3.6: Conformations of Ethane and Propane
In an organic molecule, free rotation about the carbon-carbon single bond results in energetically different conformers of the molecule. Due to this rotation, called the internal rotation, ethane has two major conformations — staggered and eclipsed.
Staggered conformation is a low energy and more stable conformation with the C-H bonds on the front carbon placed at 60°dihedral angles relative to the C-H bonds on the back carbon, leading to a reduced torsional strain. In staggered ethane, the bonding molecular orbital of one C-H bond interacts with the antibonding molecular orbital of the other, thereby further stabilizing the conformation. The rotation of farther carbon while keeping the carbon nearer to the observer stationary generates an infinite number of conformers. At 0° dihedral angles, the C-H groups cover one another to form an eclipsed conformation. This conformation has about 12 kJ/mol more torsional strain than the staggered conformation and hence is less stable. Ethane molecules rapidly interconvert between several staggered states while passing through the higher energy eclipsed states. The molecular collisions provide the energy required to cross this torsional barrier.
Similar to ethane, propane also has two major conformers: the stable staggered conformer (low energy) and the unstable eclipsed conformer (higher energy).