Haloarenas are less reactive than haloalkanes and haloalkenes. Explain.

Explanation

Haloarenes are much less reactive than haloalkanes and haloalkenes towards nucleophilic substitution reactions because of the following reasons:
1. Resonance effect: In haloarenes, there is delocalisation of electrons due to resonance. For example, chlorobenzene is considered to be a resonance hybrid of the following structures:

It is evident that the contribution of structures III, IV and V imparts a partial double bond character to the carbon-chlorine bond. This is confirmed by X-ray analysis which shows that the C-Cl bond length in chlorobenzene is `1.69 overset(@)A` while the C-Cl bond length in ethyl chloride molecule is `1.82 overset(@)A`. The shortening of bond length imparts stability to aryl halides and the bond cleavage becomes rather difficult. The aryl halides are, thereforo, less reactive than alkyl halides.
In haloalkenes also, the filled p-orbital of halogen atom cannot overlap sidewise with the unhybridised p-orbital of C atom. Therefore, C-Cl bond is not very strong and can be easily cleaved.
2. Different hybridisation states of carbon atom : In haloalkanes the carbon atom of the C-X bond is `sp^(3)` hybridised while in haloarene halides, the carbon atom is `sp^(2)` hybridised. The `sp^(2)` hybridised carbon atom with a greater s-character is more electronegative. It can hold the electron pair of the bond more tightly than the `sp^(3)` hybridised carbon atom in alkyl halides. Therefore, it has less tendency to release electrons to the halogen. As a result, the bond cleavage in aryl halides is some what more difficult than in alkyl halides.

Thus, haloarenes are less reactive towards the substitution reactions than haloalkanes.