Oxyhalogen-sulfur chemistry: Bromate oxidation of 1-methyl-2-thiourea in acidic medium

The reaction between bromate and 1-methyl-2-thiourea, CH3NH(NH2)C = S (MTU), has been studied in acidic medium. The stoichiometry of the reaction in excess MTU has been established as 4BrO(3)(-) + 3CH(3)NH(NH2)C = S + S + 3H(2)O --> 4Br(-) + 3SO(4)(2-) + 3CH(3)NH(NH2)C = O + 6H(+) (A); and in excess bromate the stoichiometry is: 8BrO(3)(-) + 5CH(3)NH(NH2)C = S + H2O --> 4Br(2) + 5SO(4)(2-) + 5CH(3)NH(NH2)C = O + 2H(+) (B). Stoichiometry B includes the oxidation of MTU (stoichiometry A) and the oxidation of Br- by the excess BrO3- : BrO3- + 5Br(-) + 6H(+) --> 3Br(2) + 3H(2)O. In excess BrO3- the reaction is characterized by an induction period followed by formation of Br-2 The reaction between Br-2 and MTU is very fast with a bimolecular lower limit rate constant of 2.0 x 10(6) M(-1) s(-1) such that the formation of Br-2 is an indicator for the complete consumption of MTU. The reaction could be followed by monitoring the depletion of MTU through its absorbance at lambda = 239 nm or the formation of Br-2 at lambda = 390 nm. The rate of reaction was deduced as -d[BrO3-]/dt = k(0)[BrO3-][MTU][H+](2) with k(0) = 14.7 +/- 1.2 M(-3) s(-1). A proposed 17-step reaction mechanism gives good agreement between experimental data and computer simulations.