Oxyhalogen–sulfur chemistry: kinetics and mechanism of oxidation of formamidine disulfide by acidic bromate

Abstract

The kinetics and mechanism of the oxidation of formamidine disulfide, FDS, a dimer and major metabolite of thiourea, by bromate have been studied in acidic media. In excess bromate conditions the reaction displays an induction period before formation of bromine. The stoichiometry of the reaction is: 7BrO₃⁻ + 3[(H₂N(HN)CS–]₂ + 9H₂O → 6NH₂CONH₂ + 6SO₄²⁻ + 7Br⁻ + 12H⁺ (A). In excess oxidant conditions, however, the bromide formed in reaction A reacts with bromate to give bromine and a final stoichiometry of: 14BrO₃⁻ + 5[(H₂N(HN)CS–]₂ + 8H₂O → 10NH₂CONH₂ + 10SO₄²⁻ + 7Br₂ + 6H⁺ (B). The direct reaction of bromine and FDS was also studied and its stoichiometry is: 7Br₂ + [(H₂N(HN)CS–]₂ + 10H₂O → 2NH₂CONH₂ + 2SO₄²⁻ + 14Br⁻ + 18H⁺ (C). The overall rate of reaction A, as measured by the rate of consumption of FDS, is second order in acid concentrations, indicating the dominance of oxyhalogen kinetics which control the formation of the reactive species HBrO₂ and HOBr. The reaction proceeds through an initial cleavage of the S–S bond to give the unstable sulfenic acids which are then rapidly oxidized through the sulfinic and sulfonic acids to give sulfate. The formation of bromine coincides with formation of sulfate because the cleavage of the C–S bond to give sulfate occurs at the sulfonic acid stage only. The mechanism derived is the same as that derived for the bromate–thiourea reaction, suggesting that FDS is an intermediate in the oxidation of thiourea to its oxo-acids as well as to sulfate.