Contributions of BrCl, Br₂, BrOCl, Br₂O, and HOBr to Regiospecific Bromination Rates of Anisole and Bromoanisoles in Aqueous Solution

Abstract

When bromide-containing waters are chlorinated, conventional wisdom typically assumes HOBr is the only active brominating agent. Several additional and often-overlooked brominating agents (including BrCl, Br2, BrOCl, Br2O) can form in chlorinated waters, albeit at generally lower concentrations than HOBr. The extent to which these additional brominating agents influence bromination rates of disinfection byproduct precursors is, however, poorly understood. Herein, the influence of BrCl, Br2, BrOCl, Br2O, and HOBr toward rates of sequential bromination of anisole was quantified. Conditions affecting bromine speciation (e.g., pH, concentrations of chloride, bromide, and chlorine) were varied, and regiospecific second-order rate constants were calculated for reactions of each brominating agent with anisole, 2-bromoanisole, and 4-bromoanisole. The regioselectivity of anisole bromination changed with pH, consistent with the participation of more than one brominating agent. Under conditions representative of chlorinated drinking water, contributions to bromination rates decreased as BrCl > BrOCl > HOBr > Br2O (Br2 negligible). The second-order rate constant determined for net bromination of anisole by HOBr is up to 3000-times less than reported in previous studies (which assumed HOBr was the only active brominating agent). Accordingly, models that assume HOBr is the only kinetically relevant brominating agent in solutions of free bromine may be insufficient for reactions involving modestly nucleophilic organic compounds.

Critical Computational Evidence Regarding the Long-Standing Controversy over the Main Electrophilic Species in Hypochlorous Acid Solution

“…Accordingly, the HOF molecule can undergo electrophilic addition with olefin [ 55 ]. Notably, a large number of studies have shown that the HOBr molecule is not a good brominating agent in water treatment compared to Br 2 and Br 2 O [ 56 ], which is also consistent with our computations.…”

Exotic Electrophiles in Chlorinated and Chloraminated Water: When Conventional Kinetic Models and Reaction Pathways Fall Short

“…HOCl and NH 2 Cl quickly oxidize Br – and I – to produce other electrophilic halogen species (e.g., HOBr and HOI). , Researchers examining electrophilic substitution often focus on reactions directly involving the most abundant halogen species [e.g., hypohalous acids (HOX), where X = Cl, Br, or I]. , However, reaction models that include only the electrophiles that are prevalent in chlor­(am)­inated water sometimes fail to accurately predict product distributions and the effects of halide ion concentration, disinfectant concentration, and pH on reaction rates (see examples in Figures S1–S3). − …”

Oxidative bromination and oxidation of organic substrates catalyzed by bis[2,2'‐hydroxyphenylbenzimidazole]Fe(III) complex entrapped in zeolite‐Y cavity

“…The [(H 2 O)(opbmz) 2 Fe‐OOH] formed in the case of oxidative bromination would later react with Br − ion to liberate HOBr that may bind to the Fe center and attack the substrate by an electrophilic attack (Br + ) to give bromo‐product …”