Hydrogen bromide

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=CH₂ + HBr → R−CHBr−CH₃

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 + CH₂Cl₂ → HCl + CH₂BrCl

HBr + CH₂BrCl → HCl + CH₂Br₂

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:

CH₂=CHCH₂OH + HBr → CH₂=CHCH₂Br + H₂O

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

RC≡CH + HBr → RC(Br)=CH₂

Also, HBr adds epoxides and lactones, resulting in ring-opening.

With triphenylphosphine, HBr gives triphenylphosphonium bromide, a solid "source" of HBr.

P(C₆H₅)₃ + HBr → [HP(C₆H₅)₃]⁺Br⁻

Inorganic chemistry

Vanadium(III) bromide and molybdenum(IV) bromide were prepared by treatment of the higher chlorides with HBr. These reactions proceed by a combination halide exchange and redox reactions:

2 VCl₄ + 8 HBr → 2 VBr₃ + 8 HCl + Br₂

Hydrogen bromide is used for the production of anhydrous ferrous bromide by oxidation of metallic iron:

Fe + 2 HBr → FeBr₂ + H₂

By contrast, use of hydrobromic acid gives the hexahydrate FeBr₂·(H₂O)₆.