What Is Hydrogen Bromide?
Hydrogen bromide is a compound of bromine and hydrogen, a colorless gas with a pungent odor.
Its official name is hydrogen bromide, which is characterized by the white smoke it produces when exposed to air moisture. Hydrogen bromide is toxic and can damage the mucous membranes of the eyes and bronchial tubes and cause breathing difficulties.
Uses of Hydrogen Bromide
Hydrogen bromide is used as a raw material for synthesizing pharmaceuticals, hydrobromic acid, and various bromides, as well as a general-purpose reagent. In the chemical industry, it is used as an alkylation catalyst and reducing agent.
Additionally, hydrogen bromide serves as a high-quality semiconductor material gas and can be used as an etching or cleaning gas. Hydrobromic acid, an aqueous solution of hydrogen bromide, is an important chemical used as a catalyst in the production of terephthalic acid, a raw material for polyester fiber, and as a raw material for various bromates and alkyl bromides.
Properties of Hydrogen Bromide
Hydrogen bromide is corrosive and nonflammable. It has a density of 3.307 g/L, a melting point of -124° F (-86.80°C), and a boiling point of -87.48° F (-66.38°C).
As one of the halogenated hydrogens, it is a compound composed of hydrogen and bromine. Hydrogen bromide is a linear molecule with the chemical formula HBr and a molar mass of 80.912. Under low-temperature conditions, hydrogen bromide will form crystals of hydrate.
Hydrogen bromide is readily soluble in organic solvents with oxygen, such as ethanol and acetone. It is also well soluble in water, and its aqueous solution is a strong acid. The aqueous solution of hydrogen bromide is called hydrobromic acid. 47.63% hydrobromic acid is an azeotrope with a boiling point of 255.7° F (124.3°C).
A similar azeotrope, 48%, is commonly used commercially and is designated as a deleterious substance, not for medicinal use. Its heat of dissolution in water is ΔH° = -85.15 kJ/mol, the largest among hydrogen halides. It is a strong monovalent acid with properties similar to hydrochloric acid and is easily oxidized.
For example, it can be oxidized by air or decomposed by light to liberate bromine, which gives it a yellowish color. Furthermore, hydrogen bromide’s reducing effect is stronger than that of hydrogen chloride (HCl).
Other Information on Hydrogen Bromide
1. Synthesis of Hydrogen Bromide
Hydrogen bromide is produced by the reaction of hydrogen and bromine over a catalyst. Hydrogen bromide can be obtained by reacting bromide with phosphoric acid or by allowing bromine to act on a mixture of red phosphorus and water.
Various other synthetic methods are available on a laboratory scale. An example is the bromination of tetralin. Anhydrous hydrogen bromide can also be obtained by refluxing triphenylphosphonium bromide (Ph3PH+Br-) in xylene and thermally decomposing it.
However, unlike hydrogen chloride and hydrochloric acid, the scale of production of hydrogen bromide and hydrobromic acid is small. Early production methods involved the reaction of hydrogen and bromine at temperatures as high as 392-752° F (200-400°C), usually using platinum or asbestos as a catalyst in industrial reactions.
2. Reaction of Hydrogen Bromide
Hydrogen bromide is used to produce bromoalkanes with alcohols. Bromoalkanes can also be obtained by the addition of hydrogen bromide to alkenes.
The addition of hydrogen bromide to alkynes yields bromoalkenes, usually with stereochemistry in the anti form. The addition of hydrogen bromide to haloalkenes follows Markovnikov’s rule, yielding gem-dihaloalkanes.
Hydrogen bromide is also used in the synthesis of bromoacetals as well as in the ring-opening reactions of lactones and epoxides. Hydrogen bromide can be used as a catalyst in a wide variety of organic reactions.