Hydrogen bromide

Hydrogen bromide is the inorganic compound with the formula HBr. It is a hydrogen halide consisting of hydrogen and bromine. A colorless gas, it dissolves 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 (255.7 °F). Boiling less concentrated solutions releases H₂O until the constant-boiling mixture composition is reached.

Hydrogen bromide, and its aqueous solution, hydrobromic acid, are commonly used reagents in the preparation of bromide compounds.

Hydrogen bromide and hydrobromic acid are important reagents in the production of organobromine compounds. In an electrophilic addition reaction, HBr adds to alkenes:

The resulting alkyl bromides are useful alkylating agents, e.g., as precursors to fatty amine derivatives. Related free radical additions to allyl chloride and styrene give 1-bromo-3-chloropropane and phenylethylbromide, respectively.

Hydrogen bromide reacts with dichloromethane to give bromochloromethane and dibromomethane, sequentially:

These metathesis reactions illustrate the consumption of the stronger acid (HBr) and release of the weaker acid (HCl).

Allyl bromide is prepared by treating allyl alcohol with HBr:

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