Vanadium(V) catalysis of perborate oxidation of substituted 5-oxo acids: a kinetic and mechanistic study

Vanadium (V) catalyzes perborate oxidation of substituted 5-oxo acid in acidic solution, being 1.6 order with respect to the oxidant, first order in the catalyst, inhibited by H⁺ and displays Michaelis–Menten kinetics on the reductant. In aqueous acetic acid solution, perborate generates hydrogen peroxide and the kinetic results reveal formation of oxodiperoxovanadium(V)-oxo acid complex. At high acidity, the ionic strength of the medium has little influence on the oxidation rates. But at low acidity, the rate increases with increasing ionic strength. The rate of oxidation increases with decreasing dielectric constant of the medium. Electron-releasing substituents in the aromatic ring accelerate the reaction rate and electron-withdrawing substituents retard the reaction. The order of reactivity among the studied 5-oxo acid is p-methoxy >> p-methyl > p-phenyl > -H > p-chloro > p-bromo > m-nitro. Activation parameters have been evaluated using Arrhenius and Eyring’s plots. A mechanism consistent with the observed kinetic data has been proposed and discussed. A suitable rate law has been derived based on the mechanism.