Hydrobromic acid
Aqueous solution of hydrogen bromide
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Through Aryl group, Sulfur and Phosphorus
Uses and reactions
Hydrobromic acid is mainly used for the production of inorganic bromides such as the hydrated bromides of iron, nickel, and zinc.
Hydrobromic acid is a useful reagent for generating organobromine compounds. It converts primary alcohols to alkyl bromides. Certain ethers are cleaved with HBr. HBr participates in anti-Markovnikov hydrohalogenation of alkenes in the presence of peroxides. The resulting 1-bromoalkanes are versatile alkylating agents, giving rise to fatty amines and quaternary ammonium salts.
In the presence of hydrogen peroxide, it gives bromine. Since elemental bromine can be dangerous to handle, the H2O2/HBr mixture is used instead, e.g. for aromatic brominations:
- 2 ArH + 2 HBr + H2O2 → 2 ArBr + 2 H2O (Ar = aryl)
Industrially significant organic compounds prepared from hydrobromic acid include allyl bromide, the fire-retardant tetrabromobisphenol A, and bromoacetic acid.
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Synthesis
Hydrobromic acid can be prepared in the laboratory via the reaction of Br2, SO2, and water.
- 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.
Alternatively the acid can be prepared with dilute (5.8M) sulfuric acid and potassium bromide:
- H2SO4 + KBr → KHSO4 + HBr
Using more concentrated sulfuric acid or allowing the reaction solution to exceed 75 °C further oxidizes HBr to elemental bromine. The acid is further purified by filtering out the KHSO4 and by distilling off the water until the solution reaches an azeotrope (124.3 °C). The yield is approximately 85%.
Hydrobromic acid is available commercially in various concentrations and purities.