Nonlinear dynamics in the oxidations of substituted thioureas I: The reaction of 4-methyl-3-thiosemicarbazide with acidic iodate [1]

The substituted thiourea, 4-methyl-3-thiosemicarbazide. was oxidized by iod ate in acidic medium. In high acid concentrations and in stoichiometric exc ess of iodate, the reaction displays an induction period followed by the fo rmation of aqueous iodine. In stoichiometric excess of methylthiosemicarbaz ide and high acid concentration,the reaction shows a transient formation of aqueous iodine. The stoichiometry of the reaction is: 4IO(3)(-) + 3CH(3)NH C(=S)NHNH2 + 3H(2)O --> 4I(-) + 3SO(4)(2-) + 3CH(3)NHC(=O)NHNH2 + 6H(+) (A) . iodine formation is due to the Dushman reaction that produces iodine from iodide formed from the reduction of iodate: IO3- + 5I(-) + 6H(+) --> 3I(2) (aq) + 3H(2)O (B). Transient iodine formation is due to the efficient acid catalysis of the Dushman reaction. The iodine produced in process B is cons umed by the methylthiosemicarbazide substrate. The direct reaction of iodin e and methylthiosemicarbazide was also studied. It has a stoichiometry of 4 I(2)(aq) + CH3NHC (=S)NHNH2 + 5H(2)O --> 8I(-) --> SO42- + CH3NHC(=O)NHNH2 + 10H(+) (C). The reaction exhibits autoinhibition by iodide and acid. Inhi bition by I- is due to the formation of the triiodide species, I-3(-), and inhibition by acid is due to the protonation of the sulfur center that deac tivates it to further electrophilic attack. In excess iodate conditions, th e stoichiometry of the reaction is 8IO(3)(-) + 5CH(3)NHC(=S)NHNH2 + H2O --> 4I(2) + 5SO(4)(2-) + 5C(3)NHC(=O)NHNH2 + 2H(+) (D) that is a linear combin ation of processes A and B. (C) 2000 John Wiley & Sons, Inc.