Electrochemical reduction of 2-cyclohexen-1-ones in a hydroethanolic medium

Abstract

Electrohydrodimerization of 2-cyclohexen-1-one, 4,4-dimethyl-2-cyclohexen-1-one and 4,4-diphenyl-2-cyclohexen-1-one in buffered hydroethanolic solutions containing 50%(v/v) ethanol over the pH range 1.0–12.1 has been studied by polarography, cyclic voltammetry and controlled-potential coulometry. All substrates give two one-electron processes corresponding to the reduction of their respective protonated and unprotonated forms. At pH values < ca. 8.5 the protonated form of each substrate is reduced in a one-electron step to generate a neutral radical which couples to give the final hydrodimers. The unprotonated form, at pH > 5.5, leads to the formation of the same final hydrodimers by an initial generation of the anion radical via a one-electron reduction step, followed by protonation and dimerization of the neutral radical. In the pH range ca. 5.5–8.5 the two processes of each substrate compete and their relative contribution is governed by the rate of establishment of the protonation equilibrium between their unprotonated and protonated forms. Voltammetric results show that the rate-determining step of each process depends on solution pH.