Mechanism of OH-induced oxidation of p-cresol to p-methylphenoxyl radical

The OH radical induced oxidation of p-cresol to p-methylphenoxyl radical was studied in aqueous solution in a wide pH range by means of pulse radiolysis combined with optical spectroscopy. OH-adduct cyclohexadienyl type radicals were identified as intermediates of the reaction. In the acidic pH range the first-order rate coefficient of phenoxyl radical formation was found linearly dependent on the H3O+ concentration yielding a bimolecular rate coefficient of 1.8 x 10^8 mol-1 dm3 s-1. In the alkaline range a linear dependence was found on the OH- concentration with rate coefficient of 4.9 x 10^10 mol-1 dm3 s-1. These findings were interpreted in terms of acid-base catalysis of the H2O elimination from the OH-adduct. With the time resolution applied, 30 ns, the radical cation p-CH3C6H4OH+· was not observed as intermediate.