Bromine species

Derbyshire and Waters192 measured the rates of bromination of sodium toluene-m-sulphonate (in water) and of benzoic acid (in aqueous acetic acid) by hypobromous acid with sulphuric or perchloric acids as catalysts, all at 21.5 °C. No bromination occurred in the absence of mineral acid and the reaction was strictly first-order in aromatic and in hypobromous acid. The function of the catalyst was considered to be the formation of a positive brominating species, according to the equilibrium... 

De la Mare and Hilton198 measured the rates at 25 °C of bromination of benzene, benzoic acid, phthalic acid, 2-nitrobenzoic acid, trimethylanilinium perchlorate and nitrobenzene by hypobromous acid with sulphuric or perchloric acids as catalysts, in some cases in aqueous dioxan, in an attempt to discover if Br+ or H2OBr+ was the appropriate brominating species since the logarithm of the rates should then follow the acidity functions H0 or HR (J0) respectively. The results, however, were inconclusive and relative rates of bromination were determined (see Table 53). 

De la Mare and Maxwell199 measured the rate of bromination of biphenyl by hypobromous acid in 75 % aqueous acetic acid, in some cases catalysed by perchloric acid, at temperatures between —3.78 and +20.1 °C. They showed that whereas when mineral acid is present the brominating species is Br+ (or a solvate), in the absence of mineral acid it is BrOAc which is a highly reactive brominating species giving Ea = 7.9 (this value is only approximate since it also includes a contribution from bromination by HOBr), and the appropriate kinetic equation is then... 

At 0.9 °C the rate of bromination of biphenyl relative to benzene was approximately 1,270, compared to 26.9 in the presence of mineral acid, and this latter value is fairly close to that obtained with 50 % aqueous dioxan. The possibility that the positive brominating species might be protonated bromine acetate, AcOHBr+, was considered a likely one since the reaction rate is faster in aqueous acetic acid than in water, but this latter effect might be an environmental one since bromination by acidified hypobromous acid is slower in 50 % aqueous dioxan than in... 

Whilst molecular hypobromous acid can be a brominating species, it is not believed to be the active species in acetic acid solution. The bromination of 4-nitroanisole by hypobromous acid in 75 % aqueous acetic acid at 19.8 °C gave a second-order rate coefficient of 0.162, so that the brominating species here appears to be more reactive than molecular bromine194. In addition, the presence of 0.05 M sodium acetate caused the rate coefficient to fall to only 0.040, and both these observations were contrary to expectation if hypobromous acid was the brominating species, but are quite consistent with it being bromine acetate, BrOAc. Also, the addition of chloride ion caused the reaction to become immeasurably slow, due to the formation of the much less reactive bromine chloride. 

Therefore, in order to obtain information about the nature of the brominating species present in the reaction mixture, and on its stability, spectroscopic measurements were carried out in the absence of olefin on methanolic Br2 solutions containing increasing amount of NaN3. (14) When bromine (4.3 x 10 3 M) and methanolic solution of NaN3 (between 4.7 x 10 2 to 2.37 xlO 1 M) were rapidly mixed in a stopped-flow apparatus, at 25 °C, no kinetic of disappearance of Br2 could be observed, but only the presence of a new absorption band (> ax 316 nm) and its subsequent decrease could be measured. The disappearance of the absorption band followed a first order rate law. The observed kinetic constants are reported in Table I. 

A double mediatory system consisting of A-oxoammonium salts and active bromine species, generated from 2,2,6,6-tetramethylpiperidine-1 -oxyl derivatives... 

The apparently first kinetic study of a metal-assisted electrophilic substitution in a Co(III) complex is recent. The bromination of Co(NH3)5imidH is complicated by the presence of different bromine species in solution (Brj, HOBr and Brj"). In addition, successive brominations of the coordinated imidazole occur. Rate data can be interpreted in terms of reaction of the conjugate base of the Co(III) complex with Brj, and a suggested mechanism for the first steps is (Rq = Co(NH3)5 ")... 

The standard potentials of the reactions involving bromine species with oxidation number of +1 or higher are calculated from thermodynamic data. 

Aranda et al. (1997) have shown in laboratory studies that CH302 also reacts with BrO in a manner analogous to the H02 reaction (123), with a rate constant at 298 K of 5.7 X 10"12 cm3 molecule 1 s 1. About 80% of the reaction generates HOBr + CH202, with the remainder forming Br + CH30 + 02. In either case, photochemically active bromine species are regenerated. 

Other species that can initiate this sulfur oxidation chemistry are N03 (discussed in Chapter 7.D.1) and ClJ. The latter radical anion is formed in sea salt particles when atomic chlorine is generated and reacts with chloride ion. In addition, Vogt et al. (1996) have proposed that oxidation of SO2- by HOC1 and HOBr in sea salt particles may be quite important. Table 8.13 summarizes the aqueous-phase chlorine chemistry that occurs in sea salt particles and Table 8.14 the oxidation of S(IV) by reactive chlorine and bromine species in solution. 

Hanson, D. R., and A. R. Ravishankara, Heterogeneous Chemistry of Bromine Species in Sulfuric Acid under Stratospheric Conditions, Geophys. Res. Lett., 22, 385-388 (1995). 

A qualitative similarity to the aqueous chemistry of chlorine will be evident. For each oxoanion of chlorine, there is a corresponding bromine species, although perbromate salts form only under certain strongly oxidizing conditions (e.g., oxidation of bromate ion in alkaline solution with F2 or XeF2) and in fact were unknown until 1968. 

Positively polarized selenation reagents for oxyselenation reactions can be substituted by an indirect electrochemical procedure in which the bromide ion acts as the redox catalyst. The active bromine species generates the phenylselenyl cation from the added diphenyl diselenide (Eq. (48) Table 4, No. 21-26)... 

Bromide and iodide ions are not only effective as redox catalyst in the indirect electrochemical formation of carbon-hetero atom bonds but also of hetero-hetero atom bonds. Thus, the nitrogen-sulfur bond in sulfenimides (Eq. (53) Table 4, No. 33) and sulfenimines (Eq. (54) Table 4, No. 34) can be generated. The electrogenerated active bromine species forms a positively polarized sulfenyl intermediate from disulfides. 

Bromine containing species, introduced from Man s release of halons, are also believed to play a significant role in the polar ozone depletion, despite the fact that the total inorganic bromine concentration in the stratosphere is typically two orders of magnitude lower than the inorganic chlorine. This is manifested in the presence of Br0N02 and HOBr, which however are less stable than the chlorine reservoirs, so that relatively more BrO, is in the active fotm [36,37]. There is a synergism between the chlorine and bromine species the oxides radicals GO and BrO react with each other to produce a series of products, G, Br, BrCl and OCIO. The latter compound is an indicator of the elevated levels of both BrO and GO [38]. 

Hanson, D.R., and Ravishankara, A.R. (1995) Heterogeneous chemistry of bromine species in sulphuric acid under stratospheric conditions, Geophys. Res. Lett. 22,385-388. 

The reaction of [Co(NH3)5(imH)]-1 + with Br2(aq) in acetate or phosphate buffer gave, in addition to ring-brominated species, the imidazole-2,4,5-trione complex [Co(NH3)5(parabanate)]2+ (71). The same reaction also occurred on treatment of the imidazole complex with Cl2(alli in the absence... 

To introduce a bromo functionality a reagent delivering a cationic bromine species is needed. 

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