Kinetics and mechanism of ruthenium(III) and osmium(VIII) catalyzed oxidation of dopamine with bromamine-B in acid and alkaline media

The kinetics of oxidation of dopamine (DPM) by bromamine-B (BAB) has been studied in HCl medium catalyzed by Ru(III) chloride and in NaOH medium with Os(VIII) as catalyst at 298 K. Despite the fact that the stoichiometry and oxidation products are same in both catalyzed reactions, the kinetic patterns are different. In acid medium the rate shows a first order dependence each on BAB, DPM and Ru(III) and fractional order on H+ concentrations. In alkaline medium, the rate is first order each on [BAB]o and [DPM]o, a fractional order on [Os(VIII)] and an inverse first order on [OH-]. The effects of addition of benzene sulfonamide and halide ions as well as variation of ionic strengths and dieletric constant of the medium did not show significant effect on the rate of the reaction. Solvent isotope effects has been studied in D2O medium. Activation parameters have been calculated. The catalytic effeciency is Os(VIII) ≫ Ru(III). The proposed mechanisms and the derived rate laws are in consistent with the observed kinetics