Flash photolysis studies of phenyl-substituted phenols, quinones, and the corresponding free radicals. Part 1. Quantitative study of the dimerization of phenyl-substituted phenoxyl radicals

Abstract

Kinetic and thermodynamic parameters of equilibrium systems of phenyl-substituted phenoxyl radicals and their dimers are described. The experimental procedure involves measurement of absorption maxima and extinction coefficients and recording of the kinetics of intermediates by a flash-photolysis apparatus. The phenoxyl radicals were generated in three different ways: (a) the classical way, i.e. oxidation of the corresponding phenol with a one-electron transfer agent, (b) by photolysis of the corresponding dimers, and (c) by photolysis of the parent phenols. The transition states for radical dimerization and decay of the less sterically hindered phenyl-substituted phenoxyl radicals are less ‘strict’ compared with those for similar reactions involving t-butyl-substituted phenoxyl radicals. ΔS^‡₁ and ΔS^‡_–1 for phenyl-substituted phenoxyl radicals (ΔS^‡₁ 0–6, ΔS^‡_–1 4 cal mol^–1 K^–1) are much higher than those for t-butyl-substituted phenoxyl radicals [ΔS^‡₁–(30–40), ΔS^‡_–1–(10–20) cal mol^–1 K^–1]. Heating the dimer of 2,6-diphenyl-4-methoxyphenoxyl in o-dichlorobenzene at 130 °C leads to a variety of reaction products, originating from intra- and inter-molecular radical coupling.