# 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 H$_2$O 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.

## Reactions
### Organic chemistry

Hydrogen bromide and hydrobromic acid are important reagents in the production of organobromine compounds. In an electrophilic addition reaction, HBr adds to alkenes:
    RCH=CH$_2$ + HBr → R−CHBr−CH$_3$
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:
    HBr + CH$_2$Cl$_2$ → HCl + CH$_2$BrCl
    HBr + CH$_2$BrCl → HCl + CH$_2$Br$_2$
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:
    CH$_2$=CHCH$_2$OH + HBr → CH$_2$=CHCH$_2$Br + H$_2$O
HBr adds to alkynes to yield bromoalkenes. The stereochemistry of this type of addition is usually anti:
    RC≡CH + HBr → RC(Br)=CH$_2$
Also, HBr adds epoxides and lactones, resulting in ring-opening.
With triphenylphosphine, HBr gives triphenylphosphonium bromide, a solid "source" of HBr.
    P(C$_6$H$_5$)$_3$ + HBr → [HP(C$_6$H$_5$)$_3$]$^+$Br$^−$

### Inorganic chemistry

Vanadium(III) bromide and molybdenum(IV) bromide were prepared by treatment of the higher chlorides with HBr. These reactions proceed via redox reactions:
    2 VCl$_4$ + 8 HBr → 2 VBr$_3$ + 8 HCl + Br$_2$

## Industrial preparation

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.

## Laboratory synthesis

HBr can be prepared by distillation of a solution of sodium bromide or potassium bromide with phosphoric acid or sulfuric acid:
    KBr + H$_2$SO$_4$ → KHSO$_4$ + HBr
Concentrated sulfuric acid is less effective because it oxidizes HBr to bromine:
    2 HBr + H$_2$SO$_4$ → Br$_2$ + SO$_2$ + 2 H$_2$O
The acid may be prepared by:
* reaction of bromine with water and sulfur:
    2 Br$_2$ + S + 2 H$_2$O → 4 HBr + SO$_2$
* bromination of tetralin:
    C$_{10}$H$_{12}$ + 4 Br$_2$ → C$_{10}$H$_8$Br$_4$ + 4 HBr
* reduction of bromine with phosphorous acid:
    Br$_2$ + H$_3$PO$_3$ + H$_2$O → H$_3$PO$_4$ + 2 HBr
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 Br$_2$, 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.

## Safety

HBr is highly corrosive and, if inhaled, can cause lung damage.