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NEET CHEMISTRYHydrocarbonsMedium

Question

The compound that has least hindered rotation around carbon-carbon bond is-

A

Ethane

B

Ethylene

C

Acetylene

D

Hexachloroethane

Step-by-Step Solution

  1. Nature of Bonds:
  • Ethylene (CH2=CH2CH_2=CH_2): Contains a carbon-carbon double bond (σ+π\sigma + \pi). The presence of the π\pi bond restricts rotation because rotation would break the sideways overlap of p-orbitals .
  • Acetylene (HC≡CHHC \equiv CH): Contains a carbon-carbon triple bond (σ+2π\sigma + 2\pi). This bond is rigid and restricts rotation .
  1. Rotation in Single Bonds: Rotation is possible around carbon-carbon single bonds (σ\sigma-bonds), as seen in Ethane and Hexachloroethane. This leads to conformational isomerism .
  2. Steric and Torsional Strain: While rotation around single bonds is described as 'free', it is actually hindered by a small energy barrier (torsional strain).
  • Ethane (CH3−CH3CH_3-CH_3): The substituents on the carbon atoms are small hydrogen atoms. The repulsive interaction (torsional strain) between the electron clouds of C-H bonds is weak (1-20 kJ mol−1^{-1}), allowing for the least hindered rotation .
  • Hexachloroethane (CCl3−CCl3CCl_3-CCl_3): The substituents are bulky chlorine atoms. The van der Waals repulsion (steric hindrance) between the large Cl atoms on adjacent carbons is significantly higher than that in ethane, making the rotation more hindered.
  1. Conclusion: Ethane possesses the single bond with the smallest substituents, resulting in the least hindered rotation.

Exam Context & Concepts Covered

This question aligns with the NEET CHEMISTRY syllabus, specifically targeting concepts from Hydrocarbons. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

CHEMISTRYHydrocarbonscompoundhinderedrotationaroundcarboncarbon

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A.Option 1 (Structure missing)
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D.The staggered conformation of ethane is less stable than eclipsed conformation because staggered conformation has a torsional strain.
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