Stabilities of Substituted Benzyl Radicals: Dissociation Rates of Amino-, Hydroxy-, and Cyanoethylbenzenes

Article abstract of DOI:10.1021/j100358a023

Rates of thermal unimolecular decomposition of amino-, hydroxy-, and cyanoethylbenzenes have been determined by the very low pressure pyrolysis technique.Assuming equal A factors, these rates, relative to the rate of dissociation of ethylbenzene, yield the following ortho, meta, and para, respectively, substituent effects on benzylic C-C bond strengths (kcal/mol): NH2, -2.7, -0.3, -1.7; OH, -1.7, -0.7, -1.1; and CN, -0.3, 0.4, -1.5.These effects are generally smaller than those for substituted anisoles and follow different trends.Along with the results of ESR studies of Nicholas and Arnold (Can.J.Chem. 1986, 64, 270), the present substituent effects indicate that a 1-G change in the benzylic hyperfine coupling constant corresponds to a 1.8 kcal mol^-1 change in bond strength.