The vanadium(V)-catalyzed oxidation of (1-hydroxyethylidene)bisphosphonic acid, CH3C(OH)(PO3H2)(2), by hydrogen peroxide in aqueous solution

The vanadium(V)-catalyzed oxidation of (I-hydroxyethylidene)bisphosphonic acid, CH3C(OH)(PO3H2)(2), by hydrogen peroxide in aqueous solution has been studied at temperatures between 50 and 80 degrees C. In contrast to vanadium(V), six-valent Mo and W are without significant catalytic action, as is SeO2, while OsO4 is only weakly catalytic. With excess substrate a limiting stoichiometry is reached in which ca. 4 mol H2O2 are consumed per mol of substrate oxidized. With excess H2O2, the reaction competes with catalytic decomposition of the peroxide, and a substantial excess of peroxide is required to consume the substrate completely. The reaction is optimal near pH 1. At higher pH it becomes slower, while at lower pH the catalytic decomposition of H2O2 comes to predominate. The principal reaction products are phosphoric and acetic acids and carbon dioxide, along with lesser quantities of CO and formic acid. The consumption of substrate in the presence of a large excess of H2O2 follows first-order kinetics, but the apparent first-order rate constant shows a weak positive dependence on initial substrate concentration, which may be pH-related, and at high substrate concentration it shows a weak negative dependence on initial [H2O2]. The principal reactant appears to be the diperoxovanadium(V) anion, OV(O-2)(2)(-), but the apparent rate shows a greater-than-first-order dependence on catalyst concentration, suggesting a secondary reaction path involving a dimeric peroxovanadium species. A free-radical mechanism has been proposed in which one-electron reduction of the vanadium accompanies oxidation of the substrate to an intermediate alkoxyl radical species that can yield either acetic acid or CO2. This mechanism is supported by the observation that vanadium(V) itself oxidizes the substrates at a measurable rate.