The complete 3D shape of a molecule cannot be specified completely by its bond lengths and bond angles. We also have to look at the conformations, or degrees of rotation, about single bonds. The barrier to rotation about a single bond is typically very low. Hence it is impossible to isolate a single conformation at room temperature.
Ethane has two note-worthy conformations. In the eclipsed form, the C-H bonds are parallel to each other and give rise to steric hindrance. The staggered form is more stable because the C-H bonds are rotated as far apart as possible at 60 degrees. This type of torsional strain means that some conformations are favored more than others.
Butane has two varieties of staggered conformations, gauche and anti. The gauche conformer is less stable because of steric hindrance between adjacent methyl substituents. In addition some higher alkanes exhibit other unique interactions: pentane and higher alkanes can experience syn interactions that occur across 5 carbon atoms.
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