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THE VAPOR-PHASE PHOTOLYSIS OF ACETIC ACID'
ABSTRACT
The photolysis of acetic acid (CHBCOOD) vapor has been investigated in the temperature range from room temperature to 285'C. Since CH3D formation is independent of temperature, it is certain that the primary process
CHSCOOD +k -+ CH3D +COr
occurs to the estent of about 10%. The results are colnples and suggest that three other primary processes may occur, viz.
CHzCOOD +hv -+ CH3CO+OD -+ CHx+COOD -+ CB,COO+D.
The abstraction reaction
CH,+CI-I,COOD-+ CH.t+CH,COOD
is of importance, and the results indicate that i t has an activation energy of 10.2 ltcal., and a steric factor of the order of 10W.
INTRODUCTION
The photolysis of acetic acid in the gas phase was first investigated by Farltas and Wansborough-Jones (j), who suggested the primary processes:
CH3COOH+hv --t CHI+COz (CH3COOH) z+ hv --t CHI+ COz+ CH3COOH (CH3COOH)z+hv --t C2H,+COZ+CO+HZO.
Burton (2) investigated the photolysis by mirror methods and suggested that radicals were formed by
CH3COOH+hv --t CHa+ COOH, followed by
COOH --t COZ+H, etc. In later worlc (3) he rejected this mechanism and suggested that H-atoms but no CH3 radicals were formed. He therefore proposed the primary step
CH3COOH+ hv --t CH,COO+H. Clusius and Schanzer (4) investigated the photolysis of CH3COOD in the
liquid phase, and concluded that two primary processes occur: CH3COOD+ hv --t CH3+ COOD CH3COOD+hv -+ CH3DfC02,
the first process being the more important. They suggested that CHI, found in the methane fraction, could be accounted for by the abstraction reaction
CH3+CH,COOD -+ CH,+CHZCOOD. In the present work the photolysis of CHBCOOD has been investigated in
the vapor phase over a more extended temperature range.
EXPERIMENTAL
The apparatus has been described previously (1). A Hanovia S-500 medium pressure mercury arc was used as a light source. The full radiation was used
'Manuscript received June 10, 1955. Contribution from the Division of Pure Clzemistry, National Researclz Council, Ottawa, Canada.
Issued as N.R.C. No. 3717. 2National Research Council of Ca?zada Postdoctorate Fellow, 1952-54.
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AUSLOOS AND STEACIE: VAPOR-PI-IASE PHOTOLYSIS 1531
t o obtain the maximum intensity. The intensity was varied by the use of calibrated screens. Two samples of CH3COOD were used, containing respec- tively 20y0 and 15y0 CH3COOH. The analysis of the products was done by taking off two fractions, one a t - 19G°C., and one a t - 150°C. These were then analyzed by a mass spectrometer. No attempt was made to determine con- densable reaction products.
One experiment was made with a Corning 7-54 filter, with a cutoff below 2400 A. This gave a rate approximately one quarter the normal rate. Since the filter has a transmission of about 40% a t 2537 A and decreases sharply below this, i t appears that the effective radiation was mainly in the range from 2400 f% t o 2537 A.
RESULTS AND DISCUSSION
The results are given in Table I. The main reaction products found were CO, COz, CHI, CHSD, and C2H6. At temperatures above 150°C. H2 and H D were also present, and they constituted about 10yo of the reaction products a t temperatures above 250°C. Very small amounts of CHzD2 and C2H5D were also detected. The amounts of these products were always less than two per cent of the methane or ethane fraction, and their formation is probably due to the presence of small amounts of CH2DCOOD in the starting material.
The Primary Process
The results in Table I indicate that for runs a t constant intensity the rate of formation of CH3D is constant within experimental error, and is independent of temperature in the range from 27 to 287°C. On the other hand, the rate of formation of CHI increases steadily with increasing temperature as might be expected if i t is formed in a secondary reaction. This constitutes a definite proof of the occurrence of the primary process
If runs 9, 12, and 15 are compared it can be seen tha t the rate of formation of CO decreases sharply below 10O0C., while a t higher temperatures (compare runs 2-7) it is nearly independent of temperature. This suggests that CH3CO is formed in the primary process
and decomposes thermally. A comparison of CH3D with COz shows that primary process [I] accounts
only for about 10yo of the COz formed. Nevertheless the rate of COz formation is nearly independent of temperature over the whole range. This excludes the formation of COz by a secondary process unless it is one with a very low activation energy. There appear t o be two possibilities:
followed by COOD -+ C02+D.
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TABLE I
Products, cc./min. X 10" 2CH4+2C?HG Run Temp., Pressure, Time, Relative k a / k ? ~ lo13 cC+COS-
"C. cm. mln. intensity CO Con CH3D CHn CZHG HS HD c1n.3'~ molecules-* CH3D
P
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AUSLOOS AND STEACIE: VAPOR-PHASE PI-IOTOLYSIS 1533
This will only account satisfactorily for C 0 2 production if [ I ] is very fast, which appears to be the case (8). The ailalogous reaction
is known to occur readily (7). Reaction [ I ] will be less exothermic than [2], but is probably feasible thermochemically.
followed by CH3CO2 --t CH,+CO?.
In solutioil the acetate radical appears to have considerable stability (6), which casts some doubt on the rapid formatioil of CO? by [3] a t low tempera- tures.
Either mechailism illvolves the formation of D-atoms. Since no D ? is formed, and H 2 and H D are o~lly formed a t higher temperatures, recombination is appareiltly negligible compared with the abstraction reaction
Hz will be formed from the CH3COOH present in the acetic acid. The quantity of H Z is, however, rather large compared with that of H D , and the possibility of the loss of an H-atom from the methyl group in the primary process is not excluded.
In ally case the very small amount of H z and H D relative t o CO? shows that most H- or D-atoms disappear in some other way to form coildellsable pro- ducts, such as HDO and D2O. The iildepeildence of CH3D formation with temperature rules out the reaction
CH,+D --t CH3D. 151
I t is also possible to explain the above results in a somewhat simpler way by postulating the occurrence of reactions such as
CH3COOD*+CH3COOD --t CH3C02+CH3CO+D20 or
( C H , C O O D ) ~ + ~ V --t C H ~ C O Z + C H ~ C O + D ~ O .
I t should be noted that a t 27OC. under the experimental coilditions used about goy0 of the acetic acid is in the form of double molecules. At 150°C., however, the number of double molecules is negligible for the pressures used in this work. I t should be pointed out that processes [j:], [I]:], and [111] are nearly illdepelldeilt of temperature. I t may therefore be concluded that double molecules do not affect the primary processes.
The Abstraction Reaction The fact that the formation of CH3D is essentially independent of tempera-
ture, while that of CHI increases rapidly with increasing temperature, indicates that only H-atoms on the methyl group are involved in abstraction reactions t o an appreciable extent, viz.
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1534 CANADIAN JOURNAL OF CHEMISTRY. VOL. 33
If this is the only way in which CH4 is formed, and if all CzH6 results from recombination,
2CH3 --t C2H6, [71
then R~H,/R~,H,[AI = kG/ki,
where RCH4 and RCZH6 are the rates of formation of the products indicated and A represents acetic acid. In applying this relationship it is necessary to correct the CH4 for the small amount formed by a process a~lalogous t o [I] from the small amount of CHSCOOH present.
The results given in Table I show that k,/k$ is not appreciably affected by a variation of the intensity by a factor of 15. The corrected values of k,/k: are given in Fig. 1 in the form of an Arrhenius plot. A good straight line is
1
FIG. 1. Arrhenius plot of ks/k;.
obtained a t higher temperatures with some curvature a t low temperatures. The curvature may be connected with the presence of double molecules and wall reactions. The slope of the curve leads t o a value of 10.2 ltcal. for E6-3E7 = E7. This is of the expected order of magnitude for such a reaction. The steric factor of reaction [7] is estimated to be -loF3.
Material Balance On the basis of the above mechanism primary process [I] implies that
All other primary processes lead to lCH3 for each COz or CO formed. Hence it would be expected that
However, nothing has been said of the fate of the CH2COOD radical formed
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I AUSLOOS AND STEACIE: VAPOR-PHASE PHOTOLYSIS 1535 I
I by abstraction. If i t is assumed tha t this disappears only by recombination with methyl, then
I This ratio will be greater than unity if some CHzCOOD radicals disappear by I dimerization, but will be low if CHzCOOD is formed by reaction [4] and is
followed by the addition of methyl. A value of [ A ] somewhat less than unity is therefore to be expected. Any other processes leading to coildensable products with elimination of CO or CO? will also lower the ratio.
The last column of Table I gives values of the ratio [ A ] . There is considerable scatter, but i t will be seen that from 80 to 90% of the methyls are accounted for, which seems quite satisfactory in the absence of a detailed 1inowledge''of the condensable products.
REFERENCES
1. Aus~oos , P. and STEACIE, E. W. R. Bull. soc. chim. Belges, 63: 87. 1954. 2. BURTON, M. J. Am. Chem. Soc. 58: 692. 1936. 3. BURTON, ivI. J. Am. Chem. Soc. 58: 1645. 1936. 4. CLUSIUS, I<. and SCHANZER, W. Ber. 75, B: 1795. 1942. 5. FARKAS, L. and WANSBOROUGH-JONES, 0. H. Z. physik. Chem. B, 18: 124. 1932. 6. FRY, A., TOLBERT, B. M., and CALVIN, M. Trans. Faraday Soc. 49: 1444. 1953. 7. KUTSCHKE, K. 0. Unpublished work. 8. WEST, W. and ROLLEFSON, G. I<. J. Am. Chem. Soc. 58: 2140. 1936.
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