I conclude my exploration of conformational preferences by taking a look at esters. As before, I start with a search definition, the ester being restricted to one bearing only sp ³ carbon centers. The result of such a search is pretty clear-cut; they all exist in just one conformation, the s-cis , in which a lone pair of electrons on the alkyl-oxygen is aligned quite precisely anti-periplanar with the axis of the C=O bond.