Question

Why is chlorobenzene less reactive than chloroethane towards nucleophiles?

Answer 1

1. Partial double bond character: In chlorobenzene, the lone pair of electrons on chlorine is delocalized into the π-electron system of the benzene ring, creating partial double bond character between the carbon and chlorine. This makes the C–Cl bond stronger and harder to break.
2. Resonance stabilization: Chlorobenzene shows resonance, which stabilizes the molecule and prevents the departure of the Cl⁻ ion easily, reducing the likelihood of nucleophilic substitution.
3. Bond strength: Due to the partial double bond, the C–Cl bond in chlorobenzene is shorter and stronger than in chloroethane, making it less susceptible to nucleophilic attack.
4. Aromatic ring stability: Breaking the C–Cl bond in chlorobenzene would disrupt the aromaticity of the benzene ring, which is energetically unfavorable.
5. Lack of positive charge on carbon: In chlorobenzene, the carbon attached to chlorine is not significantly electron-deficient, so nucleophiles are not strongly attracted to it.
6. Chloroethane lacks resonance: In chloroethane, there is no resonance or delocalization, and the C–Cl bond is purely a single bond, which makes it easier for nucleophiles to attack and displace the Cl⁻ ion. 
7. Reaction mechanism: Chloroethane undergoes SN₁ or SN₂ reactions more readily, while chlorobenzene resists these mechanisms due to resonance and aromatic stability.