Conformations of ethane

Atoms joined by a single bond can rotate about it, like two wheels on one axle. As one CH₃ group of ethane turns, the hydrogens take up different positions relative to the other group. These different spatial arrangements are called conformations (or conformers).

Two extreme conformations:

• Eclipsed: the hydrogens on the front carbon are directly in line with those on the back carbon, as close as possible.
• Staggered: the hydrogens are as far apart as possible, each front hydrogen between two back ones.

All the in-between arrangements are called skew.

The staggered form is the most stable, with the lowest energy. The eclipsed form is the least stable, because of torsional strain: repulsion between the electron clouds of the bonds that are lined up.

The energy difference in ethane is only about 12.5 kJ mol⁻¹. At room temperature, molecules have plenty of energy to cross this small barrier, so rotation is almost free, and the different conformers cannot be separated from each other.

Two drawings show conformations:

• Sawhorse projection: the C–C bond is drawn slanting, going back from the front carbon, with the three bonds on each carbon shown.
• Newman projection: you look straight down the C–C bond. The front carbon is a point where its three bonds meet; the back carbon is a circle, with its bonds coming out from the edge.