Question

Chlorobenzene is extremely less reactive towards a nucleophilic substitution reaction. Give two reasons for the same.

Answer 1

Chlorobenzene is extremely less reactive towards a nucleophilic substitution reaction.

Chlorine shows two effects viz. +R effect and -I effect.

Nucleophilic substitution reaction

That substitution reaction which are carried out by a $N{u}^{⊝}$

For eg:

$R-X+N{u}^{⊝}\to R-Nu+X^{-}$

A nucleophile is an $e^{-}$ rich species so it attacks on that compound which is $e^{-}$ deficient to donate its electron.

1) Resonance Effect:

Some pair of $e^{-}$ on chlorine is in resonance effect with the benzene ring a which increases the $e^{-}$ density on the benzene ring.

Due to the resonance effect, there is increased $e^{-}$ density on the benzene ring. The $e^{-}$ density repels the incoming Nu to attack the benzene ring.

2) Because of the development of partial double - bond character, the nucleophile cannot leave the $C=Cl$ bond for substitution.

Therefore, chlorobenzene is less reactive towards Nu substitution reaction.