Solvent effects in the reactions of free radicals and atoms—VI : Separation of polar and resonance effects in the reactions of chlorine atoms

Abstract

Aromatic solvents have a large effect upon the reactivity of chlorine atoms toward carbon-hydrogen bonds having different bond dissociation energies. When the difference in reactivity of two carbon-hydrogen bonds is determined by polar effects, aromatic solvents have little effect upon the relative reactivities toward chlorine atoms. Two polar effects are important in the reactions of atoms and radicals with carbon-hydrogen bonds. When extensive bond rupture in the transition state occurs, a resonance contribution to the transition state involving charge separation is important (e.g. bromine atoms). For an atom as reactive as chlorine where little bond breaking occurs in the transition state, the polar effect is best considered in terms of a repulsion curve between the chlorine atom and the carbon-hydrogen bonds, repulsion being higher the lower the electron density of the carbon-hydrogen bond.