Reactions 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 CÀH bonds are shown to be the major factor influencing reaction rates for benzyl-H abstraction by alkyl radicals.Introduction. -In 1982, Dütsch and Fischer published Nucleophilic Character of the tert-Butyl Radical. Absolute Rate Constants for the Reactions with Substituted Toluenes [1]. tert-Butyl radicals were produced by pulsed photolysis of di(tert-butyl) ketone at 488. The decay of tert-butyl radicals was measured by time-resolved electron paramagnetic resonance (EPR) spectroscopy in eleven meta-or para-substituted toluenes. A plot according to the Hammett equation (Eqn. 1) of the rate constants vs. Hammett substituent constants, s, was found to have a positive slope, 1. The s values reflect the electron-donating or -withdrawing abilities of the substituents, k X is the rate constant for reaction of tert-butyl radicals with the meta-or para-substituted toluene and k H is that with toluene. Because electron-withdrawing groups enhanced the rate of disappearance of tert-butyl radicals, following usual practice, these results were interpreted to mean that the transition state for benzyl-H abstraction by tertbutyl radicals is stabilized by a polar effect, with charge separation in contributing dipolar structures as shown in Eqn. 2. This conclusion was based on the assumption that the only mode of reaction between tert-butyl radicals and the toluenes is benzyl-H abstraction. The positive slope, 1 = 0.59, classified the abstracting tert-butyl radical as nucleophilic, capable of accommodating a partial positive charge, with a partial negative charge on the benzyl C-atom. Electron-withdrawing substituents, positive s, would stabilize the negative charge and speed the reaction. Electron donors, negative s, would do the reverse.