pH controlled byproduct formation in aqueous decomposition of N-chloro-α-alanine

N-chloro-amino acids are readily formed in chlorination water treatment technologies. These reactions are also important in biological systems where HOC1 plays an important role in the defense mechanism against invading pathogens. The intermediates and the products formed are of primary concern because they may have significant biological activities. In order to clarify intimate details and resolve discrepancies in the literature, the decomposition kinetics of N-chloro-alpha-alanine (MCA) was studied in the neutral - alkaline pH range by UV-vis spectrophotometry and H-1-NMA method. In contrast to earlier reports, the decomposition reaction proceeds via two distinct reaction paths: k(obs1) = k(oH)[OH-] + k, where k(oH) = (1.38 +/- 0.02) x 10(-2) M(-1)s(-1) and k = (2.95 +/- 0.09) x 10(-4) s(-1). In slightly alkaline solution, the sole product is acetaldehyde. Under alkaline conditions, the main product is pyruvate ion, however, N-acetyl-alpha-alanine is also formed in a subsequent reaction sequence. A detailed kinetic model is postulated which involves the rate determining dissociation of MCA into Cl- and ethanimine which produces acetaldehyde in further reaction steps. Via the OH- assisted path, first a carbanion is formed which undergoes dechlorination and produces iminopropionate ion. This species is transformed into pyruvate ion through hydration and deamination steps.