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Narrative
Free rotation about the bond between carbons 2 and 3 of butane reveals steric crowding when the methyl groups are eclipsed. The energy decreases as the methyl groups move farther apart. The anti-conformation of butane, the lowest energy arrangement is formed when the methyl groups are opposite each other. This may be shown in a Newman projection. If the model is twisted (120°) a higher energy staggered conformation called the gauche form is obtained. The Newman projection of the gauche form of butane is illustrated.

Discussion

Ball-and-stick and Space-filling Models of Butane

Energy versus Angle of Rotation for Butane