Direct reduction of halogenated biphenyls at mercury electrodes

Abstract

Direct electroreduction in DMF of seven halogenated biphenyls of representative structures was investigated by cyclic voltammetry. Emphasis was on the primary reduction wave, involving replacement of halide with hydrogen in an irreversible ECE reaction. For 4-bromo, 4-chloro, 4,4′-dibromo, 3,4-dichloro, 2,2′,5,5′- and 3,3′,4,4′-tetrachloro-biphenyls, the reaction is under pure kinetic control of the initial electron-transfer step. Kinetic control of the primary reduction peak of 4,4′-dichlorobiphenyl may involve decomposition of the halobiphenyl anion radical as well as slow electron transfer. Transfer of the second electron occurs at the electrode, in a homogeneous disproportionation, or by hydrogen-atom abstraction from DMF or electrolyte with reduction of the resulting radical at the electrode. Electrochemical transfer coefficients, standard potentials, and standard heterogeneous rate constants were estimated from the voltammetric data. Heterogeneous electron-transfer rates decreased with substitution of the biphenyl ring in the order F>Cl>Br. Gibbs energies of activation of homogeneous and heterogeneous reactions showed correlations according to Marcus-Hush theory.