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(mirror plane), no , 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.
|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.