Reactions of <i>N</i>,<i>O</i>- and <i>N</i>,<i>S</i>-Azoles and -Azolines with Ozone: Kinetics and Mechanisms

<i>N</i>,<i>O</i>- and <i>N</i>,<i>S</i>-azoles and -azolines are common functional groups in pharmaceuticals, agrochemicals and natural products. Their fate during ozone-based water treatment processes is unknown due to a lack of kinetic and mechanistic information on their reactions with ozone. Apparent second-order rate constants <i>k</i>_O₃ of 12 model compounds were determined at pH 7: oxazoles react 2 orders of magnitude faster (<i>k</i>_O₃ = 9 × 10²-5 × 10⁴ M^-1s^-1, depending on their substituents) than thiazoles (<i>k</i>_O₃ = 1 × 10¹-2 × 10³ M^-1s^-1). The low <i>k</i>_O₃ of thiazoles limits their degradability during ozonation. Only small yields of reactive oxygen species (^•OH, H₂O₂, and ¹O₂) were observed during ozonation of oxazoles, suggesting that all oxygen atoms from ozone are incorporated into the products. Oxazoles and thiazoles react initially by a Criegee-type reaction at the C=C double bond, followed by two reaction branches, leading to two observed product groups: (1) carboxylates and cyanate; (2) formate, amide and CO₂. For thiazoles, thiocarboxylic acids were identified as intermediates, reacting further to sulfate and carboxylic acids, forming ¹O₂. The nonaromatic 2-methyloxazoline is unreactive toward ozone. 2-Methylthiazoline reacts fast (<i>k</i>_O₃ = 2 × 10⁴ M^-1s^-1), forming ¹O₂, leading to ring-opening and formation of dimerization products which react further to <i>N</i>-acetyltaurine. These results enhance the understanding of the ozone reactivity of heterocycles and help predict transformation product formation.