Kinetics and mechanism of uncatalyzed and silver(I)-catalyzed oxidation of l-histidine by hexachloroplatinate(IV) in acid medium

Abstract

Kinetics of hexachloroplatinate(IV) (HCP) oxidation of l-histidine (His) in H2SO4 medium has been investigated in the absence and presence of silver(I) catalyst at constant ionic strength 2.0 mol dm−3 and temperature 25 °C. The progress of both uncatalyzed and silver(I)-catalyzed oxidation reactions has been monitored spectrophotometrically. Both uncatalyzed and catalyzed paths show first-order dependence on [HCP] and fractional-first-order dependence each on [His] and [acid]. The catalyzed path is first order in [AgI]. Increasing ionic strength and dielectric constant decreases the oxidation rates. The catalyzed reaction has been shown to proceed via formation of a silver(I)-histidine intermediate complex, which reacts with the oxidant by an inner-sphere mechanism leading to decomposition of the complex in the rate-determining step. The final oxidation products of histidine were identified as the corresponding aldehyde (2-imidazole acetaldehyde), ammonium ion and carbon dioxide. The mechanisms of these reactions have been proposed and the appropriate rate laws are deduced.