Chlorination Revisited: Does Cl^- Serve as a Catalyst in the Chlorination of Phenols?

The aqueous chlorination of (chloro)phenols is one of the best-studied reactions in the environmental literature. Previous researchers have attributed these reactions to two chlorine species: HOCl (at circum-neutral and high pH) and H₂OCl⁺ (at low pH). In this study, we seek to examine the roles that two largely overlooked chlorine species, Cl₂ and Cl₂O, may play in the chlorination of (chloro)phenols. Solution pH, chloride concentration, and chlorine dose were systematically varied in order to assess the importance of different chlorine species as chlorinating agents. Our findings indicate that chlorination rates at pH < 6 increase substantially when chloride is present, attributed to the formation of Cl₂. At pH 6.0 and a chlorine dose representative of drinking water treatment, Cl₂O is predicted to have at best a minor impact on chlorination reactions, whereas Cl₂ may contribute more than 80% to the overall chlorination rate depending on the (chloro)phenol identity and chloride concentration. While it is not possible to preclude H₂OCl⁺ as a chlorinating agent, we were able to model our low-pH data by considering Cl₂ only. Even traces of chloride can generate sufficient Cl₂ to influence chlorination kinetics, highlighting the role of chloride as a catalyst in chlorination reactions.