Bromination and chlorination of aromatic rings by electrophilic aromatic substitution reactions are easily achieved, but fluorination and iodination are difficult to achieve. Fluorine is so reactive that its reaction with benzene is difficult to control, resulting in poor yields of monofluoroaromatic products. To address this, Selectfluor reagent is used as a fluorine source in which a fluorine atom is bonded to a positively charged nitrogen.
Unlike fluorine, iodine is highly unreactive for electrophilic aromatic substitution reactions. The iodination of benzene is achieved in the presence of an oxidizing agent such as nitric acid, hydrogen peroxide, or a copper salt. These agents oxidize iodine to iodine cation, which behaves as a powerful electrophile, and follows the standard mechanism of the electrophilic aromatic substitution to yield the iodinated product.
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