Explain as to why haloarenes are much less reactive than halo-alkanes towards nucleophilic substitution reactions.

Explanation

Haloarenes are much less reactive than haloalkanes towards nucleophilic substitution reactions due the following reasons:
(i) Resonance effect
(ii) Difference is hybridisation of carbon atom in C-X bond
(iii) Instability of phenyl cation.
(i) Resonance effect : In haloarenes, the electron pairs on halogen atom are in conjugation with `pi`-electrons of the ring and the following resonating structure are possible

C-Cl bond requires a partial double bond character due to resonance. As a result, the bond cleavage in haloarene is difficult than holoalkane and therefore, are less reactive towards nucleophilic substitution reaction.