Figure 3

So far, all stereoisomers discussed differ in configuration but a separate class of stereoisomers also exist which are called conformational isomers. The difference between them being that in a set of conformational isomers, the isomers can be converted from one isomer to the other by mere rotation of bonds. On the other hand, configurational isomers cannot be achieved in this fashion. A bond needs to be broken and reconnected at the same stereocentre in a different spatial arrangement to obtain a configurational isomer from another.

As an example, the molecule of ethane is to be considered. Now, it cannot possibly have any configurational isomers since none of the carbons are dissymmetric. However, it may have conformational isomers. The atoms remain connected in the same order during conformational change. If the C₁-C₂-bond is considered in ethane,  then it is possible to draw two structures, one in which the hydrogen atoms on one of carbon atoms eclipse the other and another where they are as far away from each other as possible. There is a difference in energy between the two structures. The eclipsed form has a higher potential energy than the staggered form (12 kJ mol^-1) (Figure 4).

Figure 4

 The rotational barrier in this case is said to be 12 kJ mol^-1. This is the energy required to convert the stable staggered form to the unstable eclipsed form (Figure 5). The change in energy on going from staggered form to eclipsed form and vice versa is plotted in with respect to the dihedral angle. The angle between two intersecting planes on a third plane normal to the intersection of the two planes is called dihedral angle. In this case the dihedral angle is angle between the planes containing the atoms 1 and 2 and the plane containing the atoms 3 and 4.

Figure 5