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_O₃) with 13 unsubstituted/benzosubstituted aromatic heterocycles were determined and compared to reported values to identify reactivity trends. The k_O₃ of the simplest one-heteroatom compounds, pyrrole, furan, and thiophene decrease in the order N>O>S; pyrrole and furan react rapidly (>10⁶ M^-1s^-1), 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_O₃ = 1.5×10²-2.3×10⁵ M^-1s^-1) than 1,2-isomers (k_O₃ =0.5-56 M^-1s^-1), maintaining the trend N>O>S. More ring nitrogens further decrease k_O₃: 1,2,3-triazole is somewhat reactive (k_O₃ = 18 M^-1s^-1), 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_O₃ of one-heteroatom compounds (indole (k_O₃ =1.8×10⁶ M^-1s^-1), benzofuran (k_O₃ = 7.2×10⁵ M^-1s^-1), benzothiophene (k_O₃ = 1.8×10⁵ M^-1s^-1)). In contrast, 1,3-benzazoles (benzimidazole, benzoxazole, benzothiazole) react much slower with ozone (k_O₃ = 2.3-90 M^-1s^-1), consistent with the absence of a C=C double bond. Quantum-chemical descriptors were evaluated for correlation with k_O₃ of 26 aromatic heterocycles. E_HOMO and E_NBO overall correlated weakly with k_O₃. Excluding benzosubstituted 1,3-azoles and benzotriazole markedly improved the E_HOMO-k_O₃ correlation (R²=0.65, n=22), capturing k_O₃ trends across diverse subsets of heterocycles. For substituted 1,3-oxazoles and substituted 1,3-thiazoles, E_HOMO correlated strongly with k_O₃ (R²=0.95 and 1.00, n=5, respectively). E_NBO 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.