Mechanism of the Oxidation of Captopril by Diperiodatoargentate(III) in Aqueous Alkali: A Reaction Kinetics Study

Kinetics of the oxidation of captopril by diperiodatoargentate(III) in aqueous alkaline medium was studied spectrophotometrically in the visible region. The kinetics of oxidation was found to be first order in diperiodatoargentate(III) and to have a fractional order in the captopril concentration. Added periodate, alkali and one of the reaction products, silver(I), showed retarding effects on the rate of reaction. The oxidation product of captopril was identified as captopril disulfide, which was characterized by its IR and LC–ESI–MS spectra. It was observed that the reaction products, silver(I) and captopril disulfide, interacted to yield a silver(I)–captopril disulfide complex and this was confirmed by its LC–ESI–MS spectrum. Based on the kinetics results and other parameters, a suitable mechanism for oxidation of captopril by diperiodatoargentate(III) has been proposed in which an intermediate complex between oxidant and substrate results from the preceding step before generation of a free radical in a slow step. The rate law was derived and reaction constants have also been determined. The active species of the silver(III)–periodate complex was determined to be monoperiodatoargentate(III).