Q.Rotation around carbon-carbon single bond of ethane is not completely free. Justify the statement.
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Start your 14-day free trial to unlock the full solution →Rotation around the C–C single bond in ethane encounters a small but measurable energy barrier (~12 kJ/mol) due to torsional strain from electron-cloud repulsion between eclipsed C–H bonds. The staggered conformation is most stable, the eclipsed least stable, making rotation hindered rather than free.
The phrase "free rotation" suggests that a carbon–carbon single bond should allow the two methyl groups in ethane to spin past one another without any energy cost, like a frictionless axle. After all, a -bond has cylindrical symmetry about the internuclear axis, so geometrically nothing should prevent rotation. Yet experiment tells a different story: rotation is hindered.
Why rotation is not free
When you rotate one group relative to the other in ethane, the molecule passes through different conformations—spatial arrangements that differ only by rotation about the single bond. These conformations have different potential energies because of how the electron clouds of the C–H bonds on adjacent carbons interact.
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Staggered conformation: The C–H bonds on one carbon are as far apart as possible from those on the other (dihedral angle , , ). The electron clouds repel each other minimally. This is the lowest-energy, most stable arrangement.
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Eclipsed conformation: The C–H bonds on adjacent carbons line up directly (dihedral angle , , ). The electron clouds are forced close together, leading to maximum repulsion. This is the highest-energy, least stable arrangement.
The energy difference between staggered and eclipsed conformations is about 12 kJ/mol. This barrier arises from torsional strain (also called Pitzer strain)—the repulsion between bonding electron pairs when bonds are eclipsed.
A common mistake is to think the barrier comes from steric repulsion between hydrogen atoms. In ethane the hydrogens are small and far apart; the dominant effect is electron-cloud repulsion between the C–H bonds themselves.
The rotation profile
As one methyl group rotates through , the potential energy oscillates:
| Dihedral angle | Conformation | Relative energy |
|---|---|---|
| , , | Eclipsed | Maximum (~12 kJ/mol) |
| , , | Staggered | Minimum (0 kJ/mol) |
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