Hydrobromic acid catalyst

Bromoacetic acid can be prepared by the bromination of acetic acid in the presence of acetic anhydride and a trace of pyridine (55), by the HeU-VoUiard-Zelinsky bromination cataly2ed by phosphoms, and by direct bromination of acetic acid at high temperatures or with hydrogen chloride as catalyst. Other methods of preparation include treatment of chloroacetic acid with hydrobromic acid at elevated temperatures (56), oxidation of ethylene bromide with Aiming nitric acid, hydrolysis of dibromovinyl ether, and air oxidation of bromoacetylene in ethanol. 

Bromohydrins can be prepared direcdy from polyhydric alcohols using hydrobromic acid and acetic acid catalyst, followed by distillation of water and acetic acid (21). Reaction conditions must be carehiUy controlled to avoid production of simple acetate esters (22). The raw product is usually a mixture of the mono-, di-and tribromohydrins. 

A considerable amount of hydrobromic acid is consumed in the manufacture of inorganic bromides, as well as in the synthesis of alkyl bromides from alcohols. The acid can also be used to hydrobrominate olefins (qv). The addition can take place by an ionic mechanism, usually in a polar solvent, according to Markownikoff s rule to yield a secondary alkyl bromide. Under the influence of a free-radical catalyst, in aprotic, nonpolar solvents, dry hydrogen bromide reacts with an a-olefin to produce a primary alkyl bromide as the predominant product. Primary alkyl bromides are useful in synthesizing other compounds and are 40—60 times as reactive as the corresponding chlorides (6). 

In the petroleum (qv) industry hydrogen bromide can serve as an alkylation catalyst. It is claimed as a catalyst in the controlled oxidation of aHphatic and ahcycHc hydrocarbons to ketones, acids, and peroxides (7,8). AppHcations of HBr with NH Br (9) or with H2S and HCl (10) as promoters for the dehydrogenation of butene to butadiene have been described, and either HBr or HCl can be used in the vapor-phase ortho methylation of phenol with methanol over alumina (11). Various patents dealing with catalytic activity of HCl also cover the use of HBr. An important reaction of HBr in organic syntheses is the replacement of aHphatic chlorine by bromine in the presence of an aluminum catalyst (12). Small quantities of hydrobromic acid are employed in analytical chemistry. 

Bromomethylation of arenes using 1,3,5-trioxane and hydrobromic acid is catalysed by the addition of a phase-transfer catalyst. Yields in excess of 90% are attained using (tetradecyl)trimethylammonium bromide under relatively mild conditions for a range of arenes [55]. Tetra-n-butylammonium bromide is ineffective, suggesting the catalytic effect may be micellar. 

Hydrobromic acid is used in the preparation of inorganic bromide salts. The acid also is used for many organic syntheses, including alkyl bromides from alcohols or olefins, and bromophenols from phenols. The compound also is used as an acid catalyst in many alkylation, selective oxidation, isomerization, and dehydrogenation reactions. Other apphcations are in extraction of minerals and use as a reducing agent. 

By the action of chlorine and chlorine compounds on arsenic pentoxide. Arsenic trichloride is formed when chlorine is passed over the heated oxide.16 Hydrogen chloride reacts at ordinary temperatures, but not at -20° C.17 Aqueous hydrochloric acid, or sulphuric acid and a metallic chloride, also reacts with the oxide or with alkali arsenates to produce arsenic trichloride. The reaction with hydrochloric acid is greatly influenced by catalysts such as ferrous sulphate18 or chloride,19 potassium bromide20 or iodide,21 hydrobromic acid,22 methyl alcohol23... 

The resulting solution was filtered through animal charcoal and, after addition of 2 liters of methanol, it was debenzylated by hydrogenation at 60°C over palladinized charcoal as a catalyst. After removal of the catalyst by filtration, the filtrate was concentrated by evaporation, whereupon the hydrochloride of l-p-methoxyphenyl-2-(p-3, 5 -dihydroxyphenyl-p-oxo)-ethylamino-propane (MP 244°C) crystallized out. For the purpose of demethylation, the 350 grams of the hydrochloride thus produced were refluxed for 2 hours with 3.5 liters of aqueous 48% hydrobromic acid. Upon cooling of the reaction solution, 320 grams of l-p-hydroxyphenyl-2-(p-3, 5 -dihydroxyphenyl-p-oxo)-ethylamino-propanehydrobromide (MP 220°C) crystallized out. 

Broraination in the 4-position is the most facile reaction of all pyrazole electrophilic substitutions. Aqueous bromine is usually employed,208,286,555,556 or bromine in neutral solvents,208,557-574 or else hypobromous acid562 or such agents as N-bromosuccinimide.565 During bromination in aqueous solution543, 566 it is usual to neutralize the hydrobromic acid formed by the addition of an equimolar quantity of sodium acetate, but this is by no means necessary. Bromination in the presence of a catalyst such as iron may result in the introduction of two or even three bromine atoms,557 although Buchner and Fritsche94 earlier considered the introduction of more than one impossible. See also Michaelis.541 The action of bromine on 1,3-... 

The stereoselectivity in the hydrogenation of bicyclic and polycyclic a,P-unsaturated ketones with the double bond at the ring juncture has been the subject of extensive investigations.252 Formation of cA-2-decalone in the hydrogenation of A1,9-2-octalone (115) with palladium catalysts increases with increasing polarity of aprotic solvents and also in the presence of acid, especially in nonhydroxylic solvents.253,254 Hydro-bromic acid has been found to be more effective than hydrochloric acid for cA-2-de-calone, especially in tetrahydrofuran (eq. 3.72).255 Formation of alcoholic products was also completely depressed in the presence of hydrobromic acid, although the rate of hydrogenation became considerably lower than in the presence of hydrochloric acid.256... 

TABLE 3.19 Rates of Hydrogenation and Selectivities for 5p-Ketone in Hydrogenation of 3-Oxo-4-ene and 19-Nor-3-oxo-4-ene Steroids over Palladium Catalyst in Pyridine and in Tetrahydrofuran/Hydrobromic Acid"6... 

As in an example of franr-2-decalone described above, the hydrogenation of substituted cyclohexanones with platinum catalysts usually gives higher yields of axial alcohols in the presence of hydrobromic acid than in the presence of hydrochloric acid. Ruzicka et al. obtained 5a-cholestan-3a-ol in 67% yield by hydrogenating 5a-cholestan-3-one (62 g, 0.16 mol) with platinum oxide (12 g) in Bu20 (1200 ml)-HBr (48%, 3 ml) at 65-70°C. Similarly, 5P-cholestan-3P-ol was obtained in 95% yield in the hydrogenation of 5P-cholestan-3-one (40 g, 0.10 mol) over platinum oxide (8 g) in AcOH (1200 ml)-HBr (48%, 8 ml) at 60°C.155 The proportion of hydrogenolysis... 

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