ratio C2D4/CnD3Hto a value which approaches the chance value of 0.5. TABLE I GASPHASE PHOTOLYSIS --Pressure, [Ethyl
T OK. acetate1
cm.
--
[Oz]
Wave lengths,
A.
Ethylene, cc./min. x 104
Ratio
22 5 9.0 9.1 9.3 0 2 4.0 4.0
CzDa/CzD3H 0 665 0.805 0.803 0.74 1 06 2 10 2.15
315 315 315 431 315 315 315
CHsCOOCLhCDpH Full arc 0.1 3.0 2 150-2350 0.1 3 0 2150-2350 1.0 3 0 2150-2350 0.1 3.6 2250-2350 0 1 3.0 2.9 0.05 2537 Kg(jP1) 3.1 None 2537 Hg(3P1)
315 315 500 315 315
CHaCOOCzHrCH3COOC2Dj (1: 1) 3.2 0.1 Full arc 3.1 0.1 2150-2350 4.4 0 1 2150-2350 3.2 0.1 2250-2350 3 2 None 2537 Hg(3P~)
33.2 10 0 10.6 1 3
..
C&/CzD* 2 40 2 35 2 55 2 25 1.75
(6) A variation of the total ethylene rates given in the tables, for experiments performed in the same wave length range, reflect a variation in the relative quantum yields.
Vol. 67
I n the photolysis of equimolar mixtures of CH,COOCzHs and CH&OOC2D5 the intermolecular isotope effect as evidenced by the ratio CzD4/CzHd increases slightly with decrease in wave length and increase in temperature. (B) Liquid Phase Photolysis.-The incident intensity was kept constant for all experiments carried out in the wave length range 2150-2350 A. For these experiments the rates of formation of ethylene may thus be considered as relative quantum yields. The following conclusions can be drawn from the results given in Table 11. TABLE I1 LIQUIDPHASE PHOTOLYSIS T,OK.
Filter
CHsCOOCD&&H 322.5 9-54 302.5 None 302.5 7-54 302.5 9-54 273.0 9-54 270.0 9-54 244.0 9-54 224.0 9-54 198.0 9-54
Ethylene, co./min.
3.00
..
..
2. ss 2.46 2.40 2 20 2.09 1.72
273.0 273.0" 193.0
CH3COOCzHa 9-54 9-54 9-54
2.95 0.79 2.45
295.0 273.0 238.0 193.0
CHaCOOCzDb 9-54 9-54 9-54 9-54
1.62 1.58 1.02 0.56
CH3COOCzH6-CHdCOOCzDs ( I : 1) 322.5 9-54 1.86 302.5 9-54 2.05 302.5 Sone .. 273 9-54 2.03 273" 9-54 0.40 238 9-54 1.90 198 9-54 1.39 19s 7-54 a 2% by volume biacetyl added.
Ratio
CZD~/CYD~H 2.35 2.57 2.54 2.56 3.07 3.10 3.70 4.50 5.95
CzHdCzDa 1.84 1.82 1.80 1.87 7.10 2.98 4.20 4.25
A variation of the incident wave length has no effect on the ratios C Z D ~ / C ~ Dand ~ H CzH4/CD4 obtained in the photolysis of CH~COOCZD~H and CH3COOCzH5-CH3COOC2Dsmixtures, respectively. I n the photolysis of C€13COOCzD4H the total ethylene undergoes a gradual increase with increase in temperature, while the ratio C2D4/C2D3H diminishes with a raise in temperature. A plot of log C2D,/C2D3H against 1/T corresponds to 1.0 kcal/mole. The effect of temperature on the yield of ethylene is more pronounced in the photolysis of CH3COOC2D6 than in the photolysis of CH3COOC2H5. In the photolysis of equimolar mixtures of CH3COOC2H5 and CH3COOC2D5,the ratio C*Hp/CzD* decreases with increase in temperature until it reaches, around 273"K., a constant temperature independent value of -1.83. The same value is obtained by taking the ratio of the yields of CzH4 and CZD4 produced in the photolysis of C€I&OOCzH6 and CH3COOC2Db a t 273".
Jan., 1963
FORMATION OF ETHYLENE IN PHOTOLYSIS OF ETHYL ACETATE
Addition of 2% biacetyl reduces the yield of ethylene produced in the photolysis of CH&OOCzH5 by 80%. A similar reduction is observed upon addition of biacetyl to an equimolar mixture of CH3COOCpHs and CH3COOCzD6. I n the latter case addition of biacetyl also increases the ratio CzH4/C2D4. (C) Solid Phase.-In Table 111 several experiments have been presented, which have been performed a t various temperatures of deposition and irradiation. For comparkon, a few experiments carried out. on CHsCOCHzCHDCDzH have been included as well. Products other than ethylene were only observed in the photolysis of ethyl acetate a t 4°K.
165
mentsZdit could not be decided if either (IIa) or (ILIb) or both occur. However, the fact that above 124" the ratio C2H4/CO2as well as the yields of COZ and CO were within experimental error independent of temperature indicates that a t these temperatures and a t moderate intensities, COz and CO may be taken as a measure of processes (IIa) (IIb) and (I), respectively. Also the earlier observationza that addition of iodine reduces the yield of ethylene by not more than 20% demonstrates that even in absence of scavengers ethylene is mainly produced by process 111. It thus follows from a comparison of the relative yields of CO, COz, and C2H4produced in the direct photolysis a t 13.5" with those observed in the Hg(,PI) sensitized TABLE I11 photolysis, that the intramolecular rearrangement SOLIDPHASE PHOTOLYSIS process is relatively more important in the mercury Ethylene, photosensitized decomposition than in the direct T,OK. T,OK. cc./min. Wave lengths (deps.) (irrad.) X 104 Ratio photolysis. If as it has been suggested8 one assumes that the Wigner spin conservation rule can be applied CH~COOCD~CDZH CzDdCzDaH in the case of the photosensitized excitation, one is 2.0 10.4 Full arc 77 77 forced to conclude that contrary to a recent p r e d i c t i ~ n , ~ 1.9 8.9 Full arc 4 77 1.0 3.5 Full arc 77 4 an ethyl acetate molecule excited to an upper triplet 1.6 2.0 Full arc 4 4 state does decompose according to process 111. It may be argued that the independence of the yield CHaCOOC~HsCH~COOCzDs (1:1) CdL"D4 of ethylene (Table I) with a variation of the pressure 25.6 Full arc 77 77 .. of oxygen seems to rule out the occurrence of the olefin 10.0 Full arc 77 4 elimination process by way of a triplet state.'O HowCH3COCH2CHDCDtH C2DaH/C*D?H2 ever, it should be noted that the transition from the 1.2 11.4 3130 77 77 upper singlet formed in the primary photochemical act 5.0 2537 A. 77 77 to the triplet state may lead to the production of a 7.4 77 4 0.52 3130 molecule excited to a high vibrational level of the triplet .56 5.6 3130 A. 4 4 state with a short dissociative lifetime. Such a vibraThe results show that: (1) At an irradiation te:mtionally excited molecule may decompose before sufferperature of 77"K., the ratios C2D*/CzH, and C2D,/ ing a quenching collision with oxygen. In this connecCzD3H obtained in the photolysis of an equimolar tion it may be pointed out that oxygen does quench iLhe mixture of CH3COOC2H6 and CH3COOCzD5 and in intramolecular rearrangement in the photolysis of 2the photolysis of CH,COOCD&DZH, are still campentanone, provided that temperature is low enough siderably higher than those observed in the liquid and that only the longer wave lengths are absorbed by phase a t temperatures close to the melt,ing point. the ketone molecule. l1 ( 2 ) A decrease in the temperature of deposition from Other evidence for the elimination of an olefin from 77 to 4°K. apparently reduces the ratio C2D*/CzD:$H compounds containing carbonyl which are excited to a in the photolysis of CH3COOCD2CD2H. A similar triplet level follows from the recent observation'z that effect is observed in the photolysis of CH3COCHztraces of biacetyl readily quench the formation of CDHCDzH. I n the latter case there seems to be a ethylene in the photolysis of n-butyraldehyde a t 3340 decrease in quantum yield as well. (3) I n the phoA. tolysis of CH3COOCDzCDzHa decrease in temperature It is obvious that it is experimentally difficult to of irradiation from 77 to 4°K. strongly reduces the determine a t any particular wave length, temperature, ratio C2D4/C2D3H. At this temperature other prodand pressure if a certain mode of decomposition in the ucts, besides ethylene, are produced in significant gas phase occurs from a molecule excited to an upper amount^.^ singlet state or to a high vibrational level of the triplet Discussion state, or both. At any rate, the Hg(3PI)sensitized I. The Nature of the Upper Electronic State. experiments presented in this paper indicate that the (A).-The following primary processes have been reformation of ethylene from a molecule excited to an ported t o occ'ur in the vapor and liquid phase photolupper triplet state cannot be ruled out. ysis of ethyl acetate.2 From the vapor phase experi(B) Liquid Phase.-Recent gas phase photolysis studiesI3 have demonstrated that energy transfer from CH3CQQC2H6 --+ CHjCO OCzHs (I) a triplet excited ketone molecule to biacetyl occurs + CH3CO0 CzH5 (IIa) readily a t the longer wave length and low temperatures. --+ CH3 COOCzHB (IIb) ( 8 ) P Roriell and R. G W. Norrish, Proc. Roy Soc (London), A262, 19 (1961) --+ CH3COOH CzH4 (111) (9) P. Boriell Nature, 188, 1002 (1960). (7) The distribution of the gaseous products formed in the photolysis of (10) V. Brunet a?d W. A. Noyes, Jr., Bull. soc chzm France, 121 (1958).
+
4, 4.
+
+ +
+
CHaCOOCDeCDzH carried out a t 4OK. is as follows: CO, 50%; methane, 24.2%; hydrogen, 12.3%; ethylene -10%; ethane -3.5%. The isotopic compositions of the hydrogen and methane fractions were as follows: CDd, 3.1%; CDaH, 19.9%; CDzHe, 15.8%; CHaD, 23.2%; CH4, 38.0%; Dz, 13.7%; H D , 37.3%; H2, 48.9%.
(11) R. Borko-ski and P. Ausloos, J Phys Chem , 65, 2257 (1961). (12) R. Borkoiiski and P. Ausloos, J . A m Chem. floc., 84, 4044 (1962). (13) (a) H. Okabe and W. A. Noyes, Jr., zbzd., 79, 801 (1957); (b) J. Heioklen and W. A. Noyes, Jr., zb%d 81, 3858 (19.59); ( 0 ) D. S. Wen rbrd., 83, 2629 (1961).
P. Avsr,oos
166
AND
RICHARD E. REBBERT
In the liquid phase, because of extensive degradation of the vibrationa,l energy12one may expect that the energy transfer reaction between a triplet excited molecule and biacetyl will have a relatively high efficiency a t all wave lengths. This is indeed demonstrated by the pronounced reduction (Table 11) in the relative quantum yield of ethylene caused by the addit'ion of 2% bia ~ e t y 1 . l ~Although these experiments only demonstrate that transfer of electronic energy from ethyl acetate to biacetyl is an efficient process, it may be mid that on the basis of the relatively long lifet,ime of the triplet state as well as in view of the recent gas phase experiments,l1?l2 an excited triplet rather than an upper singlet st'ate is involved in this process. In this connection it may be pointed out that in an earlier investigation of the liquid phase photolysis of esters, a quenching of process (111) by a number of solvents has been observed.16 Evidence was presented which indicated that this quenching process actually consisted of a photochemical reaction involving an ester molecule excited to a higher electronic level OH
CH3COOC2HS"
+ RH --+
/ CHaC \
OCd% This reaction is analogous to the H-atom abstraction from solvent molecules by electronically excited acetone,16 biacetyl, l7 and benzophenone.18 These reactions have been ascribedl7~18~ to a triplet &ate of the compound containing carbonyl. 11. Intra- and Intermolecular Isotope Effect. (A) Vapor Phase. CH3COOCD2CD2H.--In t,he presence of oxygen, C2D4and C2D3Hare formed by the following two processes CH&OOCD&D2H
+ h~ +
+ C2D4 * CHSCOOD + CzDBH CH3COOH
temperatures. Similar observations have been made in the course of a recent investigation of the photolysis of CH3COCH2CDHCD2H.l' As may be expected, the value for the ratio C2De/ C2D3H, obtained in the Hg(3P1) sensitized experiments, is larger than that found in the direct photolysis experiments which have been carried out a t shorter wave lengths. It is interesting to note that the intramolecular isotope effect in Hg(3PI) sensitized decomposition of ethyl-& acetate is nearly twice as large as that observed under comparable experimental conditions in the photolysis of CH3COCH2CHDCD2H. This may be related to a difference in the degree of vibrational deactivation of these two compounds, prior to decomposition. As will be shown later in the discussion, in the liquid phase the ester and the ketone show about the same intramolecular isotope effect. CHsCOOC2H&H3COOC2Ds (1 :l).-From the results of Table I it can be deduced that there is an appreciable intermolecular isotope effectlg for the two processes
+ k v +CII3COOH + CzH4 CHsCOOC2D6 + hv +CHaCOOD + CzD4
CHaCOOCzH5
t R
(IV) (V)
Vol. 67
(111) (VI)
I n the direct photolysis, the variations of the ratio C ~ H ~ / C Zwith D ~ wave length and temperature are rather small and do not warrant any further discussion. (B) Liquid Phase. CH&OOCD2CD2H.-The fact that the ratio C2D4/C2D3His independent of wave length indicates that in the liquid phase, the ethyl acetate molecule excited to an upper electronic state undergoes a collision induced degradation to a lower lying energy level. The degree of excitation of the molecule which decomposes apparently is independent of the exciting wave length. However, a variation in temperature has a pronounced effect on the ratio C2D4/'C2D3Has well as on the total ethylene yield. In this respect there is a similarity between the photolysis and pyrolysis of ethyl acetate. Apparently, in the liquid phase the excited molecules undergo a sufficient number of collisions prior to decomposition t o achieve equilibration of the vibrational energy in the upper electronic state. From the results of Table 11, the following expression can be deduced
A chance occurrence of these two processes would lead to a value of 0.5 for the ratio C2D4/C2D3H. It can be seen (Table I) that this value is only approached in the gas phase photolysis a t the shorter wave lengths. It is obvious that the increase of the rat'io C Z D ~ / C ~ D ~ H k E / k ~= 0.95e1000/RT with increasing wave lengths and decreasing temperawhere JCH and k D stand for the relative rates of formatures is related to the difference in bond strength betion of CZD4 and CzD3H per p-deuterium atom, retween the C-H and C-D bond. Rupture of the C-H spectively. bond apparently is favored a t long wave lengths and low It is of interest to note that this rate expression is (14) The importance of this quenching effect for other compounds convery similar to the one obtained by Blades and Gildertaining carbonyl has been demonstrated by recent experiments carried out in this Laboratory. It was found t h a t the addition of 2% biacetyl to 2-pentason4 in their investigation of the gas-phase pyrolysis of none, photolyzed in the liquid phase a t 2537 A. and Oo, reduced the quanCH3COOCD2CDZH tum yield of ethylene by 90%. (15) I t should be noted t h a t the quenching effect of these solvents (methanol, ethanol, n-heptane) is a t least one order of magnitude smaller than the effect of biacetyl. Also, the efficiency of this process increases with decreasing C-H bond strength in the solvent molecule. This would make biacetyl a rather ineffective additive. (16) P. E. Frankenburg and W. A. Noyes, J r . , J . Am. Chem. Soc., 76,2847 (1953). (17) H.L. J. Backstrom and K. Sandros, Acta Chem. Scand., 12, 823 (1958). (18) See for instance: (a) H. L. J. BBckstrSm, Z . p h v s i k . Chem., B26, 19 (1934): (b) Y. Hirschberg and L. Farkas, J . Am. Chem. Soc., 69, 2453 (1937); (e) H. L. J. BBckstrom, A. Steneryr, and P. Perlmann, Acta Chem. Seand., 12, 8 (1958); (d) J. N. Pitts, R. L. Letsinger, R. P. Taylor, J. M. Patterson, G. Recktenwald, and R. B, Martin, J . Am. Chem. Soc., 81, 1068 (1959).
Il45/RT k ~ / = k 0.99e ~
Similarly, the liquid phase photolysis of CHaCOCH2CHDCD2H yielded a value of 1.15 i0.15 kcal./mole for the difference in activation energy between the transfer of a D and H-atome3 The actual values of CzD3H/C2H2Dz obtained in photolysis of this ketone, (19) At 2200 A. the extinction coefficients of the two compounds do not differ b y more than 5 % a s measuied on a Gary-15 spectrophotometer. The quenching cross sections in mercury sensitized decomposition have not been determined.
Jan., 1963
FORMATIOX OF E T H Y L E N E IN
a t a given temperature, agree closely with the ratios C2D4/C2D3Hgiven in Table 11. These similarities are not unexpected if one considers that in all cases, decomposition occurs by may of a similar six-centered intermediate. The actual mechanism of this process in the pyrolysis has been discussed in detail by Blades and Gildersom4 I n the vapor phase photolysis of CH&OOCD&D2H, the effect of temperature on the ratio CzD4/C2D3His much less pronounced than in the liquid phase. This is not surprising in view of the fact that in the vapor phase, as indccated by the wave length effect, collisional degradation of the vibrational energy levels is not as effective as in the liquid phase. It obviously may be expected that the magnitude of the temperature effect diminishes with a decrease of the incident wave length.3 CH3COOC2H5-CH3COOC2D5 (1:l).-The data of Table I1 demonstrate that there exists a considerable intermolecular isotope effect. In view of the fact that a t 273 and 193"K., the values obtained for the ratio CzH,/C2D4 in the photolysis of equimolar mixtures of CH3COOC2H5 and CH3COOCzD5are the same as those which can be deduced from the yields of ethylene observed in the photochemical decomposition of C€&COOCzHs and of CHaCOOC2D6, it may be concluded that the intermolecular isotope effects are not due to a transfer of energy from CH3COOC2D6to CH3COOC2H5. A comparison of the ethylene yields given in Table I1 shows that below 273"K., the magnitude of the effect of temperature on the relative quantum yields of the intramolecular processes varies according t o the following sequence CH3COOCZDS > CH3COOC2D4H > CH3COOCZH5 This trend probably is related to a diflerence in activation energy of decomposition between the various excited molecules, which can be accounted for qualitatively by considering the C-H and C-D bond strengths and the number of @-deuteriumand hydrogen atoms in the molecule. This interpretation is substantiated by the observation that biacetyl quenches the decomposition of CH,COOC2Ds more strongly than that of CH3COOCzH5. The higher activation energy for the decomposition of the deuterium labeled compound implies a longer dissociative lifetime and a larger -probability to take part in energy transfer processes. In Connection it may be noted that the more pronounced quenching effectof oxygen on the photochemical decomposition Of CH3COCH&HDCD,H3 SLS
PHOTOLYSIS OF
ETHYL ACETlTE
167
compared to CH&OC3H, also has been ascribed to a longer dissociative lifetime of the labeled compound. It may finally be pointed out that the decrease of the relative quantum yield of ethylene in the liquid photolysis of ethyl acetate and 2-pentanone3 clearly demonstrates that besides cage effects, deactivation processes have to be considered as well to account for the low quantum yields of the dissociative processes in some of the compounds containing carbonyl. (C) Solid Phase.-The results of Table I11 show that although the values of the ratio C2D4/C2D3H obtained in the solid phase photolysis of CH3COOCD2CD2H a t 77°K. are larger than those obtained in the liquid phase, they are considerably smaller than the expected value of 330 which can be extrapolated from the liquid phase data. The fact that the intermolecular isotope effect is also considerably lower than may be predicted from a consideration of the liquid phase data indicates that B hindered rotation cannot entirely account for the low intramolecular isotope effect. Apparently, an ineffective collisional deactivation prior to decomposition could explain the observed trends. I n agreement with this interpretation is the significant observation that the quantum yield of the formation of ethylene is not greatly affected by a decrease in temperature from 77 to 4"K., indicating that the energy requirement for decomposition observed in the liquid phase is of less importance in the solid phase. More direct evidence for a less efficient energy degradation in the solid phase as compared to the liquid phase follows from the observed effect of wave length on the solid phase photolysis of CH3COCH2CHDCD2H.20These experiments show that in contrast with the liquid phase data,3 the intramolecular isotope effect varies with wave length, and actually, just as in the gas phase, increases with increase in wave length. The lowering of the intra- and intermolecular isotope effects with a decrease of the temperature of irradiation probably is indicative of a less efficient energy transfer. The fact that a t constant irradiation temperature but for different temperatures of deposition similar trends are observed demonstrates that besides the irradiation temperature the structure of the solid may also affect the intramolecular isotope effect. (20) It should be noted that the appearance of non-condensable pioducts in the solid phase photolysis of ethylacetate a t 4'K. implies that a t this temperature some of the ethylene may eventually be produced by socondttry radical reactions. The ketone experiments have been carried out for comuarison because in this case no other aroducts have been observed besides ethylene, acetone, and I-methylcyclobutanol. Also, the absence of ethyl radicals in the photolysis of 2-pentanone excludes the formation of ethylene by radical disproportionation reactions.
