Equilibria, kinetics, and mechanism in the bicarbonate activation of hydrogen peroxide: Oxidation of sulfides by peroxymonocarbonate

Bicarbonate ion is an effective activator for hydrogen peroxide in the oxid ation of sulfides. Kinetic and spectroscopic results support the formation of peroxymonocarbonate ion (HCO4-) as the oxidant in the catalytic reaction s. The reaction of hydrogen peroxide and bicarbonate to form HCO4- occurs r apidly at 25 degrees C (t(1/2) approximate to 300 s) near neutral pH in aqu eous solution and alcohol/water mixtures, and an equilibrium analysis of th e reaction by C-13 NMR leads to an estimate of the electrode potential for the HCO4-/HCO3- couple (1.8 V vs NHE). Solubility of the bicarbonate cataly st is enhanced by the use of NH4HCO3 rather than by the use of group 1 salt s, which tend to have lower solubility in the mixed solvents and can lead t o phase separation. Rate laws and mechanistic analyses are presented for th e oxidation of ethylphenylsulfide and related sulfides. The second-order ra te constants for sulfide oxidations by HCO4- are similar to 300-fold greate r than those for H2O2, and this increase is consistent with expectations ba sed on a Bronsted analysis of the kinetics for other heterolytic peroxide o xidations. At high concentrations of H2O2, a pathway that is second order i n H2O2 is significant, and this path is interpreted as a general acid catal ysis by H2O2 of carbonate displacement accompanying substrate attack at the electrophilic oxygen of HCO4-. Increasing water content up to 80% in the s olvent increases the rate of oxidation. The BAP (bicarbonate-activated pero xide) oxidation system is a simple, inexpensive, and relatively nontoxic al ternative to other oxidants and peroxyacids, and it can be used in a variet y of oxidations where a mild, neutral pH oxidant is required. Variation of bicarbonate source and the cosolvent can allow optimization of substrate so lubility and oxidation rates for applications such as organic synthesis and chemical warfare agent decontamination.