Neither vinyl chloride `(CH_2=CH-Cl)` nor chlorobenzene `(C_6H_5-Cl)` can be used as alkylating agent in the Friedel-Crafts reaction -Why ?

Explanation

In vinyl chloride or in chlorobenzene, the unshared pair of electrons on Cl atom is involved in resonance interaction with the `pi`-electron system and as a result, the `C-Cl` bond in both the cases acquires some double bond character. The Lewis acid `AlCl_(3)` is incapable of breaking such a strong `C-Cl` bond. Moreover, even if the `C - Cl` bond breaks, the carbocations produced would be unstable (due to positive charge on an `sp^(2)` hybridised carbon atom). Hence such a bond is very much reluctant to undergo cleavage. For this reason, vinyl chloride or chlorobenzene cannot be used as alkylating agent in the Friedel-Crafts reaction.