Reactions of pyrrole, imidazole, and pyrazole with ozone:Kinetics and mechanisms

Five-membered nitrogen-containing heterocyclic compounds (azoles) belong to potential moieties in complex structures where transformations during ozonation can occur. This study focused on the azole-ozone chemistry of pyrrole, imidazole, and pyrazole as model compounds. Reaction kinetics and ozonation products were determined by kinetic and analytical methods including NMR, LC-HRMS/MS, HPLC-UV, and IC-MS. Analyses of reactive oxygen species (O-1(2), & x2d9;OH, H2O2), quantum chemical computations (Gibbs energies), and kinetic simulations were used to further support the proposed reaction mechanisms. The species-specific second-order rate constants for the reactions of ozone with pyrrole and imidazole were (1.4 +/- 1.1) x 10(6) M-1 s(-1) and (2.3 +/- 0.1) x 10(5) M-1 s(-1), respectively. Pyrazole reacted more slowly with ozone at pH 7 (k(app) = (5.6 +/- 0.9) x 10(1) M-1 s(-1)). Maleimide was an identified product of pyrrole with a 34% yield. Together with other products, formate, formamide, and glyoxal, C and N mass balances of similar to 50% were achieved. Imidazole reacted with ozone to cyanate, formamide, and formate (similar to 100% yields per transformed imidazole, respectively) with a closed mass balance. For pyrazole, a high ozone : pyrazole molar stoichiometry of 4.6 was found, suggesting that the transformation products contributed to the over-stoichiometric consumption of ozone (e.g., hydroxypyrazoles). Glyoxal and formate were the only identified transformation products (C mass balance of 65%). Overall, the identified major products are suspected to hydrolyze and/or be biodegraded and thereby abated by a biological post-treatment typically following ozonation. However, as substructures of more complex compounds (e.g., micropollutants), they might be more persistent during biological post-treatment

Reaction of 1-propanol with Ozone in Aqueous Media

The main aim of this work is to substantiate the mechanism of 1-propanol oxidation by ozone in aqueous solution when the substrate is present in large excess. Further goals are assessment of the products, their formation yields as well as the kinetic parameters of the considered reaction. The reaction of ozone with 1-propanol in aqueous solution occurs via hydride transfer, H-abstraction and insertion. Of these three mechanisms, the largest share is for hydride transfer. This implies the extraction of an hydride ion from the activated C-H group by O3 according to reaction: (C2H5)(H)(HO)C-H + O3 ?[(C2H5)(H)(HO)C-H+O3?]cage ?(C2H5)(H)(HO)C+ + HO3 -. The experimentally determined products and their overall formation yields with respect to ozone are: propionaldehyde-(60 ± 3)%, propionic acid-(27.4 ± 1.0)%, acetaldehyde-(4.9 ± 0.3)%, acetic acid-(0.3 ± 0.1)%, formaldehyde-(1.0 ± 0.1)%, formic acid-(4.6 ± 0.3)%, hydrogen peroxide- (11.1 ± 0.3)% and hydroxyl radical-(9.8 ± 0.3)%. The reaction of ozone with 1-propanol in aqueous media follows a second order kinetics with a reaction rate constant of (0.64±0.02)M-1·s-1 atpH = 7 and 23 °C. The dependence of the second order rate constant on temperature is described by the equation: ln kII = (27.17 ± 0.38)-(8180 ± 120) × T-1, which gives the activation energy, Ea = (68 ± 1) kJ mol-1 and pre-exponential factor, A = (6.3 ± 2.4) × 1011 M-1 s-1. The nature of products, their yields and the kinetic data can be used in water treatment. The fact that the hydride transfer is the main pathway in the 1-propanol/ozone system can probably be transferred on other systems in which the substrate is characterized by C-H active sites only. © 2019 by the authors. Licensee MDPI, Basel, Switzerland