Organic Chemistry: Conformations: Conformations of Ethane

Figure %: The eclipsed and staggered conformations of ethane.

Energetically, not all conformations are equally favored. The eclipsed conformation of ethane is less stable than the staggered conformation by 3 kcal/mol. The staggered conformation is the most stable of all possible conformations of ethane, since the angles between C-H bonds on the front and rear carbons are maximized at 60 degrees. In the eclipsed form, the electron densities on the C-H bonds are closer together than they are in the staggered form. When two C-H bonds are brought into a dihedral angle of zero degrees, their electron clouds experience repulsion, which raises the energy of the molecule. The eclipsed conformation of ethane has three such C-H eclipsing interactions, so we can infer that each eclipsed C-H "costs" roughly 1 kcal/mol.

Figure %: Eclipsing interactions in ethane.

Steric Hindrance

Eclipsing interactions are an example of a general phenomenon called steric hindrance, which occurs whenever bulky portions of a molecule repel other molecules or other parts of the same molecule. Because such hindrance causes resistance to rotation, it is also called torsional strain. The 3 kcal/mol needed to overcome this resistance is the torsional energy. Note that this figure is very small compared to the energy required to rotate around double bonds, which is 60 kcal/mol (the bond energy of a C-C $\pi$ bond). At room temperature, ethane molecules have enough energy to be in a constant state of rotation. Because of this rapid rotation, it is impossible to isolate any particular conformation in the way that cis- and trans- alkenes can be individually isolated. Although the term "conformational isomer" is sometimes used as a synonym for conformations, conformations of a molecule are not considered true isomers because of their rapid interconversion.