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Degradation Mechanisms and Substituent Effects of N‑Chloro-α-Amino Acids: A Computational Study
journal contribution
posted on 2020-02-21, 17:35 authored by Huiyuan Zhao, Yingying Zhou, Chunxiu Han, Yong Dong Liu, Rugang ZhongN-Chloro-α-amino
acids formed in the chlorination
disinfection treatment of water or wastewater and in living organisms
have attracted extensive attention due to the potential toxicities
of themselves and their decomposition products. The degradation mechanisms
of three N-chloro-α-amino acids, i.e., N-chloro-glycine, N-chloro-alanine, and N-chloro-valine, have been systematically investigated using
quantum chemical computations. The results indicate that N-chloro-α-amino acid anions undergo two competitive degradation
pathways: a concerted Grob fragmentation (CGF) and β-elimination (β-E). Generally, the former predominates
over the latter under neutral conditions and finally generates amines
and carbonyls, while the latter is preferred under base-promoted conditions
and mainly produces the respective α-keto acid anions or nitriles
in the end. To gain deeper insights into the substitution effects,
in view of the advantages of quantum chemical computations, a number
of real or designed N-chloro-α-amino acids
with traditional electron-donating groups (EDG) or electron-withdrawing
groups (EWG) have been studied. All of the substituted N-chloro-α-amino acids, regardless of the type and position
of substituents, are kinetically more favorable than N-monochloro-glycine for degradation via the CGF pathway. Moreover,
conjugated EDG substituted on the N-terminal facilitate
both CGF and β-E reactions, whereas conjugated EDG and EWG on
the α-carbon are only favorable for the CGF and β-E reactions,
respectively. These results are expected to expand our understanding
of organic N-chloramine degradation mechanisms and
chlorination reaction characteristics.
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Keywords
β- E reactionsCGF pathwaydegradation pathwaysSubstituent Effectschlorination disinfection treatmentdecomposition productsDegradation Mechanismsα- carbonchloramine degradation mechanismsEWGbase-promoted conditionsquantum chemical computationschlorination reaction characteristicschloro -α-amino acidschloro -α-amino acid anionsGrob fragmentationEDGChloro -α-amino acidsα- keto acid anionselectron-withdrawing groupsdegradation mechanismsComputational Study Nelectron-donating groupssubstitution effects
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