Summary: Conformations
The complete 3D shape of a molecule cannot be specified completely by its
bond lengths and bond angles. We also have to look at the conformations, or
degrees of rotation, about single bonds. The barrier to rotation about a single
bond is typically very low. Hence it is impossible to isolate a single
conformation at room temperature.
Ethane has two note-worthy conformations. In the eclipsed form, the C-H bonds
are parallel to each other and give rise to steric hindrance. The staggered
form is more stable because the C-H bonds are rotated as far apart as possible
at 60 degrees. This type of torsional strain means that some conformations are
favored more than others.
Butane has two varieties of staggered conformations, gauche and anti. The
gauche conformer is less stable because of steric hindrance between adjacent
methyl substituents. In addition some higher alkanes exhibit other unique
interactions: pentane and higher alkanes can experience syn interactions that
occur across 5 carbon atoms.
