Reactions of N,O- and N,S-Azoles and -Azolines with Ozone: Kinetics and Mechanisms

N,O- and N,S-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 k_O3 of 12 model compounds were determined at pH 7: oxazoles react 2 orders of magnitude faster (k_O3 = 9 × 10²-5 × 10⁴ M^-1s^-1, depending on their substituents) than thiazoles (k_O3 = 1 × 10¹-2 × 10³ M^-1s^-1). The low k_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 (k_O3 = 2 × 10⁴ M^-1s^-1), forming ¹O₂, leading to ring-opening and formation of dimerization products which react further to N-acetyltaurine. These results enhance the understanding of the ozone reactivity of heterocycles and help predict transformation product formation.