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

(i) In haloarenes, the lone pair of electron on halogen are in resonance with benzene ring. So, $\mathrm{C}-\mathrm{Cl}$ bond acquires partial double bond character which strengthen $\mathrm{C}-\mathrm{Cl}$ bond. Therefore, they are less reactive towords nucleophilic substitution reaction.

(ii) In haloarenes, the carbon atom attached to halogen is $s p^2$ hybridised. The $s p^2$ hybridised carbon is more electronegative than $s p^3$ hybridised carbon. This $s p^2$-hybridised carbon in haloarenes can hold the electron pair of $C-X$ bond more tightly and make this $\mathrm{C}-\mathrm{Cl}$ bond shorter than $\mathrm{C}-\mathrm{Cl}$ 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 $\mathrm{S}_{\mathrm{N}} 1$ mechanism ruled out.

(iv) Because of the repulsion between the nucleophile and electron rich arenes, aryl halides are less reactive than alkyl halides.

The reactivity of aryl halides can be increased by the presence of an electron withdrawing group $\left(-\left(\mathrm{NO}_2\right)\right.$ at ortho and para positions. However, no effect on reactivity of haloarenes is observed by the presence of electron withdrawing group at meta-position. Mechanism of the reaction is as depicted with ${ }^{-} \mathrm{OH}$ ion.

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 postitions not on meta position.