Homolytic bond dissociation energies of the carbon–halogen bonds in the benzyl halide radical anion intermediates formed in radical nucleophilic substitution reactions

Abstract

A simple method has been devised for estimating the homolytic bond dissociation energies (E_bd) for cleavage of the C–X bonds in the radical anion intermediates formed in radical nucleophilic substitution reactions of benzyl chloride, benzyl bromide, and m- and p-substituted benzyl bromides. The method consists of combining values for: (a) the E_bd of the C–X bond in PhCH₂X; (b) the redox potential of PhCH₂X; and (c) the redox potential of the ionic fragment (X^––). Methods of obtaining these quantities are presented. The estimates indicate: (a) that the C–Cl bond in the PhCH₂Cl˙^– radical anion is thermodynamically unstable by about 6.5 kcal mor^–1 toward cleavage to the PhCH₂˙ radical and Cl^– ion; and (b) that the C–Br bond in the PhCH₂Br˙^– radical anion is endoenergetic toward a comparable cleavage by only about 3 kcal mol^–1. On the other hand, the bond dissociation energies of the C–X bonds in these radical anions estimated for the cleavage to form a PhCH₂^– ions and X˙ atoms are not much less than those for the corresponding homolytic cleavage in the neutral parent benzyl halides. These thermodynamic estimates are consistent with various kinetic and EPR experimental results which reveal that cleavage of the C–X bonds in benzyl chloride and bromide is concerted with the addition of an electron.