Oxyhalogen-sulfur chemistry — Kinetics and mechanism of the oxidation of cysteamine by acidic iodate and iodine

Abstract

The oxidation of cysteamine by iodate and aqueous iodine has been studied in neutral to mildly acidic conditions. The reaction is relatively slow and is heavily dependent on acid concentration. The reaction dynamics are complex and display clock behavior, transient iodine production, and even oligooscillatory production of iodine, depending upon initial conditions. The oxidation product was the cysteamine dimer (cystamine), with no further oxidation observed past this product. The stoichiometry of the reaction was deduced to be IO3–+ 6H2NCH2CH2SH → I–+ 3H2NCH2CH2S-SCH2CH2NH2+ 3H2O in excess cysteamine conditions, whereas in excess iodate the stoichiometry of the reaction is 2IO3–+ 10H2NCH2CH2SH → I2+ 5H2NCH2CH2S-SCH2CH2NH2+ 6H2O. The stoichiometry of the oxidation of cysteamine by aqueous iodine was deduced to be I2+ 2H2NCH2CH2SH → 2I–+ H2NCH2CH2S-SCH2CH2NH2+ 2H+. The bimolecular rate constant for the oxidation of cysteamine by iodine was experimentally evaluated as 2.7 (mol L–1)–1s–1. The whole reaction scheme was satisfactorily modeled by a network of 14 elementary reactions.Key words: cysteamine, cystamine, Dushman reaction, oligooscillations.