Rotation around C2-C3 in n-butane

So far, we have seen the rotation around the carbon-carbon axis of ethane and propane. In this exercise, we will be looking at the rotation around the C2-C3 axis of n-butane. Going from ethane to propane, nothing really unexpected happened; the energy profile looked quite the same, except for the slightly larger rotational barrier in propane.
For butane however, we get a really different energy profile:

Whereas the energy profiles for rotation in ethane and propane were purely sinusoidal (i.e. only one maximum and one minimum energy, both repeated every 120°), the profile for butane contains local minima and maxima. Let's step through them, starting with the easy ones:

• At 0° there is an energy maximum: the hydrogens on C2 and C3 are in an eclipsed conformation, and the methyl groups attached to C2 and C3 are crowding together.
• At 180°, the absolute energy minimum is found. This is a staggered conformation in which the methyl groups connected to C2 and C3 are as far apart as possible (dihedral angle of 180°).
• At 60° and 300°, local minima in energy are found. You would indeed expect minima, because the conformation is staggered. However, the methyl groups attached to C2 and C3 are closer to each other than at 180° of rotation, which causes the higher energy. This conformation, where the functional groups attached to two adjacent carbon atoms are 60° apart, is called a gauche conformation
• At 120° and 240°, local maxima in energy exist. These maxima are not as high as the maximum at 0°, because although the conformation is eclipsed, the two methyl groups are not as close to each other as at 0° rotation.

Energies are from AM1 calculations and are for qualitative discussion only. Better calculations would give different quantitative results. This page was adapted by Bob Hanson for Internet Explorer 4 and Netscape 7 from the original, written by Hens Borkent