Why is the C–Cl bond length in chlorobenzene less than the C–Cl bond length in chloromethane?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Resonance effect in haloarenes
In chlorobenzene, resonance of a lone pair on chlorine with the ring gives the C–Cl bond partial double bond character. A bond with some double bond character is shorter and stronger than a single bond.
In chloromethane, \( \ce{CH3Cl} \), there is no ring and no conjugation, so the C–Cl bond is a pure single bond. Hence the C–Cl bond in chlorobenzene (169 pm) is shorter than in chloromethane (177 pm).
Why the other options are incorrect
Option (1) Possible repulsion between the nucleophile and the electron-rich ring of chlorobenzene — Incorrect. This explains why haloarenes react slowly with nucleophiles. It is about a reaction, not about the length of the bond in the molecule.
Option (3) Greater s-character of the carbon atom of the C–Cl bond in chloromethane — Incorrect.
It is the reverse.
The carbon in chloromethane is sp3 (less s-character); the carbon in chlorobenzene is sp2 (more s-character), which holds the bonding pair more tightly and also shortens the bond in chlorobenzene.
Option (4) Instability of the phenyl cation — Incorrect. The phenyl cation is not stabilised by resonance, so haloarenes do not react by the SN1 mechanism. This too is about reactivity, not bond length.
NCERT (2026-27 edition):
6.7.2 Reactions of Haloarenes
