title: Reactivity of BrCl, Br₂, BrOCl, Br₂O, and HOBr toward dimethenamid in solutions of bromide + aqueous free chlorine.
abstract: HOBr, formed via oxidation of bromide by free available chlorine (FAC), is frequently assumed to be the sole species responsible for generating brominated disinfection byproducts (DBPs). Our studies reveal that BrCl, Br(2), BrOCl, and Br(2)O can also serve as brominating agents of the herbicide dimethenamid in solutions of bromide to which FAC was added. Conditions affecting bromine speciation (pH, total free bromine concentration ([HOBr](T)), [Cl(-)], and [FAC](o)) were systematically varied, and rates of dimethenamid bromination were measured. Reaction orders in [HOBr](T) ranged from 1.09 (±0.17) to 1.67 (±0.16), reaching a maximum near the pK(a) of HOBr. This complex dependence on [HOBr](T) implicates Br(2)O as an active brominating agent. That bromination rates increased with increasing [Cl(-)], [FAC](o) (at constant [HOBr](T)), and excess bromide (where [Br(-)](o)>[FAC](o)) implicate BrCl, BrOCl, and Br(2), respectively, as brominating agents. As equilibrium constants for the formation of Br(2)O and BrOCl (aq) have not been previously reported, we have calculated these values (and their gas-phase analogues) using benchmark-quality quantum chemical methods [CCSD(T) up to CCSDTQ calculations plus solvation effects]. The results allow us to compute bromine speciation and hence second-order rate constants. Intrinsic brominating reactivity increased in the order: HOBr ≪ Br(2)O < BrOCl ≈ Br(2) < BrCl. Our results indicate that species other than HOBr can influence bromination rates under conditions typical of drinking water and wastewater chlorination.
authors: Department of Geography and Environmental Engineering, Johns Hopkins University, 313 Ames Hall 3400 North Charles Street Baltimore, Maryland 21218, United States.
authors: John D
authors: JD
authors: Sivey
authors: J Samuel
authors: JS
authors: Arey
authors: Peter R
authors: PR
authors: Tentscher
authors: A Lynn
authors: AL
authors: Roberts
mesh_terms: D000083:Acetanilides / Q000737:chemistry*; D001959:Bromates / Q000737:chemistry; D001965:Bromides / Q000737:chemistry; D001966:Bromine / Q000737:chemistry*; D017605:Bromine Compounds / Q000737:chemistry*; D002384:Catalysis; D002712:Chlorides / Q000032:analysis / Q000737:chemistry; D002713:Chlorine / Q000737:chemistry*; D004777:Environment; D005740:Gases / Q000737:chemistry; D054879:Halogenation; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008956:Models, Chemical; D011789:Quantum Theory; D012996:Solutions; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
chemical_list: D000083:Acetanilides; D001959:Bromates; D001965:Bromides; D017605:Bromine Compounds; D002712:Chlorides; D005740:Gases; D012996:Solutions; D014867:Water; D002713:Chlorine; C016271:bromine chloride; C120861:dimethenamid; C027664:hypobromous acid; D001966:Bromine