Halogenation of Benzene

The reaction in which a hydrogen atom of benzene is replaced by a halogen atom is called halogenation of benzene. Halogenation takes place in the presence of iron, or ferric halides (FeX₃, where X = Cl or Br).

The direct iodination of aromatic hydrocarbons is not a useful reaction since the HI produced reduces the aryl iodide back to the aromatic hydrocarbon. This is prevented by carrying out this reaction in the presence of HNO₃ or HIO₃. These acids react with HI as soon as it is formed.

Fluorobenzene cannot be prepared by direct fluorination of aromatic hydrocarbons since the reaction is very violent and cannot be controlled.