JOURNAL O F THE A M E R I C A N C H E M I C A L S O C I E T Y Registered in U. S. Palcnt O&e.
@ Cogyrinhl, 1970, by the American Chemical Society
SEPTEMBER 23, 1970
VOLUME 92, NUMBER 19
Physical and Inorganic Chemistry Kinetics of the Gas-Phase Thermal Bromination of Acetone. Heat of Formation and Stabilization Energy of the Acetonyl Radical'" Keith D. King,'b David M. Golden, and Sidney W. Benson Contribution from the Department of Thermochemistry and Chemical Kinetics, Stanford Research Institute, Menlo Park, California 94025. Received April 10, 1970 Abstract: The gas-phasethermal bromination of acetonehas been studied spectrophotometricallyover the temperature range 221-345 '. The pressure of acetone was varied from 2 to 223 Torr and of Br2from 3 to 195 Torr. Above 252", the kinetics conform to the mechanism Br2 2Br, Br CHsCOCH3 CHsCOCH2 HBr [(l) forward, (2) reverse], CH3COCH2 Br2 + CH3COCH2Br Br (3). From measurements of initial rates, log (k1lM-l sec-l) = (11.0 f 0.22) - (13.9 f 0.6)/0,where 0 = 2.303RTin kcal/mol. Inhibition by added HBr was investigated and the ratio k2/k3 is given by log (kz/k3)= (-0.64 f 0.41) - (1.8 f 1.1)/0. The results, combined with the assumption that Ea = 1 f 1 kcal/mol, lead to AHr02gs(CH3COCH2) = -5.5 f 1.8 kcal/mol and DH029s(CH8COCHrH) = 98.3 f 1.8 kcal/mol. These values agree well with previous estimates obtained from iodination studies and confirm the conclusion that there is no resonance stabilization in the acetonyl radical due to conjugation of the radical site. The C-H bond strength in acetone is the same as that in ethane, but the value of AI is significantly lower than that for the corresponding reaction involving ethane and this difference is accounted for in terms of transition-state theory.
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he stabilization energy in the allyl radical due to delocalization of the unpaired electron may be defined as DHozss(n-C3H,-H) - DHo2e~(allyl-H). The value is -10 kcal/moL2" Measurements have also been made2b on other hydrocarbon radicals in which conjugation of the radical site is possible, e.g., methylallyl, pentadienyl, and benzyl. However, until recently, there were very few data on stabilization energies in ,&carbonyl radicals. From an analysis of the Arrhenius parameters for the decompositions of cyclobutyl ketones in terms of a biradical mechanism, the stabilization energy in the methylacetonyl radical has been estimated as 7.7 f 2 kcal/mol.3 Grzechowiak, (1) (a) This work was supported in part by Grant No. AP-00353-05 from the Air Pollution Control Administration, Public Health Service; (b) Post-Doctoral Research Associate, (2) (a) D. M. Golden, N . A. Gac, and S. W. Benson, J . Amer. Chem. Soc., 91,2136 (1969); (b) D . M. Golden and S. W. Benson, Chem. Reu., 69,125 (1969). (3) H . E. O'Neal and S. W. Benson, J . Phys. Chem., 72, 1866 (1968).
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Kerr, and Trotman-Dickenson4 have measured the activation energies for the abstraction of hydrogen atoms by NFr radicals from acetone and some hydrocarbons and, from a Polanyi relationship, they arrive at DHo(CH3COCH2-H) = 92 f 3 kcal/moL5 This bond dissociation energy, when compared with DHO(n-C3H7-H), leads to a value of 6 f 3 kcal/mol for the stabilization energy in the acetonyl radical. The difference between acetonyl and methylacetonyl is in accord with the difference between allyl and methylallyl.2b Unfortunately, the above value for DHo(CH3COCH2-H) is suspect since the Polanyi type of relationship is only applicable to reactions of atoms or radicals with a series of compounds of the same class.6 More(4) J. Grzechowiak, J. A. Kerr, and A.F. Trotman-Dickenson, Chem. Commun., 109 (1965); J . Chem. Soc., 5080 (1965). (5) J. A. Kerr, Chem. Rev., 66,465 (1966). (6) S. W. Benson and W. B. DeMore, Annu. Rev. Phys. Chem., 16, 297 (1965).
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over, recent measurements of the primary C-H bond strength in acetone' and the secondary C-H bond strength in methyl ethyl ketone* by the iodine-atom technique2b have shown that the stabilization energy in the acetonyl radical is zero and that in methylacetonyl is 2.7 kcal/mol. These values imply that a concerted mechanism must make a significant contribution to the decomposition of cyclobutyl ketones. The results for the iodination of methyl ethyl ketone are clearcut, but those for acetone are somewhat less reliable owing to the occurrence of heterogeneous processes.7 Therefore, an independent check on DHCzg8(CH3COCH2-H) seemed desirable. Since bromination studies have, in general, yielded reliable values for C-H bond strength^,^ we have sought to measure DHc298(CH3COCH2-H) from a study of the direct thermal bromination of acetone using spectrophotometric techniques which have proved to be highly successful in iodination studies. lo
sure obtained from the total pressure and the Brl pressure. Reaction began a t once and the absorbance-time trace was smooth, Mixing was virtually instantaneous except a t the lower temperatures (
