Perpendicular disymmetric planes

Restricted Rotation Giving Rise to Perpendicular Disymmetric Planes. Certain compounds that do not contain asymmetric atoms are nevertheless chiral because they contain a structure that can be schematically represented as in Figure 4.2. For these compounds, we can draw two perpendicular planes neither of which can be bisected by a plane of symmetry. If either plane could be so bisected, the molecule would be superimposable on its mirror image, since such a plane would be a plane of symmetry. These points will be illustrated by examples. 

Other chiral geometries are also possible, e g. those that possess perpendicular disymmetric planes. Optical activity will occur if there are two rings that contain an even number of atoms joined in a spiro manner, with each ring substituted at the carbon furthest from the spiro junction. In this case, an elongated tetra-hedron is formed. 

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