Reactions of Alkyl Radicals with Substituted Toluenes and the Effect of Substituents on Dissociation Energies of Benzyl CH Bonds

Abstract

AbstractReactions of isopropyl and of undecyl radicals with meta‐ and para‐substituted toluenes are reported. The results demonstrate that the reactivities of toluenes are due to both benzyl‐H abstraction and addition of the alkyl radicals to the aromatic ring. Relative reactivities yield curved Hammett plots, consistent with kinetic data reported by Dütsch and Fischer. Abstractions and ring additions occur with comparable rates, but opposite Hammett slopes. Addition is favored by electron‐withdrawing and abstraction by electron‐donating substituents. The effects of substituents on the dissociation energies of benzyl CH bonds are shown to be the major factor influencing reaction rates for benzyl‐H abstraction by alkyl radicals.