S-Oxygenation of Thiocarbamides. 3. Nonlinear Kinetics in the Oxidation of Trimethylthiourea by Acidic Bromate

Abstract

The oxidation of trimethylthiourea (TMTU) by acidic bromate has been studied. The reaction mimics the dynamics observed in the oxidation of unsubstituted thiourea by bromate with an induction period before formation of bromine. The stoichiometry of the reaction was determined to be 4:3, thus 4BrO(3)- + 3R(1)R(2)C=S+ 3H(2)O --> 4Br- + 3R(1)R(2)C=O + 3SO(4)(2-) + 6H+. This substituted thiourea is oxidized at a much faster rate than the unsubstituted thiourea. The oxidation mechanism of TMTU involves initial oxidations through sulfenic and sulfinic acids. At the sulfinic acid stage, the major oxidation pathway is through the cleavage of the C-S bond to form a reducing sulfur leaving group, which is easily oxidized to sulfate. The minor pathway through the sulfonic acid produces a very stable intermediate that is oxidized only very slowly to urea and sulfate. The direct reaction of aqueous bromine with TMTU was faster than reactions that form bromine, with a bimolecular rate constant of (1.50 +/- 0.04) x 10(2) M(-1) s(-1). This rapid reaction ensured that no oligooscillatory bromine formation was observed. The oxidation of TMTU was modeled by a simple reaction scheme containing 20 reactions.

Exact Concentration Dependence of the Landolt Time in the Thiourea Dioxide–Bromate Substrate-Depletive Clock Reaction

“…In the case of the DMTU–bromate reaction, electrospray ionization experiments provided no clear evidence for the detectable formation of any S-oxygenated DMTU species . Similar to this, no S-oxygenated trimethylthiourea (TMTU) was detected in the TMTU–bromate reaction . In contrast to this, however, when tetramethylthioura is oxidized by bromate in an acidic condition, then the tetramethyl-aminoiminomethanesulfenic acid is found to be a relatively long-lived intermediate playing a key role in determining the nonlinearity of the reaction .…”

“…17 Similar to this, no S-oxygenated trimethylthiourea (TMTU) was detected in the TMTU−bromate reaction. 18 In contrast to this, however, when tetramethylthioura is oxidized by bromate in an acidic condition, then the tetramethyl-aminoiminomethanesulfenic acid is found to be a relatively long-lived intermediate playing a key role in determining the nonlinearity of the reaction. 19 Because all of the above-mentioned reactions display clock behavior, 20 it looks to be crucial to understand the kinetic role of the intermediates formed during the oxidation of substituted thioureas.…”

Recent Developments in the Chemistry of Thiourea Oxides

“…Simoyi and co‐workers studied reactions of different thioureas and their oxides with chlorine‐, bromine‐, and iodine‐containing compounds mostly in acidic aqueous solutions 13. 54b, 55 Interestingly, they observed the aging effect in the reaction of thiourea trioxide and iodine,55 attributing it exclusively to the decomposition of thiourea trioxide to sulfite, which then rapidly reacts with iodine. Indeed, the addition of formaldehyde (reacting with sulfite) decreases the rate of reaction.…”

Oxyhalogen-sulfur chemistry: Kinetics and mechanism of oxidation of n-acetylthiourea by aqueous bromate and acidified bromate

“…This is a sequence that has been suggested in several oxidations of thiols and thiocarbamides. 47 Thus the initial oxidation of ACTU would be by the generated reactive species HOBr: …”