Homogeneous and heterogeneous electron exchange in the reduction of aromatic hydrocarbons

Abstract

Homogeneous electron exchange rates, determined by a modified e.s.r. method in both the slow and fast exchange limits, are compared with published heterogeneous rates at mercury for aromatic hydrocarbon reduction in dimethylformamide; the ratio is approximately constant. This constant can be derived from theories of Marcus and Hush on a model in which image forces are negligible in the electrode process and homogeneous exchange occurs only on contact. Potential-step methods are used to study the source of electrons transferred to radicals formed by protonation of the hydrocarbon radical anions (e.c.e. mechanism) : electron transfer occurs from other radical anions for anthracene and from the electrode for naphthalene. This result is interpreted in terms of relative rates of cross-exchange reactions as predicted by Marcus theory from the “isotopic” exchange rates and redox potentials of the systems. General considerations imply that cross-exchange reactions will predominate in many aromatic systems. Proton addition from phenol to the radical anions appears to involve direct contact of acid and base rather than the free (solvated) proton.