constant boiling mixture

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.

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.

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.

Laboratory synthesis

HBr can synthesized by a variety of methods. A convenient laboratory synthesis entails the reaction between sulfuric acid and NaBr:
NaBr(s) + H2SO4(aq) → NaHSO4(s) + HBr(g)
However, this synthetic process is ineffective as HBr formed will be oxidized to bromine gas by sulfuric acid:
2HBr(g) + H2SO4(aq) → Br2(g) + SO2(g) + 2H2O(l)
Instead, non-oxidising acids like phosphoric acid or acetic acid can be used for the purpose. Alternatively, it can be prepared by the bromination of tetraline (1,2,3,4-tetrahydronaphthalene):
C10H12 + 4Br2 → C10H8Br4 + 4HBr(g)
Reacting purified hydrogen gas and bromine (in the presence of a platinum catalyst):
Br2 + H2 → 2HBr(g)
And reducing bromine with phosphorous acid:
Br2 + H3PO3 + H2O → H3PO4(s) + 2HBr(g)
Anhydrous hydrogen bromide can also be produced on a small scale (10 mmol-1 mol) through the thermolysis of triphenylphosphonium bromide in refluxing xylene.

HBr prepared by the above methods can be contaminated with Br2, which can be removed by passing the gas through Cu turnings or through phenol.


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