The chlorination of glycine and α-alanine at excess HOCl: Kinetics and mechanism

Fruzsina Simon; Mária Szabó; István Fábián

doi:10.1016/j.jhazmat.2023.130794

The chlorination of glycine and α-alanine at excess HOCl: Kinetics and mechanism

J Hazard Mater. 2023 Apr 5:447:130794. doi: 10.1016/j.jhazmat.2023.130794. Epub 2023 Jan 13.

Authors

Fruzsina Simon¹, Mária Szabó², István Fábián¹

Affiliations

¹ Department of Inorganic and Analytical Chemistry, University of Debrecen, Debrecen, Hungary; ELKH - DE Mechanisms of Complex Homogeneous and Heterogeneous Chemical Reactions Research Group, University of Debrecen, Debrecen, Hungary.
² Department of Inorganic and Analytical Chemistry, University of Debrecen, Debrecen, Hungary; ELKH - DE Mechanisms of Complex Homogeneous and Heterogeneous Chemical Reactions Research Group, University of Debrecen, Debrecen, Hungary. Electronic address: szabo.maria@science.unideb.hu.

PMID: 36669404
DOI: 10.1016/j.jhazmat.2023.130794

Abstract

The chlorination of the two simplest amino acids at HOCl excess was studied by stopped-flow, conventional spectrophotometric and time resolved ¹H NMR kinetic methods at 25 °C. These reactions show distinct characteristics under neutral and alkaline conditions. At high pH, the common feature of the two systems is that the N-dichloroamino carboxylate ion does not form and the overall process is controlled by the initial decomposition of the N-monochloro derivative. Under such conditions, carbanions form in equilibrium acid - base processes and open alternative reaction paths, resulting in enhanced complexity of the corresponding mechanisms. In the case of α-alanine, the formation of acetonitrile and N-chloro acetamide as main products; acetate ion, acetaldehyde, chloroacetaldehyde, chloroform as byproducts; acetamide and N-chloro ethanimine as intermediates was confirmed. In the case of glycine, the final products are formamide and OCN^-. Under neutral conditions, monochloroamino acid forms immediately upon mixing the reactants, and subsequently it is converted into dichloroamino acid by Cl₂O in a fast process. In considerably slower further reaction steps, acetonitrile and acetate ion form as final products in the α-alanine system, while the chlorination of glycine proceeds to full mineralization. The detailed mechanisms suggested for these reactions postulate the formation of various imines and N-chloro imines which are involved in decarboxylation, dechlorination, hydration and hydrolytic reaction steps.

Keywords: Chlorination; Hypochlorous acid; Kinetics; N-chloroalanine; N-chloroglycine; Oxidation.