Stereoisomers of Cyclic Compounds
The ring with the most stable cycloalkane conformation is usually not planar. Most of the time, however, the ring skeleton shows a high conformational flexibility; that is, the energy barrier between the different stable or metastable conformations is low enough for them to be able to interconvert.
For instance, cyclohexane has an almost strainless and therefore considerably stable conformation, called chair conformation.
In the chair conformation, common to cyclohexane and cyclohexane derivatives, two different types of C-substituent bonds exist (in cyclohexane all substiutents are H). The six bonds parallel to the C3 axis of the ring are called "axial" (Abb. 1) . On the other hand, the six bonds along the "equator" of the ring are called "equatorial" (Abb. 1) . Each equatorial bond is parallel to two ring bonds. (Put back bonds to the initial state). During the interconversion of the two chair conformations (called ring flip), a cyclohexane ring may assume various intermediate conformations, such as the metastable twist boat (or just twist) conformation, or the comparatively less stable boat conformation. As a result of the ring flip, the axial and equatorial positions are exchanged. Bonds (or substituents) that possess an axial position in one chair conformer, then have an equatorial position in the other and vice versa.