Hydrogen Bromide

The inorganic compound HBr stands for hydrogen bromide. It’s a hydrogen halide composed of two elements: hydrogen and bromine.  During the synthesis of bromide compounds, hydrogen bromide and its aqueous solutions are extensively used reagents. As well as having uses in the production of chemicals and pharmaceuticals, hydrogen bromide is also put to use in the veterinary and solvent industries. Training should be offered to employees on the correct and safe ways to handle and store hydrogen bromide before they are allowed to work with the substance.

Addition to Symmetrical Alkenes

A symmetrical alkene, such as ethene, has the same groups on both ends of the carbon-carbon double bond. At room temperature, the reaction takes place in the presence of organic peroxides or oxygen from the air. The two probable circumstances are comparable because alkenes react slowly with oxygen to form traces of organic peroxides. The hydrogen bromide is simply added to the process. For instance, consider ethene: 

CH₂=CH₂ + HBr 🡪 CH₃CH₂Br

In the absence of organic peroxides or oxygen, a symmetrical alkene produces the same result, but the mechanism is slightly different.

Peroxide Effect

Unsymmetrical alkenes, such as propene, have distinct groups at each end of the carbon-carbon double bond. The reaction occurs under the same conditions as a symmetrical alkene, but there is a complexity in that the hydrogen and bromine can combine in two ways. Whether or not organic peroxides (or oxygen) are present determines which way they contribute.

When a molecule HX attaches to a carbon-carbon double bond, the hydrogen attaches to the carbon that already has more hydrogens on it. Markovnikov’s Rule is the name for this rule. The peroxide effect, also referred to  as an anti-Markovnikov addition, occurs when HBr gets added on the “wrong way around” in the presence of organic peroxides. Hydrogen bromide combines with propene by an electrophilic addition process in the absence of peroxides. As a result, the product formed by Markovnikov’s Rule can be obtained.

Free Radical Mechanism 

The majority of hydrogen halides (hydrogen chloride, hydrogen bromide, and so on) react with alkenes via an electrophilic addition process. Hydrogen bromide, on the other hand, contributes by a different process in the presence of organic peroxides. A free radical chain reaction occurs when organic peroxides are present.

Chain Initiation

The chain is started when an oxygen-oxygen connection in the organic peroxide breaks, releasing free radicals.

R-O-O-R 🡪 R-O• + •O-R

To form bromine radicals, these free radicals take a hydrogen atom from a hydrogen bromide molecule.

R-O• + H-Br 🡪 R-O-H + •Br

Chain Propagation

A bromine radical can join to the ethene via using one of the electrons present in the π bond. This leads to the formation of a new radical with the single electron on the other carbon atom. 

CH₂=CH₂ + •Br 🡪 •CH₂-CH₂-Br

To continue the reaction, that radical combines with another HBr molecule to make bromoethane and another bromine radical.

•CH₂-CH₂-Br + HBr 🡪 CH₃-CH₂-Br + •Br

Chain Termination

Two free radicals eventually collide and form a molecule of some type. Because no new free radicals are generated, the process comes to a halt.

Conclusion

As a solvent, hydrogen bromide is used to make a variety of compounds and pharmaceuticals. It’s mostly utilised to make inorganic bromides, particularly zinc, calcium, and sodium bromides, which are employed in photography, medicines, textile finishing, industrial drying, engraving and lithography, and chemical synthesis fire retardants. It’s also used to make brominated fluorocarbons, which are used in firefighting, refrigeration, and aerosols.

Hydrobromic acid is a reagent that can be used to make organobromine compounds. With HBr, certain ethers can be cleaved. Alkylation processes and the extraction of some ores are also catalysed by it.