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 λ_max.= 420 nm, ε_max.= 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, k_f′= 130 ± 20 l² mol² s^–1 and k_f= 2 ± 1 l mol^–1 s^–1. d[Complex]/dt =(k_f′[H⁺]+k_f)[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 k_f′ 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 Cr^VI oxidises three moles of L-cysteine. The redox-reaction kinetics obey the rate law (iii)–d[Cr^VI]//_dt=K₁(k₁[H⁺]+k₂[H₂L])[H₂L][HCrO₄^–]//_{1 +K1[H2L]}(iii) where, at 25 °C, k₁=(1·2 ± 0·3)× 10^–2 l² mol²s^–1, k₂=(9·4 ±0·4)× 10^–2 l² mol² s^–1, ΔH₁^‡= 12 ± 2 kcal mol^–1, ΔS₁^‡=–27 ± 7 cal K^– mol^–1, ΔH₂^‡= 10·8 ± 0·8 kcal mol^–1, and ΔS₂^‡=–27 ± 3 cal K^–1 mol^–1. The nature of the chromium (III) products of this reaction is described. The significance of second-order reductant terms for reactions of this oxidant is discussed.