Question

Why are aryl halides less reactive towards nucleophilic substitution reactions than alkyl halides? How can we enhance the reactivity of aryl halides?

Answer 1

Part 1: Reactivity of aryl halides

Aryl halides are less reactive towards nucleophilic substitution reaction due to the following reasons

In haloarenes the lone pair of electrons on halogen are in resonance with the benzene ring So, $C-Cl$ bond acquires partial double bond character which strengthens $C-Cl$ bond Therefore, they are less reactive towards nucleophilic substitution reaction.

In haloarenes, the carbon atom attached to halogen is $s{p}^2$ hybridised. The $s{p}^2$ hybridised carbon is more electronegative than sp3 hybridised carbon. This $s{p}^2$-hybridised carbon in haloarenes can hold the electron pair of $C-X$ bond more tightly and make this $C-Cl$ bond shorter than $C-Cl$ bond of haloalkanes
Since, it is difficult to break a shorter bond than a longer bond, therefore, haloarenes are less reactive than haloarenes
In haloarenes, the phenyl cation will not be stabilised by resonance therefore $S_{N}1$ mechanism is ruled out.
Because of the repulsion between the nucleophile and electron rich arenes, aryl halides are less reactive than alkyl halides

Part 2: Ways to increase reactivity of aryl halides
The reactivity of aryl halides can be increased by the presence of an electron withdrawing group $(-N{O}_2)$ at ortho and para positions. However, no effect on reactivity of haloarenes is observed by the presence of electron withdrawing groups at meta-position. Mechanism of the reaction is as depicted with $-OH$ ions.



From the above resonance, it is very clear that electron density is rich at ortho and para positions. So, presence of EWG will facilitate nucleophilic at ortho and para positions not on meta position.