Extension of Structure-Reactivity Correlations for the Hydrogen Abstraction Reaction to Methyl Radical and Comparison to Chlorine Atom, Bromine Atom, and Hydroxyl Radical

Abstract

Recently we presented structure-reactivity correlations for the gas-phase rate constants for hydrogen abstraction from sp3-hybridized carbon by three electrophilic radicals (X• + HCR3 → XH + •CR3; X = Cl•, HO•, and Br•); the reaction enthalpy effect was represented by the independent variable ΔrH and the “polar effect” by the independent variables F and R, the Hammett-Taft constants for field/inductive and resonance effects. Here we present a parallel treatment for the less electronegative CH3•. In spite of a limited and scattered data base, the resulting least-squares fit [log k437(CH3•) = ‒ 0.0251(ΔrH) + 0.96(ΣF) ‒ 0.56(ΣR) ‒ 19.15] was modestly successful and useful for initial predictions. As expected, the polar effect appears to be minor and its directionality, i.e., the “philicity” of CH3•, may depend on the nature of the substituents.