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

Abstract

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 HOCl (at low pH). In this study, we seek to examine the roles that two largely overlooked chlorine species, Cl and ClO, 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 Cl. At pH 6.0 and a chlorine dose representative of drinking water treatment, ClO is predicted to have at best a minor impact on chlorination reactions, whereas Cl 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 HOCl as a chlorinating agent, we were able to model our low-pH data by considering Cl only. Even traces of chloride can generate sufficient Cl to influence chlorination kinetics, highlighting the role of chloride as a catalyst in chlorination reactions.

Mechanistic Investigation of Enhanced Photoreactivity of Dissolved Organic Matter after Chlorination

“…Chlorine is the most widely used disinfectant in water treatment due to its low cost and high efficiency for the inactivation of microbes . In wastewater treatment plants (WWTPs), chlorine disinfection is applied in the final stage of treatment before effluents are discharged into receiving waters .…”

Theoretical Investigation into Rate-Determining Factors in Electrophilic Aromatic Halogenation

“…All three species have been suggested to be the active electrophile in aromatic chlorination . In two recent papers, Roberts and co-workers performed a series of kinetic experiments to assess the importance of the different electrophiles in aromatic chlorination in aqueous solution. , HOCl was found to be most important electrophile only in the case of the very activated aromatic 1,3,5-trimethoxybenzene. However, for the less activated 3-methylanisole, the kinetic data investigated as a function of pH and [Cl – ] indicates that Cl 2 O is the dominating electrophile at pH > 7.5, while it is Cl 2 at pH < 6.5 .…”

“…However, for the less activated 3-methylanisole, the kinetic data investigated as a function of pH and [Cl – ] indicates that Cl 2 O is the dominating electrophile at pH > 7.5, while it is Cl 2 at pH < 6.5 . Furthermore, it was found at low pH that the chlorination rate increases when Cl – is present; this was attributed to the reaction of HOCl and Cl – to form Cl 2 . Considering that most reference data for aromatic halogenation are in aqueous acetic acid and thus under acidic conditions, we chose Cl 2 as the electrophile in our computational study.…”

“…7 Furthermore, it was found at low pH that the chlorination rate increases when Cl − is present; this was attributed to the reaction of HOCl and Cl − to form Cl 2 . 8 Considering that most reference data for aromatic halogenation are in aqueous acetic acid and thus under acidic conditions, we chose Cl 2 as the electrophile in our computational study. The reaction with Cl 2 has been shown to follow second-order kinetics.…”

Regioselective Oxidative Chlorination of Arenols Using NaCl and Oxone

“…Chlorine (Cl 2 ) or hypochlorous acid (HOCl) might be generated in situ as an active electrophilic chlorinating species from NaCl and Oxone under acidic conditions , . Interestingly, the reaction was completed within 5 minutes, as in the stoichiometric reaction with NaOCl · 5H 2 O (see Scheme ), when NaCl was pre‐mixed with Oxone for 1 h to generate active chlorinating species before the addition of 1a (Table , entry 10 vs. entry 5).…”