Question

Why is the C-Cl bond length in chlorobenzene shorter as compared to that in chloromethane?

Answer 1

1. Resonance Effect in Chlorobenzene: In chlorobenzene, the lone pair electrons on the chlorine atom can delocalize into the aromatic ring through resonance. This partial delocalization creates partial double bond character between the carbon and chlorine.
2. Partial Double Bond Character: Due to resonance, the C–Cl bond in chlorobenzene has partial double bond character, making it stronger and shorter than a typical single bond.
3. Sigma Bond in Chloromethane: In chloromethane, the C–Cl bond is a pure single sigma bond without any resonance stabilization, so it is longer.
4. Electron Delocalization: The overlap of chlorine’s lone pair p-orbital with the aromatic ring’s π-system in chlorobenzene reduces the bond length.
5. Bond Length Difference: As a result of resonance and partial double bond formation, the C–Cl bond in chlorobenzene is shorter than the C–Cl bond in chloromethane.
6. Effect on Bond Strength: The shorter bond in chlorobenzene is also stronger due to the partial double bond character.