Chlorination of Phenols: Kinetics and Formation of Chloroform

Abstract

The kinetics of chlorination of several phenolic compounds and the corresponding formation of chloroform were investigated at room temperature. For the chlorination of phenolic compounds, second-order kinetics was observed, first-order in chlorine, and first-order in the phenolic compound. The rate constants of the reactions of HOCl with phenol and phenolate anion and the rate constant of the acid-catalyzed reaction were determined in the pH range 1−11. The second-order rate constants for the reaction HOCl + phenol varied between 0.02 and 0.52 M^-1 s^-1, for the reaction HOCl and phenolate between 8.46 × 10¹ and 2.71 × 10⁴ M^-1 s^-1. The rate constant for the acid-catalyzed reaction varied between 0.37 M^-2 s^-1 to 6.4 × 10³ M^-2 s^-1. Hammett-type correlations were obtained for the reaction of HOCl with phenolate (log(k) = 4.15−3.00 × Σσ) and the acid-catalyzed reaction of HOCl with phenol (log(k) = 2.37−4.26 × Σσ). The formation of chloroform could be interpreted with a second-order model, first-order in chlorine, and first-order in chloroform precursors. The corresponding rate constants varied between k > 100 M^-1 s^-1 for resorcinol to 0.026 M^-1 s^-1 for p-nitrophenol at pH 8.0. It was found that the rate-limiting step of chloroform formation is the chlorination of the chlorinated ketones. Yields of chloroform formation depend on the type and position of the substituents and varied between 2 and 95% based on the concentration of the phenol.