3.7 : Conformations du Butane

In butane, rotations about the central carbon-carbon bond generate different conformations of the molecule. The conformers, when viewed along the C2–C3 bond, show the various Newman projections.

Energy changes that result from the C2–C3 bond rotations generate several staggered and eclipsed conformations of butane, with varying dihedral angles between the two methyl groups.

The staggered form of butane — with a dihedral angle of 180° — has the lowest energy and is the most stable form. It is called the anti conformation.

The increased stability of anti butane is due to the reduced steric strain, owing to the large spatial separation between the two methyl groups.

In the other two staggered forms, the methyl groups are separated by dihedral angles of ± 60°. These are called gauche conformations.

Since the two bulky methyl groups are relatively closer, their electron clouds strongly repel each other. This steric interaction, called a gauche interaction, increases the energy of the gauche conformer by 3.8 kJ/mol. Hence, gauche butane is less stable than anti butane.

The two gauche conformations are degenerate and mirror images of each other.

Among the three eclipsed conformers, the totally eclipsed form has a dihedral angle of 0° between the two methyl groups.

Because the methyl groups are very close in this conformation, their hydrogens experience strong van der Waals repulsions.

The resulting steric strain, along with the torsional strain, makes the totally eclipsed form of butane the least stable conformer with a high energy cost of 19 kJ/mol.

The other two eclipsed forms with dihedral angles of 120° and 240° have similar energies, and one form is a reflection of the other form.

Due to the reduced steric strain in these forms, the energy of the eclipsed conformer decreases to 16 kJ/mol.