Metal-ion oxidations in solution. Part XIII. The reaction of chromium(VI) with L-cysteine in perchlorate media
Abstract
The stopped-flow technique has been used to study the reactions between [HCrO4]– and L-cysteine, H2L, with the latter in excess, at [HClO4]= 0·020–0·100M and I= 1·00M(NaClO4) in the range 15–35 °C. The transient orange species observed is a 1 : 1 chromate ester [equation (i)] with λmax.= 420 nm, εmax.= 1 410 ± 60 l mol–1 cm–1, H2L +[(HO)CrO3]– [graphic omitted] [(HL)CrO3]–+ H2O (i) and K1= 1 030 ± 110 l mol–1 at 25 °C. Evidence regarding sulphur bonding is discussed. The kinetics of the formation reaction conform to the rate law (ii) and, at 25 °C, kf′= 130 ± 20 l2 mol2 s–1 and kf= 2 ± 1 l mol–1 s–1. d[Complex]/dt =(kf′[H+]+kf)[HCrO4–][H2L](ii) For the acid-catalysed path, ΔH‡= 7 ± 3 kcal mol–1 and ΔS‡=–24 ±11 cal K–1 mol–1. For the overall formation reaction, ΔG=–4·1 ± 0·1 kcal mol–1, ΔH=–5·0 ± 0·5 kcal mol–1, and ΔS=–3 ± 2 cal K–1 mol–1. The rate constant kf′ has a value several orders of magnitude smaller than those found for this parameter in comparable reactions. This is interpreted as evidence that the anation mechanism has some associative character. One mole of CrVI oxidises three moles of L-cysteine. The redox-reaction kinetics obey the rate law (iii)