The barrier to rotation about the C-C bond in ethane is about $12 kJ mo l^{- 1}$ . Which conformation is the energy minimum, and why is the other less stable?

AEclipsed; staggered C-H bonds repel each other
BStaggered; eclipsed C-H bonds cause torsional strain
CStaggered; the C-C bond is shorter in the eclipsed form
DEclipsed; hyperconjugation is stronger in the eclipsed form

Answer: B

In the staggered conformation the C-H bonds on adjacent carbons are 60

^{\circ}

apart, which minimises repulsion between bonding electron pairs. In the eclipsed conformation the C-H bonds are aligned and torsional strain raises the energy by about 12 kJ/mol (three H/H eclipsing interactions of about 4 kJ/mol each).

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The barrier to rotation about the C-C bond in ethane is about 12 kJ mol−1. Which conformation is the energy minimum, and why is the other less stable?

Answer: B

More questions on Conformational analysis of ethane, butane and cyclohexane

The barrier to rotation about the C-C bond in ethane is about $12 kJ mo l^{- 1}$ . Which conformation is the energy minimum, and why is the other less stable?