Hydrobromic acid is a strong acid formed by dissolving the diatomic molecule hydrogen bromide (HBr) in water. "Constant boiling" hydrobromic acid is an aqueous solution that distills at 124.3 °C and contains 47.6% HBr by mass, which is 8.89 mol/L. Hydrobromic acid has a pKa of −9, making it a stronger acid than hydrochloric acid, but not as strong as hydroiodic acid. Hydrobromic acid is one of the strongest mineral acids known.
3D model (JSmol)
|Molar mass||80.91 g·mol−1|
|Appearance||colorless/faint yellow liquid|
|Density||1.49 g/cm3 (48% w/w aq.)|
|Melting point||−11 °C (12 °F; 262 K) (47–49% w/w aq.)|
|Boiling point||122 °C (252 °F; 395 K) at 700 mmHg (47–49% w/w aq.)|
|221 g/100 mL (0 °C) |
204 g/100 mL (15 °C)
130 g/100 mL (100 °C)
|Viscosity||0.84 cP (−75 °C)|
Heat capacity (C)
Std enthalpy of
|Safety data sheet||ICSC 0282|
|R-phrases (outdated)||R34, R37|
|S-phrases (outdated)||(S1/2), S7/9, S26, S45|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Hydrobromic acid is mainly used for the production of inorganic bromides, especially the bromides of zinc, calcium, and sodium. It is a useful reagent for generating organobromine compounds. Certain ethers are cleaved with HBr. It also catalyzes alkylation reactions and the extraction of certain ores. Industrially significant organic compounds prepared from hydrobromic acid include allyl bromide, tetrabromobis(phenol), and bromoacetic acid. HBr almost uniquely participates in antiMarkovnikov hydrohalogenation of alkenes. The resulting 1-bromoalkanes are versatile alkylating agents, giving rise to fatty amines and quaternary ammonium salts.
- Br2 + SO2 + 2 H2O → H2SO4 + 2 HBr
More typically laboratory preparations involve the production of anhydrous HBr, which is then dissolved in water.
Hydrobromic acid has commonly been prepared industrially by reacting bromine with either sulfur or phosphorus and water. However, it can also be produced electrolytically. It can also be prepared by treating bromides with non-oxidising acids like phosphoric or acetic acids.
- H2SO4 + KBr → KHSO4 + HBr
Using more concentrated sulfuric acid or allowing the reaction solution to exceed 75 °C further oxidizes HBr to bromine gas. The acid is further purified by filtering out the KHSO4 and by distilling off the water until the solution reaches an azeotrope (≈ 126 °C at 760 torr). The yield is approximately 85%.
Hydrobromic acid is available commercially in various concentrations and purities.
- Henri A. Favre; Warren H. Powell, eds. (2014). Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013. Cambridge: The Royal Society of Chemistry. p. 131.
- Bell, R. P. The Proton in Chemistry, 2nd ed., Cornell University Press, Ithaca, NY, 1973.
- Dagani, M. J.; Barda, H. J.; Benya, T. J.; Sanders, D. C. (2012). "Bromine Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a04_405.CS1 maint: multiple names: authors list (link)
- Scott, A. (1900). "Preparation of Pure Hydrobromic Acid". Journal of the Chemical Society, Transactions. 77: 648–651. doi:10.1039/ct9007700648.
- Brauer, Georg (1963). Handbook of Preparative Inorganic Chemistry Vol. 1, 2nd Ed. Newyork: Academic Press. p. 285. ISBN 978-0121266011.