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

The single bond in ethane is a σ – bond, which is a coaxial overlap of orbitals, so the C ─ C bond can be rotated on its axis. But this rotation is not completely free due to the torsional strain that the bond undergoes due to rotation.

There are two conformations of ethane: the eclipsed and the staggered.

These are called as conformations or conformers or rotamers. But this rotation is limited due to an energy barrier of 20 kJ mol^- which is due to the electron repulsion of C ─ H bonds. This barrier is not large to control the rotation so these isomers co-exist and cannot be separated from one another. The staggered conformation is more stable due to minimum electron repulsion and maximum distance between atoms and C ─ H.