Mechanism of the oxidative coupling of phenols: Influence of pH upon the electrochemical oxidation of xylenol in methanol

In acidic methanol, xylenol undergoes a bielectronic one step oxidation leading to the phenoxonium cation. Above pH 5,2 the reaction takes place in two monoelectronic steps, the first of which leading to a free radical. CC coupling, leading to quinones, occurs with the cation, whereas the free monomeric radical undergoes polymerization by CO coupling. The influence of the pyridine concentration upon the orientation of the coupling may be explained by the coincidence between the pKA of pyridine in methanol and the pH which favours disproportionation of the monomer radical.