Kinetics and mechanism of oxidation of thioglycolic acid by hexachloroiridate(IV)

Oxidation of thioglycolic acid (HSCH2CO2H, TGA) by [IrCl6](2-) (Ir(IV)) in aqueous solution at 25degreesC is highly susceptible to trace metal-ion catalysis. This catalysis can be effectively suppressed by bathophenanthroline-disulfonate (bathophen), allowing the direct oxidation to be studied. Although the corresponding disulfide is produced quantitatively in the absence of bathophen, a mixture of the disulfide and a species believed to be the sulfonate is produced in its presence. In both cases the Ir(IV) is reduced to [IrCl6](3)(-). The rate law when catalysis is suppressed is first order in both Ir(IV) and TGA. The pH dependence is complex, owing to the diprotic nature of TGA. At high pH there is a term corresponding to outer-sphere oxidation of -SCH2CO2- with a rate constant of (3.52+/-0.03) x 10(6) M-1 s(-1) at mu=0.1 M. A self-exchange rate constant of 1 x 10(5) M-1 s(-1) for the (SCH2CO2-)-S-circle/ -SCH2CO2- redox couple is derived from the Marcus cross relationship. In order to account for the product mixture, a mechanism is proposed in which RSO3- is produced in a series of steps originating with the oxidation of RScircle by [IrCl6](2)(-), while RSSR is produced by association of RScircle with TGA to generate RSSRradical anion and oxidation of RSSRradical anion by [IjCl(6)](2-).