Why chloro benzene is less reactive …

Chlorobenzene is less reactive towards nucleophilic substitution reaction because of the following reasons:

(1) Resonance effect: The electron pair on chlorine atom is in conjugation with the electrons of the benzene ring which results in the following resonance structures:

This results in delocalization of the electrons of C - Cl bond and a partial double bond character develops in the bond, which makes it difficult for the Nucleophile to cleave the C - Cl bond.

(2) Polarity of the C-Cl bond: The sp2 hybridized carbon atom involved in C-Cl bond in chlorobenzene is more electronegative than the sp3 hybrid carbon atom in alkyl halide. Therefore, this sp2 hybridised carbon atom has less tendency to release electrons to the Cl atom. Thus, lower the polarity of C-Cl, lesser is the reactivity.