Insights into C-C Bond Cleavage Mechanisms in Dichloroacetonitrile Formation during Chlorination of Long-Chain Primary Amines, Amino Acids, and Dipeptides

• Published in: Environmental science & technology 2023-11, Vol.57 (47), p.18834-18845
• Main Authors: Zhou, Yingying, Jiao, Jia-Jia, Huang, Huang, Liu, Yong Dong, Zhong, Rugang, Yang, Xin
• Format: Article
• Language: English
• Subjects: Amines; Amino acids; Carbon dioxide; Carbonyl compounds; Carbonyl groups; Carbonyls; Chlorination; Cleavage; Covalent bonds; Electron transfer; Hydroxylation; Isocyanates; Nitriles; Precursors
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/acs.est.2c07779

Dichloroacetonitrile (DCAN) as one of the potentially prioritized regulated DBPs has drawn great attention; however, understanding its formation, especially the C-C bond cleavage mechanisms, is limited. In this study, DCAN formation mechanisms from long-chain primary amines, amino acids, and dipeptides during chlorination were investigated by a combined computational and experimental approach. The results indicate that nitriles initially generate for all of the above precursors, then they undergo β- -hydroxylation or/and α- -chlorination processes, and finally, DCAN is produced through the C -C bond cleavage. For the first time, the underlying mechanism of the C-C bond cleavage was unraveled to be electron transfer from the O anion into its attached C atom in the chlorinated nitriles, leading to the strongly polarized C -C bond heterocleavage and DCAN formation. Moreover, DCAN molar yields of precursors studied in the present work were found to be determined by their groups at the γ-site of the amino group, where the carbonyl group including -CO , -COR, and -CONHR, the aromatic group, and the -OH group can all dramatically facilitate DCAN formation by skipping over or promoting the time-consuming β- -hydroxylation process and featuring relatively lower activation free energies in the C-C bond cleavage. Importantly, 4-amino-2-hydroxybutyric acid was revealed to possess the highest DCAN yield among all the known aliphatic long-chain precursors to date during chlorination. Additionally, enonitriles, (chloro-)isocyanates, and nitriles can be generated during DCAN formation and should be of concern due to their high toxicities.