. Can. J. Chem. 67, 1541 (1989). Rate constants for the abstraction of hydrogen from benzene, toluene, and cyclopentane by methyl and ethyl radicals have been measured relative to the corresponding abstraction reaction from ethylene. The method is based on the effect on the rates of formation of methane and ethane of the addition of small quantities of the reactants to the thermal chain reactions of ethylene in the temperature range 650-770 K. Taking the following values of the rate constants for the reference reactions (R = 8.314 J mol-' K-I):[4] C2Hs + C2H4 -* C2H6 + C2H3; log k4 (L mol-' s-I) = 8.2 t-0.2 -(62200 t-3000)/2.3RT the following rate constants were measured:[8] C2Hs + C6H6 -* C2H6 + C6HS; log kg (~m 0 1 -' S-') = 8.8 i 0.3 -(62200 i 5000)/2.3RT [ l l ] CH3 + C7Hg + CH4 + C7H7; log kll (L mol-' s-') = 7.4 i 0.5 -(29000 ? 7000)/2.3RT[12] C2HS + C7H8 'C2H6 + C7H7; log k12 ( L m~l -' s -~) = 7.7 * 0.5 -(37000 ? 7000)/2.3RT[15] CH3 + c-CsHlo -CH4 + c-C5Hg; log klS (L mol-' s-I) = 8.3 2 0.5 * (36000 ?7000)/2.3RTThe values of the activation energies are discussed in relation to the dissociation energies of the C-H bonds in the reactants.

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This paper is dedicated to Professsor J . A . Morrison S. I. AHONKHAI and M. H. BACK. Can. J. Chem. 66,578 (1988). A method is described for the measurement of relative rate constants for abstraction of hydrogen by methyl radicals from ethane and from ethylene based on the effect of the addition of small quantities of ethane on the rates of formation of products in the thermal reactions of ethylene. The ratio of rate constants for reactions over the temperature range 673-733 K was given by the following expression: 3500 * 500 log k4lk3 = 0.0 2 0.1 + 2.3RT (R = 1.987 cal mol-I deg-I). Using a value of k4 from the literature the following expression was obtained for k3, over the same range of temperature:It was concluded that a previous measurement of k3 in a lower range of temperature was too close to the value of k4

A method is described for the measurement of relative rate constants for abstraction of hydrogen from ethylene at temperatures in the region of 750 K. The method is based on the effect of the addition of small quantities of propane and isobutane on the rates of formation of products in the thermal chain reactions of ethylene. On the assumption that methane and ethane are formed by the following reactions, (2 1 (7)