Oxyhalogen-Sulfur Chemistry: Kinetics and Mechanism of Oxidation of N-acetylthiourea by Aqueous Bromate and Acidified Bromate

Abstract

The oxidation of N-acetylthiourea (ACTU) by acidic bromate has been studied by observing formation of bromine in excess bromate conditions. The reaction displays an induction period before formation of bromine. The stoichiometry of the reaction was determined to be 4:3: 4BrO₃ ^–+3(CH₃CO)NH(NH₂)C=S+3H₂O®4Br^–+3(CH₃CO)NH(NH₂)C=O+3SO₄^2–+6H⁺ (A) with a complete desulfurization of ACTU to its urea analogue. In excess bromate conditions the stoichiometry was 8:5: 8BrO₃^– + 5(CH₃CO)NH(NH₂)C=S + H₂O ® 4Br₂ + 5(CH₃CO)NH(NH₂)C=O + 5SO₄²⁺ + 2H⁺ (B). Bromine is derived from an extraneous reaction in which bromide fromstoichiometry (A) reacts with excess acidic bromate. The oxidation of ACTU by aqueous bromine gave stoichiometry (C): 4Br₂(aq)+(CH₃CO)NH(NH₂)C=S+5H₂O®8Br^–+CH₃CO)NH(NH₂)C=O+SO₄ ^2–+¹⁰H⁺.Reaction (C) is much faster than reactions (A) and (B), with a lower limit bimolecular rate constant of 2.1 ×10⁵ M^–1 s^–1 such that appearance of bromine signals complete consumption of ACTU. We were unable to trap any intermediate sulfur oxo-acids of ACTU on its oxidation pathway to N-acetylurea. As opposed to other substituted thioureas, none of its intermediates were stable enough to be isolated and detected.

KEYWORDS Kinetics, mechanisms, oxyhalogen chemistry, s-oxygenation, bioactivation.