Metal-ion oxidations in solution. Part XVIII. Characterization, rates, and mechanism of formation of the intermediates in the oxidation of thiols by chromium(VI)

The stopped-flow technique has been used to study the reactions of [HCrO₄⁻] and penicillamine, glutathione and β-mercaptoethylamine (RSH) in perchlorate media, [H⁺] = 0.02−0.100 M, I = 1.00 M over the temperature range 10–30 °C. The transient orange species formed as intermediates are 1:1 chromate esters,[Formula: see text]with λ_max ∼ 420–430 nm and K₁ ∼ 700 (DL-penicillamine) to 1440 M⁻¹ (glutathione) at T = 25 °C. Thermodynamic parameters for complex formation have been derived from initial absorbance data and the evidence for sulphur bonding is discussed. Kinetics of the reactions conform to the rate law[Formula: see text]Activation enthalpies lie in the range 6–10 kcal/mol but ΔS^≠ values vary considerably possibly reflecting solvation effects. The rate constants k_f^H+ are several orders of magnitude lower than those found for this parameter in other reactions of this type. The complex formation mechanisms are discussed and the possibility of some associative character in the present reactions is considered.