The vanadium(V)-catalyzed oxidation of (1-hydroxyethylidene)bisphosphonic acid, CH 3C(OH)(PO 3H 2) 2, by hydrogen peroxide in aqueous solution

Abstract

The vanadium(V)-catalyzed oxidation of (1-hydroxyethylidene)bisphosphonic acid, CH 3C(OH)(PO 3H 2) 2, by hydrogen peroxide in aqueous solution has been studied at temperatures between 50 and 80°C. In contrast to vanadium(V), six-valent Mo and W are without significant catalytic action, as is SeO 2, while OsO 4 is only weakly catalytic. With excess substrate a limiting stoichiometry is reached in which ca. 4 mol H 2O 2 are consumed per mol of substrate oxidized. With excess H 2O 2, 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 H 2O 2 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 H 2O 2 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 [H 2O 2]. 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 CO 2. This mechanism is supported by the observation that vanadium(V) itself oxidizes the substrate at a measurable rate.