Electrophilic substitution

Aryl chlorides and bromides can be prepared by direct halogenation of benzene and its derivatives through electrophilic aromatic substitution. The reaction is conveniently carried out in the dark at ordinary temperature, in the presence of a suitable Lewis-acid catalyst such as iron, ferric chloride (FeCl3), or anhydrous aluminium chloride (AlCl3), which polarises the halogen molecule enough to generate the electrophile that attacks the ring. When the substrate already carries an activating, ortho/para-directing group -- for example toluene -- the product is a mixture of the ortho- and para-substituted arenes rather than a single compound (bromination of toluene in the presence of iron, in the dark, gives a mixture of o-bromotoluene and p-bromotoluene). Aromatic electrophilic substitution with iodine is reversible, because the HI formed as a byproduct can add back across the ring; adding an oxidant such as HNO3 or HIO4 removes the HI by oxidising it to I2, which pulls the equilibrium to the right and lets the iodo product form. Fluorine, in contrast, is far too reactive …