Hydrogen bromide

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Hydrogen bromide
Skeletal formula of hydrogen bromide with the explicit hydrogen and a measurement added
Ball-and-stick model of hydrogen bromide Hydrogen-bromide-3D-vdW.png
CAS number 10035-10-6 YesY
PubChem 260
ChemSpider 255 YesY
EC number 233-113-0
UN number 1048
KEGG C13645 N
MeSH Hydrobromic+Acid
ChEBI CHEBI:47266 YesY
RTECS number MW3850000
Jmol-3D images Image 1
Molecular formula HBr
Molar mass 80.91 g mol−1
Appearance Colorless gas
Odor Acrid
Density 3.6452 kg/m3 (0 °C, 1013 mbar)[2]
Melting point −86.9 °C (−124.4 °F; 186.2 K)
Boiling point −66.8 °C (−88.2 °F; 206.3 K)
Solubility in water 221 g/100 mL (0 °C)
204 g/100 mL (15 °C)
193 g/100 mL (20 °C) 130 g/100 mL (100 °C)
Solubility soluble in alcohol, organic solvents
Vapor pressure 2.308 MPa (at 21 °C)
Acidity (pKa) ~–9[3]
Basicity (pKb) ~23
Molecular shape Linear
Dipole moment 820 mD
350.7 mJ K-1 g-1
198.696-198.704 J K-1 mol-1[4]
-36.45--36.13 kJ mol-1[4]
MSDS hazard.com


GHS pictograms The corrosion pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS) The exclamation-mark pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)
GHS signal word DANGER
H314, H335
P261, P280, P305+351+338, P310
EU Index 035-002-00-0
EU classification Corrosive C
R-phrases R35, R37
S-phrases (S1/2), S7/9, S26, S45
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
TWA 3 ppm (10 mg/m3)[5]
Related compounds
Related compounds Hydrogen chloride

Hydrogen fluoride
Hydrogen iodide

Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N (verify) (what is: YesY/N?)
Infobox references

Hydrogen bromide is the diatomic molecule HBr. HBr is a gas at standard conditions. Hydrobromic acid forms upon dissolving HBr in water. Conversely, HBr can be liberated from hydrobromic acid solutions with the addition of a dehydration agent, but not by distillation. Hydrogen bromide and hydrobromic acid are, therefore, not the same, but they are related. Commonly, chemists refer to hydrobromic acid as "HBr", and this usage, while understood by most chemists, is imprecise and can be confusing to the non-specialist.

General description[edit]

At cold temperature, HBr is a nonflammable gas with an acrid odor, fuming in moist air because of the formation of carbon monoxide

HBr is very soluble in water, forming hydrobromic acid solution, which is saturated at 68.85% HBr by weight at room temperature. Aqueous solutions that are 47.6% HBr by weight form a constant-boiling mixture (reverse azeotrope) that boils at 124.3°C. Boiling less concentrated solutions releases H2O until the constant boiling mixture composition is reached.

Uses of HBr[edit]

There are many uses of HBr in chemical synthesis. For example, HBr is used for the production of alkyl bromides from alcohols:

ROH + HBr → RBr + H2O

HBr adds to alkenes to give bromoalkanes, an important family of organobromine compounds:

RCH=CH2 + HBr → RCH(Br)–CH3

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

RC≡CH + HBr → RC(Br)=CH2

HBr adds to the haloalkene to form a geminal dihaloalkane. (This type of addition follows Markovnikov's rule):

RC(Br)=CH2 + HBr → RC(Br2)–CH3

Also, HBr is used to open epoxides and lactones and in the synthesis of bromoacetals. Additionally, HBr catalyzes many organic reactions.[6][7][8][9]

HBr has been proposed for use in a utility-scale flow-type battery.[10]

Industrial preparation[edit]

Hydrogen bromide (along with hydrobromic acid) is produced on a much smaller scale than the corresponding chlorides. In the primary industrial preparation, hydrogen and bromine are combined at temperatures between 200-400 °C. The reaction is typically catalyzed by platinum or asbestos.[7][11]

Laboratory synthesis[edit]

HBr can be synthesized by a variety of methods. It may be prepared in the laboratory by distillation of a solution of sodium or potassium bromide with phosphoric acid or diluted sulfuric acid:[12]

2 KBr + H2SO4 → K2SO4 + 2HBr

Concentrated sulfuric acid is ineffective because HBr formed will be oxidized to bromine gas:

2 HBr + H2SO4 → Br2 + SO2 + 2H2O

The acid may be prepared by several other methods, as well, including reaction of bromine either with phosphorus and water, or with sulfur and water:[12]

2 Br2 + S + 2 H2O → 4 HBr + SO2

Alternatively, it can be prepared by the bromination of tetraline:[13]

C10H12 + 4 Br2 → C10H8Br4 + 4 HBr

Alternatively bromine can be reduced with phosphorous acid:[7]

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.[6]

HBr prepared by the above methods can be contaminated with Br2, which can be removed by passing the gas through Cu turnings or through phenol.[11]


  1. ^ "Hydrobromic Acid - Compound Summary". PubChem Compound. USA: National Center for Biotechnology Information. 16 September 2004. Identification and Related Records. Retrieved 10 November 2011. 
  2. ^ Record in the GESTIS Substance Database from the IFA
  3. ^ Perrin, D. D. Dissociation constants of inorganic acids and bases in aqueous solution. Butterworths, London, 1969.
  4. ^ a b Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. ISBN 0-618-94690-X. 
  5. ^ NIOSH Pocket Guide to Chemical Hazards 0331
  6. ^ a b Hercouet, A.;LeCorre, M. (1988) Triphenylphosphonium bromide: A convenient and quantitative source of gaseous hydrogen bromide. Synthesis, 157-158.
  7. ^ a b c Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Butterworth-Heineman: Oxford, Great Britain; 1997; pp. 809-812.
  8. ^ Carlin, William W. U.S. Patent 4,147,601, April 3, 1979
  9. ^ Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry: Structure and Function; 4th Ed.; W. H. Freeman and Company: New York, NY; 2003.
  10. ^ http://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/30535ag.pdf
  11. ^ a b Ruhoff, J. R.; Burnett, R. E.; Reid, E. E. "Hydrogen Bromide (Anhydrous)" Organic Syntheses, Vol. 15, p.35 (Coll. Vol. 2, p.338).
  12. ^ a b Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
  13. ^ WebElements: Hydrogen Bromide