Bromination and chlorination of aromatic rings by electrophilic substitution reactions are easily achieved. However, fluorination and iodination are exceptional.

Among all halogens, fluorine is highly reactive towards aromatic rings. Therefore, it is difficult to limit the direct fluorination of benzene, resulting in poor yields of monofluoroaromatic products.

To overcome this limitation, other sources of fluorine such as Selectfluor, which has a fluorine atom bonded to a positively charged nitrogen, are used. Selectfluor acts as a fluorine donor and facilitates electrophilic fluorination.

Unlike fluorine, iodine is unreactive towards aromatic rings.

Therefore, aromatic iodination must be carried out using oxidizing agents such as nitric acid, hydrogen peroxide, or a copper salt.

These agents oxidize iodine to an iodine cation, which acts as a powerful electrophile and follows the mechanism of a typical electrophilic aromatic substitution to give the iodinated product.