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Alkanes: Introduction and Chemical Bond

Alkanes: Conformation of Butane

Energy profiles get more complicated when H atoms are replaced with other atoms, thereby destroying the symmetry of the molecule (e.g. in butane, H3C-CH2-CH2-CH3). A conformational energy diagram plotting the torsional angle against the energy results in the following profile:

Energy of butane as a function of rotation around the central C-C bond

Two different eclipsed conformations and two staggered ones can be distinguished. The conformations are called rotamers, as they can be interconverted by rotation.

Conformations of butane
C-C-C-C Dihedral angleRotamer
fully eclipsed
60° gauche
120°partly eclipsed
180° anti
240°partly eclipsed
300° gauche
360°fully eclipsed

The energy profile is symmetrical with a mirror plane at the dihedral angle of 180°. The most stable conformation of butane is the one in which the two terminal methyl groups are the farthest removed from each other, i.e. the anti conformation. Somewhat less favorable is the gauche conformation in which the methyl groups assume a dihedral angle of 60°. The fully eclipsed conformation is unpopulated and separates the two gauche conformations at 60° and 300°. The two partly eclipsed conformations form the barriers between the gauche and anti conformations.

Although the rotational barriers in the eclipsed conformations of butane are higher than the ones in ethane, butane too is an entirely flexible molecule at room temperature.

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