Chlorination of a chlorothalonil metabolite (R471811) and aniline: Kinetics and mechanisms

Chlorothalonil metabolites (CMs) have been detected ubiquitously and persistently in European groundwater, despite the ban of chlorothalonil in the European Union and Switzerland in 2019. Among the six frequently detected CMs, CM (R471811), which contains two amide groups and one sulfonic acid moiety, has been detected most frequently and at the highest concentrations. This metabolite has been shown to be partially abated during mild chlorination, despite its limited removal by ozonation or hydroxyl radical-driven advanced oxidation processes, thereby warranting further investigation of its chlorination kinetics and mechanisms. CM was found to react primarily with hypochlorite via nucleophilic attack at the two amide nitrogen atoms, exhibiting distinct species-specific second-order rate constants of 24 M −1 s −1 and 0.19 M −1 s −1 , respectively for the two reaction steps. The reaction initially yielded N -chloramides, which subsequently underwent a Hofmann rearrangement to form an aniline-type compound. The rearrangement from aromatic N -chloramide to aniline was confirmed during chlorination of the model compound 2,4,5-trichlorobenzamide with the detection of 2,4,5-trichloroaniline. To elucidate the fate of aniline, its kinetics and mechanisms for the reaction with chlorine were further investigated. Chlorination of aniline was dominated by hypochlorous acid, with an apparent second-order rate constant of 4.9 × 10 4 M −1 s −1 at pH 7. The reaction is proposed to initially form N -chloroaniline, followed by cleavage of the N - Cl bond generating phenylnitrenium cations, leaving the ortho and para positions available for nucleophilic substitution. As a consequence, 2-chloroaniline, 4-chloroaniline, and 4-aminophenol were detected as initial chlorination products. Subsequently, 2-chloroaniline was transformed to 4-amino-3-chlorophenol, while a fraction of 4-aminophenol was further oxidized to the potentially toxic products, 4-nitroso- and 4-nitrophenol, with yields of ≤ 4.5%.