As we advance down the group, the electrophilicity and electronegativity of halogens drop. Hence Iodine is the least reactive element, while fluorine is the most reactive. Fluorination is a highly reactive process, whereas iodination is a highly non-reactive process.
Moving down the group also results in a drop in the exothermic rates. Fluorination produces explosive, extremely exothermic compounds that are polyfluorinated. As a result, fluorinating benzene directly is risky and complicated.
Fluorine is added to aromatic rings in indirect methods. Through the Balz-Schiemann reaction, fluorobenzene can be produced from benzene diazonium chloride. In this reaction, a diazonium tetrafluoroborate intermediate transforms an aromatic amine into an aryl fluoride. It is usual practise to obtain fluorobenzene and its derivatives using this approach.

The conversion has the following form: Nitrous acid is used to treat aniline to create benzene diazonium chloride, which is then transformed into benzene diazonium fluoroborate by a reaction with ${\mathbf{HBF}}_{\mathbf{4}}$. It undergoes breakdown upon additional heating, producing fluorobenzene. The response is displayed below: