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Mechanistic Studies of the Reaction between Thioglycolic Acid and Chromium(VI): Substitution, Isomerisation, and Electron Transfer

Thioglycolic Acid, H2TGA, containing a reactive sulphydryl group, has been shown to reduce Cr(VI). The experimental conditions were [Cr(VI)]T = 0.20 mM, [TGA]T = 22 – 100 mM, 15.0 ≤ T ≤ 30.0 °C, 2.66 ≤ pH ≤ 4.66 (TGA self-buffer) or 2.74 ≤ pH ≤ 6.05 (citric acid-phosphate) at a constant ionic strength of 0.50 M (NaClO4). The reaction was studied using rapid scanning stopped-flow spectrophotometry at λ = 425 nm. The kinetics of the reaction showed distinct spectral evidence for the formation of an intermediate that was subsequently followed by a slower, bimolecular redox process leading to the formation of the final products. Based on the analysis of the experimental data, the formation of the intermediate involved the conversion of an O-bonded Cr(VI)-thioglycolate intermediate, O3CrOC(O)CH2SH, to the Sbonded type O3CrSCH2COOH, prior to the redox step. Normal acid-base equilibria of thioglycolate ( H2TGA / HTGA) and chromate species ( HCrO4 / CrO4 2) were found to be inadequate to explain the overall mechanism. Instead, the facile reactions of both O-bonded and S-bonded Cr(VI)-thioglycolate intermediates were seen to be influenced by their own proton dependency. It was found that depending on the nature of the buffer used, some catalytic effects were evident, but the complete pH profile obtained was inherently peculiar to the reaction involving the main reactants. Added metal ions such as Cu2+ and Zn2+ are implicated in the overall course of the reaction, and their effects also seem to be determined by the concentration of [H +] utilized.