Explain why fluorobenzene is more reactive than chlorobenzene toward electrophilic aromatic substitution but chloromethylbenzene is more reactive than fluoromethylbenzene.

Explanation:

The order of positive mesomeric effect (+M) is as follows

F>Cl>Br>I.

This implies that electron density in the respective Halobenzene is of the order

Fluorobenzene > Chlorobenzene > Iodobenzene.

This means that electrophilic aromatic substitution will also follow the same order, more the electron density more will be chances of electrophilic aromatic substitution. This means  fluorobenzene is more reactive than chlorobenzene toward electrophilic aromatic substitution

However, methyl group is attached in ring, beacuse of starric hindrance, chloromethylbenzene is more reactive than fluoromethylbenzene.