Factors controlling reactivity in hydrogen abstractions by free radicals

Roberts and Steel have proposed an empirical calculation for obtaining energies of activation for hydrogen abstractions by radicals, A(.) + H-B. The results of their calculations led them to: (a) conclude that, in identity exchanges (A = B and Delta H = 0), the bond dissociation enthalpy D(A-H) is a significant factor; and (b) suggest that antibonding between A and B is not an important factor, Both findings are in conflict with our conclusions from results of our model for hydrogen abstractions. It is shown here that an examination of experimental results for several identity hydrogen exchanges does not support (a) and (b). Antibonding is important and, in identity exchanges, D(A-H) is not an important factor.