Polarity of Free Radicals in Hydrogen Abstraction Reactions

Abstract

Abstract The electrophilicity of the CF3 radical in hydrogen abstraction reactions was studied theoretically by the CNDO/2 method. The activation energy for hydrogen abstraction by the CF3 radical from aliphatic hydrocarbons decreases with an increase in the electron density on the hydrogen atom to be abstracted. The origin of the electrophilicity of the CF3 radical can be well understood by the stabilization energy due to delocalization of electrons (SEDE) between the radical and substrate. The electrophilic stabilization energy denned by the SEDE from substrate to the radical governs the relative reactivities of aliphatic hydrocarbons, CH4<CH3CH3<CH3CH2CH3<(CH3)3CH, which corresponds to the order of the increase in the electron density. The results indicate that the radical is an electrophile if the relative reactivities of substrates are governed by the electrophilic stabilization energy, and a nucleophile when the reactivities are governed by the nucleophilic stabilization energy (the SEDE from the radical to substrate). SEDE was calculated for the reactions of the OH and the CH3 radicals with CH4and CH3CN; the results indicated that the OH radical is electrophilic and the CH3 radical nucleophilic. These polarities of radical reagents are discussed in connection with their physicochemical properties such as ionization potentials and electron affinities.