Oxyhalogen−Sulfur Chemistry: Bromate Oxidation of 1-Methyl-2-thiourea in Acidic Medium

The reaction between bromate and 1-methyl-2-thiourea, CH 3 NH(NH 2 )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(NH 2 )C S + 3H 2 O → 4Br - + 3SO 4 2- + 3CH 3 NH(NH 2 )C O + 6H + (A); and in excess bromate the stoichiometry is: 8BrO 3 - + 5CH 3 NH(NH 2 )C S + H 2 O → 4Br 2 + 5SO 4 2- + 5CH 3 NH(NH 2 )C O + 2H + (B). Stoichiometry B includes the oxidation of MTU (stoichiometry A) and the oxidation of Br - by the excess BrO 3 -: BrO 3 - + 5Br - + 6H + → 3Br 2 + 3H 2 O. In excess BrO 3 - 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 × 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 λ = 239 nm or the formation of Br 2 at λ = 390 nm. The rate of reaction was deduced as -d[BrO 3 - ]/d t = k 0 [BrO 3 - ][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.