Ruthenium(III)- and osmium(VIII)-catalyzed oxidation of 2-thiouracil by bromamine-B in acid and alkaline media: a kinetic and mechanistic study

The kinetics of oxidation of 2-thiouracil (TU) by sodium N-bromobenzenesulphonamide or bromamine-B (BAB) have been studied in an HCl medium, catalyzed by RuCl3, and in a NaOH media with OsO4 as catalyst, at 313 K. The stoichiometry and oxidation products are the same in both cases, but their kinetic patterns were found to be different. In acid medium the rate shows a first order dependence in each of [BAB] and [TU], and is dependent on [RuIII]. The reaction rate is inversely dependent on [H+]. In alkaline medium, the rate is first order in [BAB] and in [OsVIII] and zero order in [TU]. The reaction rate is dependent on [NaOH]. Activation parameters have been evaluated, solvent isotope effects have been studied in D2O medium, and equilibrium constants were calculated. The activation parameters and rate constants indicate that the catalytic efficiency is: OsVIII > RuIII. The proposed mechanisms and the derived rate laws are consistent with the observed kinetics.