Stereoisomers of Cyclic Compounds
Cycloalkenes: Physical Properties
As is the case with open-chain alkenes, cycloalkenes exhibit cis,trans isomerism of the double bond. The unequivocal (E,Z) notation is the official notation when naming these types of isomers according to IUPAC rules. However, frequently cis,trans is being used. In order for the (E,Z) notation to be applied, the substituents at the double bond have to be sorted according to the CIP priority rules (or CIP sequencing rules). If the two substituents with the highest CIP priority are on the same side of the double bond, the isomer is called (Z) (or cis). If they are on opposite sides from each other, the isomer is called (E) (or trans).
Contrary to open-chain alkenes, cis cycloalkenes in general are more stable than their trans isomers. The trans double bond causes strong twisting of the ring. Because of the resulting high ring strain small trans cycloalkenes have not been observed and cis isomers show considerable ring strain. However, the latter are sufficiently stable in order to exist. To form a trans isomer the cycloalkene ring must contain at least eight carbons. The energy difference between cis- and trans-cyclooctene is approximately 38.5 . Eventually, trans isomers become more stable than cis isomers once the ring contains more than eleven carbons,
Interactive three-dimensional molecular models of cis- and trans-cyclooctenes illustrate the ring strain of cycloalkenes and the bond twist in trans cycloalkenes.
Trans-cyclooctene is an example of a compound that is chiral even though a chirality center is absent. The chirality of trans-cyclooctene is caused by a chirality helix, another chirality element. Trans-cyclooctene is chiral because of its internal helical twist caused by the strong ring strain of the planar form. The enantiomers of trans-cyclooctene are conformers that do not interconvert. This is largely due to the fact that the conformational change generates a substantially high energy barrier (151 ) to prevent interconversion. This, in turn, is the result of the strong ring strain of the planar form that must be passed through while interconversion occurs. Therefore, racemization is not observed.
The ring strain of cycloalkenes and the high energy barrier of the interconversion of trans-cyclooctene enantiomers can be observed during the generation and manipulation of molecular models.