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By starting with simple alkanes we can prepare ourselves for the more complex world that lies beyond.....

Methane: CH₄

Although sigma bonds are able to rotate about the internuclear axis, the spherical symmetry of H atoms means that methane has only a single unique conformation.

Ethane: CH₃-CH₃

Rotation about the C-C bond in ethane (see animation) produces different conformations. Although an infinite number of conformations are possible, the staggered conformation and the eclipsed conformation which represent the most and least stable respectively are the most important. These can be seen by rotating the ethane molecule to view it directly down the C-C bond, as in a Newman projection.
STAGGERED Look at how the each H-C-H bond angle is bisected by a C-H bond on the adjacent C atom. This is the most stable conformation for ethane since the torsional strain is minimized.
ECLIPSED Think like an astronomer....each of the C-H bonds is aligned with a C-H bond on the adjacent C atom so that the H attached to the front C obscures (eclipses) those on the rear C

Propane: CH₃-CH₂-CH₃

Although there are 2 C-C bonds in propane, they are equivalent and rotation produces conformations that are similar to those of ethane except that the "extra" methyl group is interacting with the H atoms. These can be seen by rotating the propane molecule to view directly down the C-C bond, similar to a Newman projection.

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