S-Oxygenation of Thiocarbamides IV: Kinetics of Oxidation of Tetramethylthiourea by Aqueous Bromine and Acidic Bromate

Abstract

The kinetics and mechanism of oxidation of tetramethylthiourea (TTTU) by bromine and acidic bromate has been studied in aqueous media. The kinetics of reaction of bromate with TTTU was characterized by an induction period followed by formation of bromine. The reaction stoichiometry was determined to be 4BrO(3)(-) + 3(R)(2)C═S + 3H(2)O → 4Br(-) + 3(R)(2)C═O + 3SO(4)(2-) + 6H(+). For the reaction of TTTU with bromine, a 4:1 stoichiometric ratio of bromine to TTTU was obtained with 4Br(2) + (R)(2)C═S + 5H(2)O → 8Br(-) + SO(4)(2-) + (R)(2)C═O + 10H(+). The oxidation pathway went through the formation of tetramethythiourea sulfenic acid as evidenced by the electrospray ionization mass spectrum of the dynamic reaction solution. This S-oxide was then oxidized to produce tetramethylurea and sulfate as final products of reaction. There was no evidence for the formation of the sulfinic and sulfonic acids in the oxidation pathway. This implicates the sulfoxylate anion as a precursor to formation of sulfate. In aerobic conditions, this anion can unleash a series of genotoxic reactive oxygen species which can explain TTTU's observed toxicity. A bimolecular rate constant of 5.33 ± 0.32 M(-1) s(-1) for the direct reaction of TTTU with bromine was obtained.

Recent Developments in the Chemistry of Thiourea Oxides

“…The structures of di‐ and trioxides of unsubstituted thiourea are stabilized by intermolecular hydrogen bonds. The decisive role of hydrogen bonds may partly be related to the fact that tetramethylthiourea dioxide has not been yet prepared in solid state 13. Hydrogen bonds strongly influence the density of thiourea oxides as well: the density of N , N ′‐dimethylthiourea dioxide (1.496 g cm −3 )44 is much lower than that recorded for thiourea di‐ (1.70 g cm −3 )42 and trioxide (1.948 g cm −3 ),28 which is probably a consequence of the absence of extensive hydrogen bonding in the first compound.…”

“…Numerous studies have been devoted to the reactions of thioureas13, 54 or thiourea oxides10a,b,d, 55 with halogens and oxyhalogen compounds. The reactions of oxyhalogens (chlorite, chlorine dioxide, bromate, bromite, iodate) with organosulfur compounds are usually a rich source for the generation of exotic dynamical phenomena.…”

“…Very often kinetic models with 25 or more reactions are proposed, but even these large sets of reactions are unable to explain quantitatively the experimental behavior observed. 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.…”

Oxidation and Reduction

“…There was no evidence for formation of the sulfinic and sulfonic acids, which implicated the sulfoxylate anion as a precursor to formation of sulfate ion. 176 The BrO 3 − oxidation of l-methionine in aqueous acidic medium was first order in substrate and oxidant and second order in H + ion. The rate increased with increase in ionic strength and anions, and decreasing dielectric constant.…”

Oxyhalogen–Sulfur Chemistry: Kinetics and Mechanism of Oxidation of Chemoprotectant, Sodium 2-Mercaptoethanesulfonate, MESNA, by Acidic Bromate and Aqueous Bromine

“…The general driving force for this reaction is controlled by the well-established oxyhalogen kinetics which involve a square dependence term in acid: 30 (R6)…”