Question

Why is the C-Cl bond in chlorobenzene less polar as compared to that in chloromethane?

Answer 1

1. Resonance Effect:
   • In chlorobenzene, the lone pair of electrons on the chlorine atom delocalizes into the benzene ring through resonance.
   • This delocalization spreads out electron density, reducing the difference in electron density between carbon and chlorine.
   • Hence, the C–Cl bond polarity decreases as resonance reduces the partial charges.
2. Hybridization Difference:
   • The carbon bonded to chlorine in chlorobenzene is sp² hybridized, whereas in chloromethane it is sp³ hybridized.
   • The sp² carbon holds the bonding electrons more tightly due to higher s-character, lowering the polarity of the C–Cl bond.
3. Electron Density Distribution:
   • The conjugation in chlorobenzene distributes electron density over the aromatic ring, lessening the localized dipole of the C–Cl bond.
   • In chloromethane, the electron density remains more localized on the C–Cl bond, increasing polarity.
4. Effect on Bond Character:
   • Due to resonance, the C–Cl bond in chlorobenzene has partial double bond character.
   • This partial double bond character stabilizes the bond and makes it less polar.
5. Lack of Resonance in Chloromethane:
   • Chloromethane lacks an aromatic ring or π system, so no resonance delocalization occurs.
   • Therefore, the full electronegativity difference between chlorine and carbon manifests as bond polarity.