Question

Why are haloarenes less reactive towards nucleophilic substitution reaction? Give two reasons.

Hint:

Haloarenes are compounds where a halogen atom is attached directly to a benzene ring, for example chlorobenzene (\(C_6H_5Cl\)).

Answer 1

Concept:
Haloarenes are compounds where a halogen atom is attached directly to a benzene ring, for example chlorobenzene (\(C_6H_5Cl\)). In a nucleophilic substitution, a nucleophile must attack the carbon holding the halogen and push the halogen out. Anything that makes that C-X bond stronger or makes the carbon harder to attack will slow this reaction down.

Reason 1 (Resonance / partial double bond character):
In a haloarene, the lone pair of electrons on the halogen overlaps with the pi electrons of the benzene ring (resonance). Because of this, the carbon-halogen bond gains partial double bond character and becomes shorter and stronger. A stronger C-X bond is harder to break, so substitution is difficult.

Reason 2 (sp2 carbon holding the halogen):
In haloarenes the carbon bearing the halogen is sp2 hybridised. An sp2 carbon holds its electrons more tightly (more s-character) than an sp3 carbon, so the C-X bond is shorter and stronger and the carbon is less easily attacked by a nucleophile.

Answer: Haloarenes are less reactive towards nucleophilic substitution because (1) resonance gives the carbon-halogen bond partial double bond character, making it short and strong, and (2) the carbon bearing the halogen is sp2 hybridised, which also makes the C-X bond stronger and harder to break. (Other accepted reasons: the ring carbon is electron rich and repels the incoming nucleophile, and the possible instability of the phenyl cation.)