Hydrobromic acid

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 8.77 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.

Hydrobromic acid
Ball-and-stick model of hydrogen bromide
Ball-and-stick model of water
Ball-and-stick model of the bromide anion
Ball-and-stick model of the hydronium cation
Names
Other names
Hydronium bromide
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.240.772
EC Number
  • 233-113-0
RTECS number
  • MW3850000
UNII
Properties
HBr(aq)
Molar mass 80.91 g·mol−1
Appearance colorless/faint yellow liquid
Odor acrid
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)
Acidity (pKa) −9[1]
Viscosity 0.84 cP (−75 °C)
Thermochemistry
29.1 J/(K·mol)
198.7 J/(K·mol)
−36.3 kJ/mol
Hazards
Safety data sheet ICSC 0282
Corrosive (C)
R-phrases (outdated) R34, R37
S-phrases (outdated) (S1/2), S7/9, S26, S45
NFPA 704 (fire diamond)
Flash point Non-flammable
Related compounds
Other anions
 Hydrofluoric acid
Hydrochloric acid
Hydroiodic acid
Related compounds
 Hydrogen bromide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

Uses

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.[2]

Synthesis

Hydrobromic acid can be prepared in the laboratory via the reaction of Br2, SO2, and water.[3]

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.[3] It can also be prepared by treating bromides with non-oxidising acids like phosphoric or acetic acids.

Alternatively the acid can be prepared with dilute (5.8M) sulfuric acid and potassium bromide:[4]

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%.[4]

Hydrobromic acid is available commercially in various concentrations and purities.

References
  1. Bell, R. P. The Proton in Chemistry, 2nd ed., Cornell University Press, Ithaca, NY, 1973.
  2. 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.
  3. Scott, A. (1900). "Preparation of Pure Hydrobromic Acid". Journal of the Chemical Society, Transactions. 77: 648–651. doi:10.1039/ct9007700648.
  4. Brauer, Georg (1963). Handbook of Preparative Inorganic Chemistry. 1 (2nd ed.). New York: Academic Press. p. 285. ISBN 978-0121266011.

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