Additional Chirality Elements
Chirality does not result only from a chirality center or chirality axis, but may also arise from another chirality element, namely a chirality plane. A chirality plane is the plane of a structural fragment in a chiral molecule that cannot lie in a symmetry plane because of restricted rotation or structural requirements. The enantiomers of such a chiral molecule differ in the spatial arrangement of the remaining atoms of the molecule with respect to the chirality plane.
In (E)-cyclooctene, for instance, the chirality plane is spread out by the double bond and its two vicinal carbon atoms. The (E)-cyclooctene enantiomers I and II differ in their rotational sense of the internally twisted carbon chain that is located, as depicted in the illustration below, in front of the chirality plane.
Furthermore, chirality planes may be found in paracyclophanes. Paracyclophanes consist of a parasubstituted aromatic ring whose substituents are bound together forming an aliphatic bridge above the plane of the ring. If the bridge is small enough, or if the aromatic ring carries an additional third substituent, the rotation of the aromatic ring through the aliphatic ring may be restricted. In this case, the paracyclophane is optically active because the enantiomers cannot rapidly interconvert. Due to their handle-like aliphatic ring above the plane of the aromatic ring, cyclophanes are also called ansa compounds (from the Latin ansa, handle).
13-Bromo-1,10-dioxaparacyclophane is an example of a chiral and optically active ansa compound. The chirality plane lies inside the plane of the aromatic ring. The rotation of the aromatic ring is restricted by steric interactions between the aliphatic bridge and the bromine at the aromatic ring.