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Kinetic studies on the oxidation of hydroquinone and catechol by the heteropoly 10-tungstodivanadophosphate anion, [PV V V V W 10 O 40 ] 5-, have been carried out in aqueous acidic medium at 25°C by UV-visible spectrophotometry. The oxidation of hydroquinone shows simple second-order kinetics overall, with first-order dependence of the rate on both [oxidant] and [hydroquinone] at constant [H ? ]. For catechol oxidation, the order of the reaction with respect to [oxidant] is unity, while the order with respect to [catechol] is variable; this reaction shows Michaelis-Menten-type kinetics at constant [H ? ]. The rate of the reaction is insensitive to [H ?] in the pH range 1.2-1.7. Rate retardation for deuterated hydroquinone and catechol (C 6 H 4 (OD) 2 ) in D 2 O indicates breaking of the -OH bond in the rate-limiting step. Based on the observed kinetic isotope effect and calculated ground-state free energy change (DG 0 ) values, a hydrogen atom transfer mechanism is suggested for the reaction; i.e., in the ratelimiting step, one electron and one proton are transferred from the reductant to the oxidant in a concerted manner. Rates of oxidation of hydroquinone by this oxidant in neat acetonitrile at 25°C have also been measured. By applying the Marcus equation, the self-exchange rate constant of the oxidant (V V V IV OH/H Á ) in acetonitrile has been evaluated.

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