Hypobromous acid, aqueous

Acids nd Sa.lts. The oxygen acids of bromine are strong oxidants but at ordinary temperatures are stable only in solution. An aqueous solution of hypobromous acid [13517-11-8] may be prepared by treating bromine water with silver oxide or mercuric oxide (69) ... 

The mechanism of oxidation has not been studied in detail. In aqueous media it may be assumed that hypobromous acid is formed and that this is the true oxidizing agent. Indeed, Deno" has shown that the rate of oxidation of alcohols by aqueous bromine closely parallels the HOBr concentration and that the alkyl hypohalide is a most likely intermediate. Rate studies and... 

The method of Fried and Sabo for the in situ preparation of hypobromous acid from A-bromoacetamide (or A-bromosuccinimide) in aqueous dioxane or acetone containing perchloric acid is commonly used, e.g., (81) (82). 

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... 

The positive bromination of aromatics ethers was first studied by Bradfield et al.193 and by Branch and Jones194. The reaction of hypobromous acid in 75 % aqueous acetic acid with benzyl 4-nitrophenyl ether and 4-nitrophenetole at 20 °C was very rapid and approximately second-order193. The value of k2/[H+] remained constant in the [H+] range 0.005-0.090 M for the effect of added mineral acids on the bromination of 4-nitroanisole and 4-nitrophenetole (at 19.8 °C)194. The variation in reaction rate with the percentage of acetic acid in the medium was also studied and showed a large increase in the 0-10 % range with a levelling off at approximately 25 % acetic acid (Table 52) this was attributed... 

Aqueous dioxan (50%) has been used as a medium for bromination with acidified hypobromous acid and de la Mare and Harvey195 showed that, with perchloric acid as catalyst, the bromination of toluene followed the usual kinetic equation (89). At 25 °C, in ca. 0.0013 M hypobromous acid, the average value of fc2/[H+] for toluene (0.008-0.15 M) was 21.7 and for benzene (0.0011-0.016 M) was 0.60, so that ktoWtnc/kbtaztae was 36.2. The bromination of f-butylbenzene196 and biphenyl197 gave k2/[H+] = 7.25 and 7.52, and hence relative rates of 12.1,... 

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... 

Christen and Zollinger303 have made an extensive study of kinetic isotope effects in bromination of the disodium salt of 2-naphthol-6,8-disulphonic acid with hypobromous acid and with bromine in aqueous buffers at 20 °C. Both bro-minating agents give the same rate (within 20 %) and the reactions are first-order... 

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. 

The bromination of /-butylbenzene by acidified hypobromous acid in 50% aqueous dioxan at 25 °C follows the kinetic equation (89) (p. 84)196, and the kinetic form of bromodebutylation is assumed to be the same since only 1.9% of the total reaction (k3 = 7.25) is debutylation, leading to a partial rate factor of 1.4 the same conclusions apply as outlined above. 

The hydroxyl in (42-6) is then acylated with p-toluenesulfonyl chloride exposure of this to a base leads to elimination to form the 9,11 olehn (43-1). It should be noted that the hydroxyl group in the hrst-obtained fermentation product is equatorial and would eliminate only with great difficulty as it lacks a transoid proton at the adjacent position. Reaction of (43-1) with A -bromosuccinimide in an aqueous base leads to the addition of the elements of hypobromous acid. The stereochemistry of the reaction... 

The elements of hypobromous acid are added across the 2,3-double bond when chromones (394) are treated with NBS in aqueous DMSO (75JHC981). 

Hypobromous acid HOBr results from the hydrolysis of bromine with H2O and exists only in aqueous solution. The compound finds limited use as a germicide and in water treatment also it can be used as an oxidizing or brominating agent in the production of certain organic compounds. Although hypobromous acid is low in bromine content, concentrated hypobromite solutions can be formed by adding bromine to cooled solutions of alkalis. 

Barbituric acid reacted with bromine to give in turn the 5-bromo and 5,5-dibromo derivatives. Subjection of the latter to base-induced dehydro-bromination or reduction converted it back into the monobromo species. iV-Alkylbarbituric acids behaved similarly [62HC(16)172 74JCS(P1)2095]. Uracil was brominated in aqueous solution initially in much the same way. A 5-bromo derivative was formed at first, and then (38), apparently a consequence of addition of hypobromous acid. Although quite stable, (38) can be converted back into the monobromo derivative when boiled with mineral acid. 1,3-Dimethyluracil, substituted at C-6 by chlorine, bromine, or iodine, was both mono- and di-brominated at C-5 by bromine (81S701). Cytosine and related compounds behaved similarly [59JOC11 90AHC(47)325],... 

Halogenation by aqueous solutions of hypochlorous acid and hypobromous acid in dilute mineral acids has the kinetic form (42).19 As with the analogous... 

Rates of bromination of several methylpyridines by hypobromous acid in aqueous perchloric acid at 25°C (Table 9.9) have been measured. Rate-acidity profiles and comparison with model compounds showed that the conjugate acids were the reacting species (74JOC3481). From this work the partial rate factor for bromination of the 3-position of the pyridinium ion was estimated as 2-6 x 10 which gives a o+3 value of —2.0. 

Perbromic acid is a strong monobasic acid. Its aqueous solutions are stable up to about 6 M (55% HBr04), even at 100°. Fairly concentrated solutions may develop a yellow bromine color from the decomposition of traces of bromate ion and hypobromous acid. If a 6 M perbromic acid solution is allowed to stand for several months, the bromate and hypobromite will have all decomposed, and the resulting bromine can be flushed out with pure nitrogen, leaving a colorless solution. 

Addition of hypobromous acid to l-methyl-4-phenyl-3-piperideine hydrobromide (127) was accomplished by the action of bromine in aqueous sodium bromide. Treatment of the resulting l-methyl-3-bromo-4-phenyl-4-piperidinol hydrobromide (128) with 10% aqueous sodium hydroxide gave l-methyl-4-phenyl-3,4-epoxypiperidine (129).114 (Compound 128 was formerly given the erroneous structure of l-methyl-5-bromo-4-phenyl-3-piperideine hydrobromide.7)... 

Hypobromous acid, HOBr, is found to be a poorer brominating agent than Bra at pll 3 (aqueous solutions). However, at higher concentrations of strong acids a third-order rate law is observed/... 

To achieve a clean addition of the elements of hypobromous acid (Br and OH ) it is advantageous to use reagents such as fV-bromosuccinimide 19 as the source of Br in an aqueous medium. So we see, the reactions given in the textbooks to illustrate the characteristic reactivity patterns of functional groups and the synthetic methods elaborated to realize their potential in practice can be vastly different. 

In aqueous solution hypobromous acid is in equilibrium with molecular bromine, viz. 

The exact nature of the brominating species released by the enzyme is unknown. Hypobromous acid is known (35) to be in rapid equilibrium with molecular bromine and tribromide ions in aqueous solutions. Further, it is important to note that at neutral pH values a very fast reaction occurs between H2O2 and OBr to yield singlet oxygen (36). This rapid side reaction hampers direct observation of brominating species at neutral pH values and is why rapid-reaction kinetics had to be used by De Boer and Wever (33) to detect Bri formation. 

Preparation. An aqueous solution of hypobromous acid prepared from siiver sulfite and bromine water is shaken with r-butanoi, and the product is extracted with trichlorofluoromethane (Freon 11) and distilled yield 42%. It is a reddish orange liquid with a bromine-like odor and is stable at 0° in the dark for long periods. It is rapidly destroyed at 85° or on irradiation, or by sodium bicarbonate. The reagent reacts with an alkane and bromotrichloromethane to form an alkyl bromide. 

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