Reactions of Aromatic Compounds (overall)
In connection with activated aromatic compounds, such as aniline and phenol, the halogens chlorine and bromine react without an additional catalyst yielding the corresponding aryl halide. In contrast, their reaction with benzene requires the application of a Lewis acid catalyst, such as or . The Lewis acid promotes the electrophilic aromatic substitution by polarizing the halogen molecule. As a result of the halogen molecule's polarization, the nucleophilic attack by the aromatic π electron system is made easier.
Iodine is a relatively poor electrophile. Therefore, it only reacts with strongly activated aromatic compounds, such as aniline. In the presence of silver(I) salts or other oxidants, iodine cations, which can also be attacked by less activated aromatic compounds, are formed. As the iodination is reversible, it may be accelerated by removing , one of the reaction products.
If fluorine that is extremely reactive is applied to benzene, carbon-carbon bonds are completely broken and the aromatic ring is destructed. Fluorine, therefore, cannot be introduced into an aromatic compound through the electrophilic aromatic substitution. Instead, the Sandmeyer or Schiemann reaction may be applied in this case.