Hydrogen bromide is the inorganic compound with the formula . 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 . 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.
Reactions
Organic chemistry
Hydrogen bromide and hydrobromic acid are important reagents in the production of organobromine compounds.
[Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Butterworth-Heineman: Oxford, Great Britain; 1997; pp. 809–812.][Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry: Structure and Function; 4th Ed.; W. H. Freeman and Company: New York, NY; 2003.] In an electrophilic addition reaction, HBr adds to alkenes:
The resulting alkyl bromides are useful
, 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:
- RC≡CH + HBr → RC(Br)=CH2
Also, HBr adds and , resulting in ring-opening.
With triphenylphosphine, HBr gives triphenylphosphonium bromide, a solid "source" of HBr.[Hercouet, A.; LeCorre, M. (1988) Triphenylphosphonium bromide: A convenient and quantitative source of gaseous hydrogen bromide. Synthesis, 157–158.]
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:
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:
[M. Schmeisser "Chlorine, Bromine, Iodine" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 282.]
- KBr + H2SO4 → KHSO4 + HBr
Concentrated sulfuric acid is less effective because it oxidizes HBr to bromine:
- 2 HBr + H2SO4 → Br2 + SO2 + 2 H2O
The acid may be prepared by:
-
reaction of bromine with water and sulfur:
-
: 2 Br2 + S + 2 H2O → 4 HBr + SO2
-
bromination of tetralin:
-
: C10H12 + 4 Br2 → C10H8Br4 + 4 HBr
-
reduction of bromine with phosphorous acid:
-
: Br2 + H3PO3 + H2O → H3PO4 + 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 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.[Ruhoff, J. R.; Burnett, R. E.; Reid, E. E. "Hydrogen Bromide (Anhydrous)" Organic Syntheses, Vol. 15, p. 35 (Coll. Vol. 2, p. 338).]
Safety
HBr is highly corrosive and, if inhaled, can cause lung damage.
