Hydrogen bromide

Hydrogen bromide is the diatomic molecule HBr. Under standard conditions, HBr is a gas, but it can be liquified. The aqueous solution hydrobromic acid forms upon dissolving HBr in water. Conversely, HBr can be liberated from hydrobromic acid solutions upon the addition of a dehydration agents. Hydrogen bromide and hydrobromic acid are, therefore, not the same, but they are related. Commonly, chemists refer to hydrobromic acid as "HBr", and this usage, while understood by most chemists, is imprecise and can be confusing to the non-specialist.

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General Description

At room temperature, HBr is a nonflammable gas with an acrid odor, fuming in moist air because of the formation of hydrobromic acid. HBr is very soluble in water, forming hydrobromic acid solution, which is saturated at 68.85% HBr by weight at room temperature. Hydrobromic acid is almost completely dissociated into H⁺ and Br^– in aqueous solution. Aqueous solutions that are 47.38% HBr by weight form a constant-boiling mixture (reverse azeotrope) that boils at 126°C. Boiling a solutions less concentrated causes H₂O to boil in excess until the constant boiling mixture is reached.

Uses of HBr

There are many uses of HBr in chemical synthesis. For example, HBr is used for the production of alkyl bromides from alcohols:

ROH + HBr → R⁺OH₂ + Br^– → RBr + H₂O

HBr adds to alkenes to give bromoalkanes:

RCH=CH₂ + HBr → RCH(Br)–CH₃

HBr adds to alkynes to yield haloalkenes. The stereochemistry of this type of addition is usually anti):

RC≡CH + HBr → RCH(Br)=CH₂

And adds to the haloalkene to form a geminal dihaloalkane This type of addition follows Markovnikov's rule):

RC(Br)=CH₂ + HBr → RC(Br₂)–CH₃

Also, HBr is used to open epoxides and lactones and in the synthesis of bromoacetals. Additionally, HBr catalyzes many organic reactions.^[1][2][3][4]

Industrial preparation

Unlike hydrogen chloride and hydrochloric acid, which are major industrial chemicals, hydrogen bromide (along with hydrobromic acid) is produced on a much smaller scale. In the primary industrial preparation, hydrogen and bromine are combined at elevated temperatures (200-400 °C). The reaction is typically catalyzed by platinum or asbestos.^[2][5]

References

1. ^ Hercouet, A.;LeCorre, M. (1988) Triphenylphosphonium bromide: A convenient and quantitative source of gaseous hydrogen bromide. Synthesis, 157-158.
2. ^ ^{a b} Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Butterworth-Heineman: Oxford, Great Britain; 1997; pp. 809-812.
3. ^ Carlin, William W. U.S. Patent 4,147,601 , April 3, 1979
4. ^ Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry: Structure and Function; 4th Ed.; W. H. Freeman and Company: New York, NY; 2003.
5. ^