Conformations of ethane

Electron density in a C–C σ bond is symmetrical about the bond axis, so turning one carbon relative to the other does not break the bond. The resulting interconvertible arrangements are called conformations, conformers or rotamers.

Rotation is not perfectly free. A small barrier of 1–20 kJ mol⁻¹, due to repulsion between bonds on neighbouring carbons, opposes it; this repulsion is called torsional strain.

In ethane, the staggered form keeps the H atoms on the two carbons as far apart as possible, the eclipsed form puts them as close as possible, and anything in between is a skew form. Bond lengths and angles are the same in all of them.

The angle of rotation about the C–C bond is the dihedral (torsional) angle. Torsional strain is least in the staggered form and greatest in the eclipsed form, so staggered is the more stable, preferred conformation.

The energy gap between the eclipsed and staggered forms of ethane is about 12.5 kJ mol⁻¹. Collisions at ordinary temperature supply this easily, so rotation is almost free in practice and the conformers of ethane cannot be separated.

Sawhorse projection: the C–C bond is drawn as a longer slanted line, front carbon at the lower end and rear carbon at the upper end, each carrying three bonds 120° apart.

Newman projection: looking straight down the C–C bond, the front carbon is a point with three bonds at 120°, and the rear carbon is a circle with three shorter bonds at 120°.