The introduction of halogens to benzene is called halogenation. It is an electrophilic aromatic substitution reaction. There are four types of halogenation reaction on the basis of halogen atom used; Chlorination and bromination occurs at normal conditions while fluorination is so much fast and exhaustive that it can break all the bonds and iodination is so slow that it is reversible.
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Mechanism of reaction:
Step:-1 Generation of electrophile:
Electrophile used here is halonium ion. The methods for generation of electrophile is different for different halogen atoms. This is discussed in the later sections separately in this article.
Step:-2 Formation of sigma complex:
After the generation of electrophile the electrophile approaches the aromatic substrate which shifts its pi-electron density towards the electrophile and there is a small pi interaction between aromatic substrate and electrophile. Then, there occurs a complete transfer of electrons from benzene ring and electrophile and this results in the generation of sigma bond between electrophile and aromatic substrate and hence the name sigma complex. It is also known as Arenium ion or Whealand intermediate. It is stable and have various contributing structures. i.e.