OXIDATION OF METHOXYHYDROXYCYCLOHEXADIENYL RADICALS BY QUINONES - INFLUENCE OF THE ISOMERIC STRUCTURE OF THE RADICAL ON THE RATE CONSTANT FOR ELECTRON-TRANSFER

o-, m-, and p-Methoxyhydroxycyclohexadienyl radicals, produced in aqueous solution by the reaction of OH with anisole, react with p-benzoquinone by electron transfer with rate constants of 1.2 × 109, ≤8 × 105, and 4.4 × 109 M-1 s-1, respectively, to yield p-benzosemiquinone and o-, m-, and p-hydroxyanisole. The lifetime of intermediate carbocations produced by oxidation of the methoxyhydroxycyclohexadienyl radicals is ≤1 × 10-6 s and the production of the hydroxyanisoles occurs synchronously with the reduction of p-benzoquinone. Substitution of H by methyl in p-benzoquinone leads to a decrease in the rate constants for oxidation of the methoxyhydroxycyclohexadienyl radicals. This decrease, which reflects the decrease in the redox potential of the quinone, is much stronger for the ortho isomer than for the para isomer. The distribution of isomeric methoxyhydroxycyclohexadienyl radicals, as determined by analysis of the build-up kinetics of different benzosemiquinones, is equal with that of isomeric hydroxyanisoles formed as final products from the cyclohexadienyl radicals by oxidation. © 1979 American Chemical Society.