Question

Aryl halides are less reactive in nucleophilic substitution reactions.
(i) Write any two reasons for less reactivity.
(ii) Give one example for nucleophilic substitution reactions of aryl halides.

Answer 1

(i) Aryl halides are less reactive in nucleophilic substitution reactions. This is due to following two reasons.
(1) $C-X$ bond in aryl halides is $s{p}^2$ hybridised. s character is more. Bond length is shorter. Bond is strong and requires greater energy. Hence, aryl halides are less reactive.
$C-X$ bond in alkyl halides is $s{p}^3$ hybridised. s character is less. Bond length is longer. Bond is weak and requires lesser energy. Hence, alkyl halides are more reactive.
(2) The lone pair of electrons of halogen atom in aryl halides is in conjugation with pi electrons of the ring. This resonance imparts partial double bond character to $C-X$ bond.
(ii) Example for nucleophilic substitution reactions of aryl halides.
$Chlorobenzene$ on heating with $NaOH$ at 623 K under pressure gives phenol. Here, $-Cl$ is replaced with $-OH$
$C_6{H}_5-Cl+O{H}^{-}\underset{H^{+}}{\overset{NaOH623K,pressure}{\to }}{C}_6{H}_5-OH+C{l}^{-}$