Why is the C-Cl bond length in chlorobenzene is less than the C-Cl bond length in chloromethane? 

Resonance effect in haloarenes

The correct answer is option 2. Resonance effect in haloarenes.

Let us delve deeper into why the C-Cl bond length is shorter in chlorobenzene compared to chloromethane:

Chlorobenzene is a haloarene where chlorine is directly bonded to a benzene ring. The benzene ring in chlorobenzene exhibits aromaticity and resonance stabilization. Here's how resonance affects the C-Cl bond length:

Resonance Stabilization: In benzene, the six \(\pi\)-electrons are delocalized over the entire ring due to resonance. The chlorine atom is directly attached to this aromatic ring.

Electron Delocalization: The lone pair of electrons on the chlorine atom can participate in resonance with the benzene ring. This delocalization spreads the negative charge over the ring, including the carbon atom bonded to chlorine.

Bond Strength and Length: Resonance stabilization in benzene makes the carbon-chlorine bond in chlorobenzene stronger and shorter. The electrons are delocalized, reducing the electron density on the chlorine atom and making the C-Cl bond more stable and shorter in length.

Chloromethane (\(\text{CH}_3\text{Cl}\))

Chloromethane, on the other hand, is a simple alkyl halide where chlorine is bonded to a methyl group (CH\(_3\)).

Electron Density: In chloromethane, the lone pair of electrons on chlorine is localized more on the chlorine atom itself. There is no significant resonance stabilization effect as seen in benzene.

Bond Length: The C-Cl bond in chloromethane is longer compared to chlorobenzene because the electron density around the chlorine atom is higher. This makes the bond weaker and longer in chloromethane.

Comparison:

Chlorobenzene: Shorter C-Cl bond due to resonance stabilization and delocalization of electron density over the aromatic ring.

Chloromethane: Longer C-Cl bond due to the absence of significant resonance effects and higher electron density localized on the chlorine atom.

Conclusion: The difference in C-Cl bond length between chlorobenzene and chloromethane is primarily due to the resonance stabilization in chlorobenzene, which spreads electron density and stabilizes the bond, making it shorter. Chloromethane lacks this resonance stabilization, leading to a longer C-Cl bond.