Metal-ion oxidations in solution. Part XIV. The formation of intermediates in the reaction of iron(III) with 2-mercaptocarboxylic acids

Abstract

The oxidation of thiols by iron (III) in perchlorate media proceeds via formation of transient blue-purple intermediates. Stopped-flow studies on the reactions with mercaptoacetic (thioglycolic), 2-mercaptopropionic (thiolactic), and 2-mercapto-2-methylpropanoic acids have been made over the temperature range 10–25 °C. Under conditions of high acidity, [H⁺] > O.3M, and in excess of oxidant, optical-density changes have been interpreted by considering the formation of chelated 1 : 1 complexes, the kinetic parameters being consistent with the rate-determining step (i)(HORSH = thiol acid). Rate and thermodynamic parameters are described in terms of a predominantly [Fe(OH)]²⁺+ HORSH [graphic omitted] [Fe(ORS)]⁺+ H⁺(i) dissociative mechanism involving rate of water loss on the Fe^III. Visible-absorption spectra for the intermediates have been derived, the maxima occurring in the region 610–640 nm. Considerable variation of the absorption coefficients with methyl substituents on the ligand has been observed, the data being interpreted in terms of increasing strain in the chelate ring. Thermodynamic parameters obtained from optical-density changes are in accord with those predicted from activation data derived from rate constants.