Among the following compounds, which one has the shortest C – Cl bond?

Solution

Chemistry ma’am · 3 steps

Look only at the carbon holding the Cl: more s character and resonance pull the bond shorter.

1. Hybridisation rule. An sp² orbital has 33% s character, against 25% for sp³. More s character keeps the electrons closer to the nucleus, so the bond it forms is shorter.
2. Resonance bonus. When Cl sits directly on a C=C, a lone pair on Cl delocalises into the π bond: CH₂=CH-Cl ↔ ⁻CH₂-CH=Cl⁺. The C–Cl bond gains partial double-bond character and shortens further.

   Compound	Carbon bonded to Cl	Resonance with Cl	C–Cl bond
   (a) CH₃-Cl	sp³	no	normal single bond
   (b) (CH₃)₃C-Cl	sp³	no	single bond, slightly longer (crowded tertiary carbon)
   (c) CH₂=CH-CH₂-Cl	sp³ (CH₂ group)	no, the C=C is one carbon away	normal single bond
   (d) CH₂=CH-Cl	sp²	yes	partial double bond, shortest

3. So. Only vinyl chloride, option (d), has Cl on an sp² carbon with resonance, so it has the shortest C–Cl bond.

The idea to keep. To compare C–X bond lengths, check the carbon carrying X. An sp² or sp carbon, plus lone-pair resonance into a π bond, gives a shorter, stronger bond than an sp³ carbon.

Answer from the NTA final answer key. Solution by PrepWiser.