Pyrolysis of Alkyl Benzenes. Relative Stabilities of Methyl-Substituted Benzyl Radicals

Article abstract of DOI:10.1021/j100454a007

Decomposition rates for β C-C bond scission in several methyl-substituted ethylbenzenes were measured in a very low pressure pyroly (VLPP) system from 1050 to 1200 K, and relative stabilities of the resulting substituted benzyl radicals were derived.Relative to ethylbenzene, rates were highest for o-methyl-substituted ethylbenzenes, and little affected by meta and para substitution.Activation energy differences were approximately separable into ortho and nonortho contributions, amounting to a lowering of the activation energy, relative to ethylbenzene, of 1.3-1.7 kJ/mol per m- or p-CH3, and 5.0-6.3 kJ/mol per o-CH3.Rate differences were explained in terms of a partial relaxation of steric interaction in the activated complex.Methyl inductive effects were found to be very small, amounting to a decrease of < 1.7 kJ/mol per CH3 in the Gibbs energy of formation of the radicals produced.