The following reaction method
is not suitable for the preparation of the corresponding haloarene products, due to high reactivity of halogen, when X is :

- Aryl chlorides and bromides can easily be prepared by electrophilic substitution of arenes (toluene) with $\mathrm{Cl}_2$ and $\mathrm{Br}_2$ respectively in the presence of Lewis acid catalyst ( Fe in dark). - Reaction with $\mathrm{I}_2$ is reversible and requires the presence of oxidising agent. - Corresponding fluoroarene is not prepared by this method due to high reactivity of fluorine. Hence, ' X ' is $F$.