Cycloalkenes

A cycloalkene may be chiral even if it contains no asymmetric carbon. Such a cycloalkene enantiomer's chirality is the result of another chirality element, the chirality helix. (E)-cyclooctene is chiral due to its internal helical twist, which is caused by the strong ring strain of the planar form. The enantiomers of (E)-cyclooctene (enantiomer I and II in the table below) differ in their rotational sense of the helical shape. During the interconversion of these enantiomers by a conformational change, a strong ring strain is built in the planar form. As interconversion then takes place, the conformational change incurs a very high energy barrier (151 $\text{kJ}{\text{mol}}^{-1}$). This would explain some characteristic qualities of these enantiomers, such as their optical activity and their ability to be isolated at room temperature.

Animation

In addition, (E,Z) isomerism of the ring's double bond and cis,trans isomerism of the substituents are common in cycloalkenes.