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

Step 1: Understanding the Concept:
Nucleophilic substitution involves the breaking of the C-X bond. In haloarenes, this bond is particularly strong and difficult to break compared to haloalkanes.
Step 2: Detailed Explanation:
1. Resonance Effect: The lone pair of electrons on the halogen atom is in conjugation with the \(\pi\)-electrons of the benzene ring. This results in resonance, which gives a partial double bond character to the C-X bond. Breaking a partial double bond is much harder than breaking a single bond.
2. Difference in Hybridization: In haloarenes, the halogen is attached to an \( sp^2 \) hybridized carbon, whereas in haloalkanes it is attached to an \( sp^3 \) carbon. An \( sp^2 \) carbon is more electronegative (has more s-character), which holds the electron pair of the C-X bond more tightly, making the bond shorter and stronger.
3. Instability of Phenyl Cation: If the bond were to break, the resulting phenyl cation would be highly unstable because it cannot be stabilized by resonance.
Step 3: Final Answer:
Haloarenes are less reactive due to the partial double bond character from resonance and the stronger bond from \( sp^2 \) hybridization.