The kinetics and primary products of uracil chlorination

Abstract

The reaction between free available chlorine (FAC) and the pyrinidine base uracil, was studied over a pH range of 5–9 and at a range of FAC to uracil ratios between 0.5 and 5 mol mol −1. Rate constants for the reaction were obtained and the primary uracil chlorination products identified. The reaction was found to adhere very well to mixed second order kinetics at FAC to uracil molar ratios ⩾ ∼ 1.0. At a ratio of 5 mol mol −1, the reaction demonstrated a reasonable empirical fit to the same kinetic model. The rate constant was markedly pH dependent increasing from approx. 0.3 M −1 s −1 at pH = 5 to 5 M −1 s −1 above pH = 7 at low molar ratios while the respective values at high molar ratios were approx. 7 and 90 M −1 s −1. At low FAC uracil ratios the chlorination reaction yielded exclusively 5-chlorouracil while at the higher ratios, a dichlorouracil was isolated in a pure form and characterized spectrometrically. This material was the only organic chloramine detected in any of the systems. While evidence was obtained for breakdown of the uracil ring, little evidence for formation of secondary chloramines was found. The characteristic odor of nitrogen trichloride was noted at the higher FAC/uracil ratios. FAC consumption increased from 1 mol of FAC per mol of uracil consumed at the low molar ratios to 3 mol mol −1 at initial ratios of ≈ 5 mol FAC per mol uracil.