Axial chirality

In some molecules with nonsuperimposable mirror images it is not possible to identify a chiral centre. The optical activity of several of these molecules is due to an axis of symmetry, thus we name it axial chirality. A few molecules are shown in the next figure:

The first molecule is a typical allene, the third one is a spirocyclic system. In the fourth and fifth examples the source of the axial chirality is the hindered rotation about the central C-C bond caused by the bulky substituents. In the case of biphenyls this is called atropisomerism.

The axis of axial chirality could be for example a C₂ axis of symmetry (rotation by 180°), depending on the substituents. Its recognition is not always straighforward for the unskilled eyes. In the next figures you can see that in the case of penta-2,3-diene there is a 45° angle between the C₂ axis and the planes defined by the atoms of the two ends of the molecule:

To determine the configuration of a molecule with axial chirality:

1) The four substituents are regarded to the vertex corners of a tetrahedron.
2) The tetrahedron is rotated that the axis of chirality starts from our viewpoint and is perdendicular to the plane of the paper:

3) When applying the CIP sequence rules, an additional rule is needed: the two near groups precede the two far groups.
4) Orient the molecule such that the lowest priority atom (substituent) is pointing away from the us and assign the configuration as described earlier (R or S):

Excercises

Determine the configuration of the compounds:

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