Metal-ion oxidations in solution. Part XV. Rate-determining dimerisations in redox reactions of iron(III) with some α-mercaptocarboxylic acids

Abstract

The kinetics of decomposition of iron(III)–sulphur-bonded complexes with α-mercaptocarboxylic acids (HORSH) have been investigated. A rate-determining dimerisation is considered to take place with a two-electron transfer Fe^III+ HORSH [graphic omitted] [Fe^III(ORS)]⁺+ 2H⁺(i), 2[Fe^III(ORS)]⁺ [graphic omitted] 2Fe^II+ HORS·SROH (ii) leading directly to the disulphide product. The effect of methyl substituents on the ligands suggests an associative mechanism. At 25 °C, for mercaptoacetic acid, k₀= 167 ± 18 l mol^–1 s^–1(ΔH^‡= 5 ± 1.5 kcal mol^–1, ΔS^‡=–31 ± 5.5 cal K^–1 mol^–1); 2-mercaptopropionic acid, k₀= 130 ± 10 l mol^–1 s^–1(ΔH^‡= 9 ± 2 kcal mol^–1, ΔS^‡=–18 ± 3 cal K^–1 mol^–1); 2-mercapto-2-methylpropanoic acid, k₀= 47 ± 8 l mol^–1 s^–1(ΔH^‡= 14 ± 4 kcal mol^–1, ΔS^‡=–5 ± 1.5 cal K^–1 mol^–1); and 2-mercaptosuccinic acid, k₀= 68 ± 7 l mol^–1 s^–1(ΔH^‡= 10 ± 1.5 kcal mol^–1, ΔS^‡=–16 ± 2 cal K^–1 mol^–1). Apparent differences in the overall rates of the redox reaction with methylation are explicable in terms of variation in the equilibrium constant K₁.