Kinetics, Mechanisms, and Particle Formation from Gas-Phase OH Oxidation of 2-Methylthiobenzothiazole

Abstract

Benzothiazole (BTH) derivatives are used widely. In particular, 2-methylthiobenzothiazole (2-MeS-BTH), used in rubber products and textiles and formed in the degradation of other BTH compounds, is found in air, particles, and water systems. In air, it reacts with OH radicals. We report here the first studies of the kinetics, products, and mechanisms of the gas-phase reaction of OH with 2-MeS-BTH. Relative rate studies carried out in air at 1 atm and 298 K give a rate constant for 2-MeS-BTH reaction with OH of (5.4 ± 0.9) × 10^–12 cm³ molecule^–1 s^–1 (1σ), indicating a lifetime of ∼2 days in air at 1 × 10⁶ cm^–3 OH. The reaction proceeds through (1) the abstraction of hydrogen atoms from the thiomethyl group, (2) the addition of OH to the thiomethyl sulfur atom, and (3) the addition of OH to benzene ring as a minor path. Sulfuric acid and benzothiazole-2-sulfonic acid are major products, contributing to particle formation. This is consistent with contributions from the oxidation of SO₂ formed in the reaction as well as the direct formation of SO₃ in the oxidation pathways. Thus, not only the parent compound but also its oxidation products should be included in assessments of 2-MeS-BTH environmental impacts.