Kinetics and mechanism of the decomposition of N-brominated alanine in aqueous solution

Aqueous bromine reacts with alkyl‐sidechain amino acids through a series of steps resulting in the formation of the corresponding alkyl aldelydes and nitriles. The kinetics and the mechanism of the interaction of bromine with alanine are examined. The products and the rates of this reaction are dependent in a complex way on the initial reactant concentration and pH. Acetaldeyde production is favored at low bromine‐to‐alanine ratios, low bromine concentrations, and pH values above 6. The first‐order rate constant for the formation of acetaldelyde from alanine under these conditions is k4 = 1.98 × 1015 e−22,500/RT min−1. At higher concentration the nitrile is formed through a bromoimine intermediate. Under most conditions the nitrile appears to form from a catalyzed decomposition of the bromoimine which is too fast to be followed by the methods used in this study. However, residual amounts of the bromoimine decay by a slower first‐order mechanism. The rate constant for this slower reaction in the case of alanine at pH 6.8–6.9 and alanine concentrations of 1 × 10−4M is k6 = 1.75 × 105 e−10,400/RT min−1.