staggered and eclipsed conformation

Picture ethane (two carbons, six hydrogens) and look along its C-C bond as a Newman projection. As one carbon rotates relative to the other, the molecule passes between two extreme arrangements. In the staggered conformation the front and back C-H bonds are offset by sixty degrees, each back bond nestled into the gap between two front bonds. In the eclipsed conformation they line up directly, every front bond hiding a back bond behind it.

These two are not equal. The eclipsed form is higher in energy — for ethane, about 12 kilojoules per mole higher — and the staggered form sits in the energy valley. The cost of eclipsing is torsional strain: when bonding electron pairs are forced to line up, their slight repulsion (plus the way overlapping bonds resist) raises the energy. Spread over three eclipsing pairs in ethane, that is roughly 4 kJ/mol per eclipsed H-H pair.

Because the staggered form is the comfortable one, molecules spend most of their time there and only briefly visit the eclipsed peaks as they tumble between staggered minima. The barrier is small enough that rotation is essentially free at room temperature, but it is real, and it is the simplest example of the universal principle that molecules avoid eclipsing. Every later conformational story — butane's anti and gauche, the cyclohexane chair — is this same staggered-good, eclipsed-bad idea made richer.