Question

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

Hint:

Haloarenes can be made more reactive by adding electron-withdrawing groups (like \( -NO_2 \)) at ortho and para positions.

Answer 1

1. Resonance Effect:
In haloarenes, the halogen atom is attached to the aromatic ring. The lone pair of electrons on the halogen can participate in resonance with the π-electrons of the aromatic ring. This delocalization of electrons makes the carbon-halogen bond stronger and more stable, as the electron density on the carbon attached to the halogen decreases. As a result, the carbon becomes less electrophilic, making it less susceptible to nucleophilic attack. This resonance effect significantly reduces the reactivity of haloarenes in nucleophilic substitution reactions.

2. Partial Double Bond Character:
The resonance between the halogen and the aromatic ring also imparts partial double bond character to the C–X bond in haloarenes. This partial double bond character makes the bond stronger and more difficult to break compared to a simple single bond. Because of this strengthened bond, the halogen atom is less likely to leave, and the nucleophilic substitution reaction becomes slower and less favorable. This contributes to the reduced reactivity of haloarenes towards nucleophilic substitution reactions.