Kinetics and mechanism of general-acid-assisted oxidation of bromide by hypochlorite and hypochlorous acid

Abstract

ascribe the difference in reaction enthalpies to a bonding interaction more favorable in CuT than in H[CuT] compared with the change in overall bonding between Cu(EDTA)2' and H [Cu-(EDTA)]". This might result from a reorientation of metal-ligand bonds in the CuT species resulting in either release of strain or more effective metal-ligand interaction. Some internal reorientation is consistent with the more negative AS0 value for H [CuT] acidic dissociation compared with that for H[Cu(EDTA)]'. Both processes restrict internal motion by binding a carboxylate group, but this restriction seems larger in the case of H[CuT]. AS°for the acidic dissociation reactions consists of contributions from several processes: (1) solvent ordering around H+, common to both reactions; (2) increased secondary solvation around the copper complexes, a process that provides a larger negative contribution to H[Cu(EDTA)]" dissociation than to H[CuT] dissociation; (3) positive contributions due to solvent release from the sixth copper coordination; (4) loss of internal freedom. Of these factors the last might account for the more negative AS°value observed for H[CuT] dissociation. That is, differences in solvent binding at the sixth copper coordination site might result in a relatively more negative contribution to AS°for H[CuT] dissociation. However, the sixth coordination position on Cu2+ is known to be only weakly solvated so that differences between weakly solvated sites are not likely to provide for the observed result. Consequently, it appears possible that some distortion of Cu2+ ligation occurs upon CuT formation. This proposition seems supported by the X-ray crystallographic data, which indicate an unusually large trigonal distortion in CuT in which the locus of nitrogen atoms is twisted away from the carboxylate locus by about 30°from the idealized octahedral orientation. A large trigonal distortion of this kind would certainly influence the ligand field near Cu2+ and might profoundly effect the magnitude of the ligand field, thus accounting for the unusual difference between spectral properties of H[CuT] and CuT. As a final comparison we note that Xmax = 660 nm2c for aqueous Cu[9]aneN32+, which we presume exists as a hexacoordinate species. Unfortunately, no crystallographic data is available for this complex. Nevertheless it seems unlikely that large trigonal distortions would be present and that the similar Xmax values of H[CuT] and Cu[9]aneN3(H20)32+ reflect approximately octahedral coordination. Thus, it appears that the large displacement of Xmax from 660 nm for H[GuT] to a value near 750 nm for CuT results from a substantial distortion of the ligand field in CuT, which we connect with a release of strain interactions in H[CuT], The importance of these interactions is consistent with thermodynamic, X-ray crystallographic, and spectral evidence.

Enhanced Bromate Formation during Chlorination of Bromide-Containing Waters in the Presence of CuO: Catalytic Disproportionation of Hypobromous Acid

“…The reaction between HOCl and Br − dominates because the reaction rate constant between OCl − and Br − is low (eq 2). 13 In the presence of CuO, the HOBr decay rate is significantly enhanced (e.g., 10 4 times with 0.2 g L −1 CuO at pH 8.6), via two reaction pathways (eqs 11 and 12). For pH between 7.6 and 9.6, the disproportionation of HOBr to bromate is the major pathway, and the reaction rate is highest near the pK a of HOBr.…”

“…12 Bromate formation during chlorination of bromide-containing waters is a slow process. In a first step (eqs 1 and 2), bromide is oxidized by hypochlorous acid (HOCl) to form hypobromous acid (HOBr), 13 which is in equilibrium with OBr − with a pK a of 8. …”

Production of Brominating Intermediates by Myeloperoxidase

“…Based on the chemistry of the interhalogen compounds (40,41), we propose that BrCl is were performed as described in the legend to Fig. 3.…”

Kinetics and mechanism of dimethoate chlorination during drinking water treatment

“…Because bromide is quite ubiquitous and 90% of drinking water systems use FC for disinfection in China, the presence of bromide in source waters can affect the transformation rate of organophosphorus pesticides in chlorinated potable water (Duirk et al, 2008). In the presence of aqueous FC, bromide can be easily oxidized into hypobromous acid (HOBr), which was found to oxidize organics at a much faster rate than HOCl (Kumar and Margerum, 1987;Duirk et al, 2008). Experiments to determine whether bromide accelerates the rate of DMT degradation were conducted at pH 8.29 with increasing bromide concentrations from 0 to 5 mg L À1 (Fig.…”