Conformational isomerism 1.
In as much as unrestricted rotation about a C-C (sigma) bond is possible it can give rise to different conformers. In saturated compunds the rotation, in the first approximation, is completely free. Two joined tetrahedral atoms are visualized in front-on Newman projection in the next figure, showing the molecule along its axis of rotation. During the rotation the atoms acquire alternating eclipsed and staggered conformations.
In fact, there is always a small barrier to rotation of about 3.0 kcal/mol (12 kJ/mol) when passing through the eclipsed conformation, arising from the Van der Waals repulsion of the H-atoms. This energy barrier is not big enough to prevent the free rotation about the C-C bond, therefore the conformers cannot be separated even at low temperature. The next figure shows butane, the naming of the possible conformers and the energy changes during the rotation:
The naming of the conformers of butane and the energy changes associated with the rotation.
Conformer (conformational isomers): one possible 3D structure of a molecule. The conformers can freely interconvert by the rotation about a single bond in the molecule. As the conformers have usually different conformational energies, their equilibrium mixture is composed of different distributions of the components.
Sometimes the energy barrier is high enough to restrict the rotation. In this case the presence of the isomers can already be detected with appropriate techniques such as NMR. Fully restricted rotation leads to atropisomerism.
Excercise:
Draw the possible conformers of 1,1,2-tribromo-2-chloroethane (Br2CH-CHBrCl) and predict which of the conformers is energetically the most favourable? (Guess which interaction is the stronger one: H···H or H···halogene?)
