Question

Explain the Resonance effect in nucleophilic substitution reactions of Haloarenes.

Hint:

Haloarenes are less reactive towards nucleophilic substitution due to both resonance stabilization and the partial double bond character of the C–Cl bond.

Answer 1

Step 1: Understanding Resonance in Haloarenes.
In haloarenes (like chlorobenzene), the lone pair of electrons on the halogen atom interacts with the $\pi$-electrons of the benzene ring. This delocalization leads to resonance structures.
Step 2: Resonance Structures.
The $p$-orbital of the halogen atom overlaps with the $p$-orbitals of the benzene ring, giving rise to multiple resonance structures. As a result, the C–Cl bond acquires partial double bond character.
Step 3: Effect on Nucleophilic Substitution.
Because of resonance: - The C–Cl bond in haloarenes is shorter and stronger than a normal C–Cl bond.
- This makes the bond difficult to break, thereby making nucleophilic substitution reactions of haloarenes much less reactive compared to haloalkanes.
Conclusion:
Resonance stabilizes the C–Cl bond in haloarenes, giving it partial double bond character and reducing the tendency for nucleophilic substitution.