Sept. 20, 1955
PHOTOLYSIS AND
experimental results in this direction. There are several possible ways in which catalase can decompose H202 without causing a detectable 018-enrichment in the oxygen produced. First, the oxygen may be produced through an action of the enzyme on H202without breaking the peroxide bond, e.g., by dehydrogenation process. This, however, seems most unlikely, because the peroxide bond is the weakest link in the HzOz molecule. Since catalase is the most efficient of all enzymes, it is very unlikely that such an enzyme would skip the weak peroxide bond and try to break the much stronger 0-H bonds. Secondly, one may suggest that the reaction is diffusion-controlled, so that no isotope effect is not detected. But this suggestion contradicts experimental evidence. Chance estimated the activation energy for the catalytic decomposition of HzOz by catalase to be 1400 cal./mole or less,6 whereas for a diffusion-controlled reaction in aqueous solution a t room temperatures the activation energy should be close to 4.6 kcal./mole. The third possibility is that the splitting of the peroxide bond by catalase is preceded by a slower and hence rate-determining step in which the substrate combines with the enzyme; ;.e., once the substrate succeeded in combining with catalase, the peroxide
[CONTRIBUTION FROM
THE
PYROLYSIS O F ACETONE-d,q
4719
bond is immediately split irrespective of whether i t is 016-016 bond or 016-018 bond. The transition of compound I to compound I11 in Fig. 1 has some resemblance to this possibility. Finally, the peroxide bond may not be completely broken until fairly stable bonds have been formed between the two 0 atoms and catalase. The energy consumed in breaking the stronger 016-018 bond is mostly compensated by the energy gained in forming the stronger FelI1-Ols bonds, etc.; and if the splitting of the peroxide bond and the formation of the new enzyme-substrate bonds are not isolated events but occur in a more or less continuous transition, this would minimize the activation energy as well as the isotope effect. The mechanisms of most catalytic processes, including the one illustrated in Fig. 1, probably have some feature of this continuous transition from old to new bonds. I n general, the efficiency of the catalyst depends largely on the nature of this kind of transition. Acknowledgment.-In preparing this work, the present author benefited through conversations with Professors H. G. Cassidy, L. Onsager, A. Patterson, s.J. Singer, J. M. Sturtevant, H . H. Wasserman and Dr. E. H . White. SEW HAVEN,CONNECTICUT
COMBUSTION BRANCH, u.
s. NAVALORDNANCE TESTSTATION]
The Photolysis and Pyrolysis of Acetone-d6 in the Presence of Ethane and of Acetone in the Presence of Ethane-d6 BY JAMES R. MCNESBYAND ALVINS. GORDON RECEIVED MARCH1, 1955 The photolysis and pyrolysis of mixtures of acetone-& and ethane, and of acetone and ethane-& have been studied with particular emphasis on abstraction of H and D and by CD3 and CHs. respectively (reaction 1and 2 ) . The activation energies found are El = 11.5 kcal. and EZ = 14.8 kcal. Changing light intensity and introducing additional surface has no measurable effect on the relative rates of (1) and (3). Small amounts of ethylene-&, H D and Dz, but no HZwithin the accuracy of the mass spectrometer are found in the products of the photolysis and pyrolysis of the ethane-&-acetone mixture. HD, Hz but no DI were found in the acetone-&-ethane reaction, A fifty-fold increase in surface had no measurable effect upon the H D / Dzratio. Similarly, argon does not appear to affect the ratio measurably. Both the HD/Hz and HD/Dz ratios increased with temperature. The most reasonable interpretation of the hydrogen analyses appears to be that a H (or D) atom is formed by decomposition of the ethyl radical and the H (or D) atom subsequently either abstracts H or D from acetone or ethane, or recombines with a similar atom.
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Introduction very highly deuterated acetone-de. The acetone-& The measurement of activation energies of meta- used by Trotman-Dickenson, Birchard and Steaciel in their work on abstractions by CD3 of hydrogen thetical reactions of the type CH3 f H R CH4 R has been greatly facilitated in recent years by a from hydrocarbons contained some 30y0 acetonetechnique first introduced by Steaciel which in- dg. This results in large percentages of the total volves the use of the CD3 radical. The rate of for- CD3H arising from the partially deuterated acemation of CD3H is compared with the rate of forma- tone. We calculate about 80% in the case of CPH6. tion of CDk in the reaction of CD3 with acetone-de Recently the reaction between CD3 and CH4* was from which the CDs radicals are generated. This studied in this Laboratory. Since methane has only comparison is made over a temperature range and one kind of hydrogen, the significance of these exthe pre-exponential factors and activation energies periments is clear. In the present investigation the reactions are obtained relative t o those for the reaction of CD3with acetone-&. T o obtain a meaningful measCD3 CZH6 +CD3H + CiHj (1) urement, the amount of CD3H arising from the inCH3 CZDBCHiD + CzDj (2) completely deuterated acetone must represent a were studied since they can be interpreted unambigvery small proportion of the CD3H produced by uously by the observation of the CD3H/CDd and the abstraction of hydrogen from the compound CH4/CH3D ratios, respectively. Previous work3 under investigation. This requirement demands a (2) J. R. McNesby and A. S. Gordon, THISJOURNAL, 76, 4186
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(1) A. F.Trotman-Dickenson, J. R. Birchard and E.W.R. Steacie, J . Chcm. Phrs., 18, 163 (1951).
(1954). (3) J. R. McNesby and A. S. Gordon, ibid.. 1 6 , 1416 (1954).
J. R. MCNESBYAND ALVINS . GORDON
4720
showed that when acetone and acetone-& reacted with CD3, the same activation energy difference (1.7 kcal.) existed as for the reactions with CHI. I t was of interest to determine whether this activation energy diflerence is dependent upon the nature of the compound from which H and D are abstracted. Experimental Acetone-& was prepared by the method described prev i o ~ s l y . ~Mass spectrometer analysis showed the product t o be 97.5% acetone-d~.and 2.5% acetone-dj. The ethane was Phillips research grade material and contained a trace of methane. Acetone was distilled from commercial material. The ethane-& was kindly given to us by Drs. R. E. Varnerin and F. 0. Rice. It contained 97.6% ethane-de, 2.2% ethane-&, 0.2% CD4 and a trace of CD3H. Two master mixtures were prepared. For the study of the reaction of CD3with ethane, the mixture contained ethane and acetone46 in the ratio 2.84, and for the investigation of the C ~ D Bthe , acetonelethane-de ratio was reaction of CHI 0.53. The light source was a flat spiral Hanovia SC-2537 lamp. The photolyses were carried out in aluminum block furnaces, fitted with double windows, one of fused silica and the other a Corning filter to remove the 1849 mercury line. The fused silica, cylindrical reaction vessels were 4 cm. i.d. X 4 cm. high. The reaction vessels were protected from mercury vapor with a gold leaf trap. The reaction vessel in one furnace was charged with enough silica rod to increase the surface area sixfold. The total pressure was 100 mm. for the acetone-deethane mixtures, and 80 mm. in the case of acetone-ethanede mixtures at each of the temperatures a t which the mixture was photolyzed. The mixtures were pyrolyzed in Pyrex reaction vessels using the technique described e l ~ e w h e r e . The ~ vessels were charged a t room temperature with about 100 mm. of the master mixture in the case of acetone-de and ethane, and 50 mm. in the case of acetone and ethane-&. I n experiments with added surface the reaction vessels were packed with Pyrex wool and the surface area estimated from the weight of the Pyrex wool and the approximate diameter of the fibers. The analysis for CDIH and CD4 in the investigation of reaction (1) has been described previously;% and involves analyzing the fraction of the products volatile a t -195'. The analysis for CH8D and CHI in the study of reaction (2) again involved a low temperature fractionation and presented a considerable problem. When a mass spectrometer is in general use, complete removal of mass 18 water vapor from the instrument becomes a virtual impossibility. Since small amounts of CD4 and CD3H were present in the initial mixture and some CHIDz was formed in the reaction, the mass 18 peak in the mixture had contributions from a number of compounds. In addition, a large amount of CO is formed which has a considerable contribution a t mass 16, while both CHzDz and H 2 0 have contributions a t masses 16 and 17. The Consolidated Engineering mass spectrome-
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Vol. 77
ter in use in this Laboratory is equipped with a high resolution slit arrangement which facilitates separating the mass 17 due to OH and the mass 16 due to 0, from the corresponding 17 and 16 peaks arising from CHID, CD2H, and CH4, CDa, CHzD, respectively. The mass spectrum of CHI measured on the high resolution instrument compared very well with the cracking pattern in the literature after suitable adjustment of the instrument, and with that previously measured on our instrument without high resolution. It was therefore assumed that the cracking pattern of CD4, CDIH, CD2H2 and CHPDagreed with the literature with the most recent CD3H pattern being used.3 A typical methane analysis is given in Table I.
Results and Discussion Two methane producing reactions occur when a mixture of acetone-ds and ethane are photolyzed or pyrolyzed. CD:
CD, t C2Hs --+ CDIH + CZHS + CDaCOCD3 +CD4 f CDzCOCD3
(1) (3)
Nixtures of acetone-de and ethane were photolyzed in the range 246-405' and pyrolyzed from 377-321°. Since the CD3HjCDd ratio for the photolysis and pyrolysis of acetone-do is only about 0.03, no correction in the CD3H,/CD4 ratios was applied when mixtures were run. The results are given in Table 11. No trend with time was observed in the CD3H,'CD4 ratios since the reactions were stopped after about ly0reaction. A least squares evaluaticn of the Xrrhenius plot gave E1 - E3 = 0.21 0.04 kcal. (standard deviation). Since E3 = 11.3 * 0.2 k ~ a l . , " ~ E =1 11.5 f 0.2 kcal. The Arrhenius plot gives the ratio of pre-exponential factors AljA3 = 1.21. It is of interest that the cruder experiments of Trotman-Dickinson and Steacie' gave E , - R3 = 0.1.
*
REACTION T, 'C.
246 246 275 275 309 309 309 336 336 405 405 405 477 498 50 1 524 524 320" 320" 319 R1
