Formation mechanism of chloropicrin from amines and free amino acids during chlorination: A combined computational and experimental study

Abstract

Chloropicrin as one of the most frequently detected N-DBPs has drawn great attention due to its high toxicity. However, our understanding of its formation mechanism is still very limited. A combined computational and experimental approach was used in this study to reveal chloropicrin formation mechanism during chlorination. Ethylamine, n-propylamine, alanine and tryptophan along with the above two amines and their four derivatives substituted by -OH or/and -NO₂ groups were chosen as computational and experimental model precursors, respectively. The results indicate that primary amines and free amino acids are more likely to share the same chloropicrin formation pathway including N-chlorination, imidization, β-C-alcoholization, N-nitration, α-C-chlorination and dealdehydation processes. Moreover, elimination of hydrochloric acid from N,N-dichloro-amine and electrophilic addition of N-chloroalkylimide with hypochlorous acid were found to be the rate-limiting steps among all the elementary reactions. By skipping over both of the above rate-limiting steps, RCH(OH)CH₂NO₂ and RCH(OH)CH₂NH(OH) compounds were proposed to be potent chloropicrin precursors, and experiments confirmed that 2-nitroethanol and N-methylhydroxylamine have the highest chloropicrin yields in the chlorination among all the precursors reported to date. The findings of this work are helpful for expanding the knowledge of chloropicrin formation mechanisms and predicting the potential chloropicrin precursors.

Keywords: Chlorination; Chloropicrin; DFT; Free amino acids; Primary amines.