• Aryl chlorides and bromides can easily be prepared by electrophilic substitution of arenes (toluene) with Cl2 and Br2 respectively in the presence of Lewis acid catalyst ( Fe in dark). • Reaction with I2 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 .