Kinetics of aqueous phase ozone reactions of 5-membered aromatic heterocycles.

5-membered aromatic heterocycles are abundant in natural organic compounds, pesticides and pharmaceuticals. Systematic kinetic data of their aqueous reactions with ozone is scarce, hindering predictions on their fate during ozonation processes. Therefore, second-order rate constants for reactions of ozone (k) with 13 unsubstituted/benzosubstituted aromatic heterocycles were determined and compared to reported values to identify reactivity trends. The k of the simplest one-heteroatom compounds, pyrrole, furan, and thiophene decrease in the order N>O>S; pyrrole and furan react rapidly (>10 Ms), whereas thiophene is 35-fold less reactive. In aromatic heterocycles, sulfur seems completely deactivated. For two-heteroatom compounds, both nitrogen in the 2- and 3-position are deactivating, with 1,3-isomers being significantly more reactive k = 1.5×10-2.3×10 Ms than 1,2-isomers k =0.5-56 Ms, maintaining the trend N>O>S. More ring nitrogens further decrease k: 1,2,3-triazole is somewhat reactive k = 18 Ms, while 1,2,4-triazole and tetrazole show no measurable reaction, likely due to the absence of an attackable C=C double bond. Benzo-substitution increases k of one-heteroatom compounds indole (k =1.8×10 Ms, benzofuran k = 7.2×10 Ms, benzothiophene k = 1.8×10 Ms). In contrast, 1,3-benzazoles (benzimidazole, benzoxazole, benzothiazole) react much slower with ozone k = 2.3-90 Ms, consistent with the absence of a C=C double bond. Quantum-chemical descriptors were evaluated for correlation with k of 26 aromatic heterocycles. E and E overall correlated weakly with k. Excluding benzosubstituted 1,3-azoles and benzotriazole markedly improved the E-k correlation R=0.65, n=22, capturing k trends across diverse subsets of heterocycles. For substituted 1,3-oxazoles and substituted 1,3-thiazoles, E correlated strongly with k R=0.95 and 1.00, n=5, respectively. E showed weak correlations even within subsets, suggesting that aromaticity of these compounds dominates their ozone reactivity. Overall, the data presented provides novel insights into the ozone-reactivity of 5-membered aromatic heterocycles to better assess their fate during ozonation.