Question

The energy barrier to free rotation about the carbon-carbon bond of ethane is about.
(A) $30\text{ kJ/mo}{{\text{l}}^{-1}}$
(B) $\text{3 kJ/mo}{{\text{l}}^{-1}}$
(C) $\text{3 kcal/mo}{{\text{l}}^{-1}}$
(D) $\text{40 kJ/mo}{{\text{l}}^{-1}}$

Answer 1

The energy barrier can be defined as the energy which must be overcome in order to complete one rotation around the bond. The structure of the carbon-carbon bond of ethane is staggered and eclipsed.

Complete Step by step solution:
The chemical formula of ethane is ${{\text{C}}_{2}}{{\text{H}}_{6}}$.
${{\text{H}}_{3}}\text{C}-\text{C}{{\text{H}}_{3}}$ is cylindrically symmetrical therefore rotation of bond is possible.
In ethane molecules both staggered and eclipsed conformation takes place. In eclipsed conformation two substitutes $\text{X}$-$\text{Y}$ and adjacent atoms $\text{A}$-$\text{B}$ are closest to each other which implies that the torsion angle $\text{X-A-B-Y}$ is $\text{0}{}^\circ$. This configuration generally exists in any open chain and single chemical bond. The maximum energy level is often explained as steric hindrance. The steric hindrance is very high in the eclipsed conformation because the bonds are close to each other and at least when the conformation is staggered as the bonds are far from each other.
In ethane, the rotation around the carbon-carbon bond is not entirely free but that an energy barrier exists. The energy barrier of $3\text{ kcal/mol}$ exists when the structure of the ethane is in eclipsed form. The ethane molecule needs to overcome the energy barrier in order to rotate around the bond. The reason behind this energy barrier is torsional strain. In staggered conformation around $12.5\text{ J/mol}$ of torsion energy is released.

Note: The torsion strain can be defined as the strain caused by the close approach of atoms or groups separated by three covalent bonds. The conceptual knowledge about staggered conformation and eclipsed conformation in Newman projection is required.