Borate-catalyzed reactions of hydrogen peroxide: Kinetics and mechanism of the oxidation of organic sulfides by peroxoborates

The kinetics of the oxidation of substituted phenyl methyl sulfides by hydrogen peroxide in borate/boric acid buffers were investigated as a function of pH. total peroxide concentration, and total boron concentration. Second-order rate constants at 25 degrees C for the reaction of methyl 4-nitrophenyl sulfide and H2O2, monoperoxoborate, HOOB(OH)(3), or diperoxobo-rate. (HOO)(2)B(OH)(2), are 8.29 x 10(-5), 1.51 x 10(2) and 1.06 x 10(-2)m(-1)s(-1), respectively. Peroxoboric acid, HOOB(OH)(2), is unreactive. The Hammett rho values for the reactions of it range of substituted phenyl methyl sitlfides and hydrogen peroxide. monoperoxoborate or diperoxoborate are -1.50 +/- 0.1, -0.65 +/- 0.07 and -0.48 (two points only). respectively. The rho values for the peroxoborales are (if significantly lower magnitude than expected from their reactivity compared to other, peroxides, Nevertheless the negative rho) values indicate positive charge development on the sulfur atom ill the transition state consistent with nueleophilie attack by the organic sulfides oil the peroxoborates its with the other peroxides. The kinetic parameters, including the lack of reactivity of peroxoboric acid. ire discussed in terms of the differences in the transition state of reactions involving peroxoboron species with respect to those of other peroxides.