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Despite what we’ve just said, we actually don’t observe perfectly free rotation in ethane. Experiments show that is a small (12kj/mol;2.9kcal/mol) barrier to rotation and that some conformers are more stable than others. The lowest-energy, most stable conformer is the one in which all six C-H bonds are as far away from one another as possible –staggered when viewed end-on in a Newman projection. The highest-energy, least stable conformer is the one in which the six C-H bonds are as close as possible—eclipsed in a Newman projection. At any given instant, about 99% of ethane molecules have an approximately staggered conformation and only about 1 % are near the eclipsed conformation.
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