Metal-ion oxidations in solution. Part XIX. Redox pathways in the oxidation of penicillamine and glutathione by chromium(VI)

Abstract

Three moles of penicillamine or glutathione are required to reduce chromium(VI) to chromium(III). The kinetics and mechanism of the redox reaction have been studied using the stopped-flow method. The reaction proceeds via the formation of a transient intermediate (K1) which decomposes either in a proton-catalyzed pathway or by reaction with a second mole of thiol. The rate law[Formula: see text]where n = 1 for penicillamine and 2 for glutathione has been shown to hold over a range of thiol and hydrogen-ion concentrations. At 25 °C k2 = 14.3 ± 1.0 M−1 s−1 for penicillamine (ΔH≠ = 9 ± 2 kcal mol−1, ΔS≠ = −33 ± 6 cal K−1 mol−1) and 12.1 ± 0.4 M−1 s−1 for glutathione (ΔH≠ = 7 ± 2 kcal mol−1, ΔS≠ = −40 ± 5 cal K−1 mol−1). Several chromium(III) products have been identified by ion-exchange methods. The significance of the second-order pathways in these reactions is discussed.