THERMODYNAMIC SIGNIFICANCE OF RHO+ AND RHO- FROM SUBSTITUENT EFFECTS ON THE REDOX POTENTIALS OF ARYLMETHYL RADICALS

The electrochemical oxidation and reduction potentials of substituted benzyl, cumyl, and diphenylmethyl radicals are found to give linear correlations with σ+ and σ- respectively (ρ+ =-9.3,-6.8, and-5.6 respectively for the oxidations; ρ- = 13.5, 11.0, and 7.4 respectively for the reductions). Although the redox potentials reflect the thermochemical properties of both the radicals and the product ions, substituent effects on the ion dominate leading to a simple relationship between the oxidation potential and pKR+ and the reduction potential and pKa(R-H). The p values obtained from thermodynamic measurements are surprisingly insensitive to the nature of the solvent or the method of generation of the ion, implying that differential solvation effects in the various media are small. The relationship between the redox potentials and the charge separation in the transition state for solvolysis and the substituent effects on the Gibbs solvation energy of the ions are discussed. © 1990, American Chemical Society. All rights reserved.