Kinetics and mechanism of the oxidation of l-methionine by aqueous solution of chromium(VI)

Abstract

Abstract The kinetics of the reduction of Cr(V1) by l -methionine (represented by RSCH 3 ) have been studied in aqueous perchloric acid. The data are consistent with the rate law: −d[Cr(VI)] d t = ( k 1 + k 2 [H + ] 2 [RSCH 3 ])[Cr(VI)] with k 1 = (3.1 ± 0.5) x 10 −4 s −1 ; k 2 = 12.3 ± 1.1 mol −3 dm 9 s −1 at I = 1.0 mol dm −3 (NaClO 4 ), [H + ] = 40.04–0.10 mol dm −3 and T 25.1°C. The corresponding activation parameters are ΔH 1 * = 26.2 ± 0.4 kJ mol −1 ; ΔS 1 * = −224± 1 JK −1 mol −1 ; ΔH 1 * = 33.9±0.01 KJ mol −1 ; ΔS 1 *  −111±1 JK −1 mol −1 . The above rate is very much slower than those of other related reactions. This is interpreted in terms of a rate-determining CrS bonded intermediate which undergoes hydrolysis after electron transfer to yield Cr(III) and methionine sulphoxide. The significance of pathways involving second-order substrate dependence is also discussed.