Cis and Trans Isomers of Cycloalkenes

Abstract

As a rule, a trans disubstituted alkene is more stable than the corresponding cis isomer. For cycloalkenes of fewer than eleven members, cis isomers are more stable than their trans counterparts. Although this exception to the normal rule is occasionally noted in beginning organic chemistry textbooks, it is often done without a careful analysis of the reasons behind it. The purpose of this article is to provide that analysis. In order for a cycloalkene to accommodate a trans double bond one or more of the following nonideal geometries must occur: a twisted π bond; pyramidal sp2-carbon atoms; nonideal sp3 bond angles; or longer than normal C–C single and double bonds. This article provides a list of experimentally determined relative energies of the cis and trans isomers within the series cycloheptene–cycloundecene, along with computationally derived energies at several levels of theory. It also examines the geometric distortions through which cycloalkenes relieve the strain introduced by a trans double bond.