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Hydrogen bromide is the heteronuclear diatomic molecular compound with the formula HBr, 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:
HBr has been proposed for use in a utility-scale flow-type battery.
Hydrogen bromide (along with hydrobromic acid) is produced by combining hydrogen and bromine at temperatures between 200 and 400 °C. The reaction is typically catalyzed by platinum or asbestos.
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 (producing 2,4,6-tribromophenol and generating more HBr in the process) or through copper turnings or copper gauze at high temperature.
HBr is highly corrosive and irritating to inhalation.