Hydrogen bromide


Hydrogen bromide is the diatomic molecular compound with the formula, a hydrogen halide consisting of hydrogen and bromine. In pure form it is a colorless gas.
Hydrogen bromide is very soluble in water, forming hydrobromic acid, which is saturated at 68.85% HBr by weight at room temperature. Aqueous solutions that are 47.6% HBr by mass form a constant-boiling azeotrope mixture that boils at 124.3 °C. Boiling less concentrated solutions releases H2O until the constant-boiling mixture composition is reached.
Both the anhydrous and aqueous solutions of HBr are common reagents in the preparation of bromide compounds.

Uses of HBr

Hydrogen bromide and hydrobromic acid are important reagents in the production of inorganic and organic bromine compounds. The free-radical addition of HBr to alkenes gives alkyl bromides:
These alkylating agents are precursors to fatty amine derivatives. Similar free radical addition to allyl chloride and styrene gives 1-bromo-3-chloropropane and phenylethylbromide, respectively.
Hydrogen bromide reacts with dichloromethane to give bromochloromethane and dibromomethane, sequentially:
Allyl bromide is prepared by treating allyl alcohol with HBr:

Other reactions

Although not widely used industrially, HBr adds to alkenes to give bromoalkanes, an important family of organobromine compounds. Similarly, HBr adds to haloalkene to form a geminal dihaloalkane. :
HBr also adds to alkynes to yield bromoalkenes. The stereochemistry of this type of addition is usually anti:
Also, HBr is used to open epoxides and lactones and in the synthesis of bromoacetals. Additionally, HBr catalyzes many organic reactions.

Potential applications

HBr has been proposed for use in a utility-scale flow-type battery.

Industrial preparation

Hydrogen bromide is produced by combining hydrogen and bromine at temperatures between 200 and 400 °C. The reaction is typically catalyzed by platinum or asbestos.

Laboratory synthesis

HBr can be synthesized by a variety of methods. It may be prepared in the laboratory by distillation of a solution of sodium bromide or potassium bromide with phosphoric acid or sulfuric acid:
Concentrated sulfuric acid is less effective because it oxidizes HBr to bromine:
The acid may be prepared by:
Anhydrous hydrogen bromide can also be produced on a small scale by thermolysis of triphenylphosphonium bromide in refluxing xylene.
Hydrogen bromide prepared by the above methods can be contaminated with Br2, which can be removed by passing the gas through a solution of phenol at room temperature in tetrachloromethane or other suitable solvent or through copper turnings or copper gauze at high temperature.

Safety

HBr is highly corrosive and irritating to inhalation.