Question

Rotation around a carbon-carbon single bond of ethane is not completely free. Justify the statement.

Answer 1

Alkanes contain carbon-carbon sigma $\left(\sigma \right)$ bonds. Electron distribution of the sigma molecular orbital is symmetrical around the internuclear axis of the $C–C$ bond which is not disturbed due to rotation about its axis. This permits free rotation of $C–C$ single bonds.

This rotation results in different spatial arrangements of atoms in space which can change into one another.
Such spatial arrangements of atoms which can be converted into one another by rotation around a $C–C$ single bond are called conformations or conformers.

However, the rotation around a $C–C$ single bond is hindered by a small energy barrier due to weak repulsive interaction between the adjacent bonds. Such a type of repulsive interaction is called torsional strain. In all the conformations of ethane, the staggered form has the least torsional strain, and the eclipsed form has the maximum torsional strain. So, it creates a small energy barrier between these conformations.

Thus, rotation around a carbon-carbon single bond of ethane is not completely free.