Question

Explain why aryl halides are less reactive towards nucleophilic substitution reactions?

Answer 1

Aryl halides are less reactive towards nucleophilic substitution reactions due to following reasons:
(1) The $C$ atom of $C-X$ bond is $s{p}^2$ hybridized. This results in more s character, shorter bond length and higher bond strength. Breaking of $C-X$ bond requires greater energy.
(2) There is conjugation between lone pair of electrons on halogen atom and pi electrons of aromatic ring. Resonance results in double bond character for $C-X$ bond.
(3) Self ionization of aryl halide will give phenyl cation which will not be stabilized by resonance. Hence, SN${}^1$ attack is not possible. Pi electrons of aromatic ring block backside attack of nucleophile. Hence, SN${}^2$ mechanism is ruled out.