Halogenation

Reaction

Halogenation Reaction

Mechanism

Halogenation Mechanism

Notes

Halogenation is an example of electrophilic substitution of benzene. Benzene is normally unreactive in the presence of halogens because halogens are not electrophilic enough to disrupt its aromaticity. However, the halogen may be activated by Lewis acid catalysts, such as ferric halides (FeX3) or aluminium halides (AlX3), to become a much more powerful electrophile.

The characteristic of Lewis acids is the ability to accept electron pairs. When a halogen such as bromine is exposed to FeBr3, the two molecules combine as Lewis acid and base. In this complex, the Br-Br bond is polarized, thereby imparting electrophilic character to the bromine atoms. Electrophilic attack on benzene is at the terminal bromine, allowing the other bromine atom to depart with a good leaving group FeBr4-.

Generation of Electrophile

Halogenation Mechanism

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Copyright 2002 Nick Barwell.