Oxidation of branched chain amino acids by HOCl: Kinetics and mechanism

The kinetics of the chlorination of leucine, isoleucine, and valine (BCAAs) was studied in excess HOCl by stopped-flow and spectrophotometric methods (25 ^◦C, I = 1.0 M NaClO₄). The intermediates and products were identified and monitored by ¹H NMR spectroscopy. It was established that these reactions are fully analogous and proceed according to distinct mechanisms under alkaline and neutral conditions. At high pH, the formation and subsequent rate determining decomposition of N-monochloroamino acid control the process. The decomposition occurs via competing pH-independent and OH^--assisted reaction paths and the sequence of chlorination, dichlorination and decarboxylation steps leads to the formation of N-chloroimines and their carbanionic forms, which are in fast acid - base equilibria. The dechlorination of the carbanions yields nitriles as the main products. The hydration of the N-chloro imines produces chloramine and aldehydes which are involved in further oxidation reactions with HOCl. The formation of chloroform and chloroacetaldehyde was confirmed in each system. At pH 7.0, the N-chloro derivatives of BCAAs form immediately and are converted into the corresponding N,N-dichloro species within a few seconds after mixing the reactants. In this reaction, the reactive form of the oxidant is Cl₂O. The first-order decomposition of the dichloroamino acids occurs on stopped-flow timescale (k = 0.5 - 0.7 s^-1) and yields N-chloroimines which slowly decompose with a characteristic first-order rate constant on the order of a few times 10^-5 s^-1. The main products are the corresponding nitriles that account for about 80% and 60% of the original amounts of amino acids under neutral and alkaline (c_OH = 5.00 × 10^-2 M) conditions, respectively. Aldehydes, carboxylic acids, chloroform and NCl₃ were also identified as by-products. The results unequivocally confirm that harmful chlorinated species may form from amino acids long after the chlorination step in water treatment technologies that deteriorates the quality of the finished water. ENVIRONMENTAL IMPLICATION: In source waters, amino acids account for about 75% of the total dissolved nitrogen. Therefore, it is an essential issue how the reactions of these compounds with hypochlorite ion can be controlled to avoid the formation of toxic compounds. The compounds formed from BCAAs are considered to be harmful both under alkaline and neutral conditions (chloroacetaldehyde, chloroform, nitriles). However, some of the intermediates have extended lifetime in these systems and they may also react with other components of raw water during water treatment processes.