3.7: Conformations of Butane
Unlike ethane and propane that have only two major conformations, butane has more than two conformers. The staggered form of butane in which the bulky methyl groups on the two carbons are placed on opposite sides, that is, at a dihedral angle of 180°, is the lowest energy, most stable form — called the anti conformer. This conformation is stabilized due to the absence of steric repulsion between the largely spaced out methyl groups. The other two staggered conformations are degenerate and have 3.8 kJ/mol more energy than the anti conformation, emerging from the steric repulsion between the methyl groups that are now positioned at 60° dihedral angles. These unfavorable steric interactions are known as gauche interactions, and the conformers, as such, are called gauche conformers.
The totally eclipsed butane, due to the two eclipsing methyl groups at 0° dihedral angle, has the highest energy (19 kJ/mol) and is the most unstable form. The other two eclipsed conformations — with two CH3-H eclipses and one H-H eclipse — are degenerate, each with an energy cost of 16 kJ/mol.