Oxidation of 4,4′-diphenoquinones giving p-benzoquinones

4,4′-Diphenoquinone 1a can be oxygenated to give p-benzoquinone 2a when treated with PbO2 in a solvent containing an aqueous acid. The conversion of 1a is facilitated when the acid is strong and the solvent is of low basicity. The concentration of H2O in the medium also has a profound effect on the rate of conversion of 1a. These facts may indicate that protonation of 1a is the first step in the conversion into 2a. 4,4′-Diphenoquinones 1b and 1c can be analogously converted into p-benzoquinones 2a and 2b, respectively. Benzoquinones 2 can thus be the ultimate products of both oxygenation and dehydrogenative dimerization of phenols 3.