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Chirality and Symmetry Elements

Nonsuperimposable molecules that have a mirror-image relationship are called enantiomers (from the Greek enantio, opposite and meros, part).

Enantiomers are always chiral molecules (see: Definition of Chirality). They always occur in pairs because each object has exactly one mirror image, not more or less.

How can one ascertain whether or not a molecule is chiral?

A time-consuming method is to build models of the molecule and its mirror image. If they can be superimposed by trial and error, they are not chiral, if not, they are chiral. A more scientific method without building models is to analyse the symmetry elements of the molecule. Chiral molecules show no symmetry plane (mirror plane), no symmetry center, and no rotation-reflection axis. If a molecule shows one of these symmetry elements, it is not chiral because it is superimposable with its mirror image.

Example of a symmetry plane (mirror plane)

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Example of a symmetry center

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This molecule has a mirror plane. It is achiral. This molecule is chiral. The plane shown is not a mirror plane.
In this molecule the left side of the plane shown is the mirror image of the right side. In this molecule no symmetry plane or center can be found.
If an sp3 carbon atom carries four different substituents, it is a so-called chirality center or asymmetric carbon. A molecule with one chirality center is chiral. When a molecule contains more than one chirality center it can be chiral or not. We shall see why later.

Chirality is subdivided into different types with the following characteristic chirality elements:

  • Chirality center.
  • Chirality axis.
  • Chirality plane.
Chirality center

An asymmetric carbon is an example of a chirality center. It is the most frequent chirality element in organic molecules.

Helical molecule - hexahelicene

Spiral staircases, snail shells and screws are well-known examples of helical objects with a chirality axis, which also occurs in the helical molecule hexahelicene.

Chirality axis

Disubstituted allenes are examples of molecules with a chirality axis.

Chirality plane

The plane of a structural fragment in a chiral molecule is called a chirality plane if it cannot lie in a symmetry plane because of restricted rotation or structural requirements. The plane of the benzene ring in the chiral cyclophane (an ansa-compound) cannot lie in a symmetry plane. It is a chirality plane.


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