Equilibria, Kinetics, and Mechanism in the Bicarbonate Activation of Hydrogen Peroxide: Oxidation of Sulfides by Peroxymonocarbonate

Abstract

Bicarbonate ion is an effective activator for hydrogen peroxide in the oxidation of sulfides. Kinetic and spectroscopic results support the formation of peroxymonocarbonate ion (HCO 4 - ) as the oxidant in the catalytic reactions. The reaction of hydrogen peroxide and bicarbonate to form HCO 4 - occurs rapidly at 25 °C ( t 1/2 ≈ 300 s) near neutral pH in aqueous solution and alcohol/water mixtures, and an equilibrium analysis of the reaction by 13 C NMR leads to an estimate of the electrode potential for the HCO 4 - /HCO 3 - couple (1.8 V vs NHE). Solubility of the bicarbonate catalyst is enhanced by the use of NH 4 HCO 3 rather than by the use of group 1 salts, which tend to have lower solubility in the mixed solvents and can lead to phase separation. Rate laws and mechanistic analyses are presented for the oxidation of ethylphenylsulfide and related sulfides. The second-order rate constants for sulfide oxidations by HCO 4 - are ∼300-fold greater than those for H 2 O 2, and this increase is consistent with expectations based on a Brønsted analysis of the kinetics for other heterolytic peroxide oxidations. At high concentrations of H 2 O 2, a pathway that is second order in H 2 O 2 is significant, and this path is interpreted as a general acid catalysis by H 2 O 2 of carbonate displacement accompanying substrate attack at the electrophilic oxygen of HCO 4 - . Increasing water content up to 80% in the solvent increases the rate of oxidation. The BAP (bicarbonate-activated peroxide) oxidation system is a simple, inexpensive, and relatively nontoxic alternative to other oxidants and peroxyacids, and it can be used in a variety of oxidations where a mild, neutral pH oxidant is required. Variation of bicarbonate source and the cosolvent can allow optimization of substrate solubility and oxidation rates for applications such as organic synthesis and chemical warfare agent decontamination.