Molecular Level -- Stereoisomerism

Many compounds are incapable of showing optical activity, and are called achiral. They usually have a "mirror plane of symmetry" (a plane passing through the molecule making each side of the plane the mirror image of the other. For example, chlorofluoromethane (right) has such a plane defined by the C, F, and Cl. One result of the mirror plane is that the mirror-image of this compound is superimposable on the original.

However, mirror images that are not superimposable are called enantiomers. Compounds (or portions thereof) not superimposable on their mirror image are called chiral. The most common example of chiral molecule: carbon bearing 4 different substituents:

•Not superimposable

•Chiral (each compound)

•The carbon is called "the chiral center" or stereogenic center.

We can define the shape here. Called configuration or absolute configuration, it is written as "D", or "L, or as "R" or "S". Note that this is a human definition, not that of the molecules. The former configuration designation (D/L) is based on arcane transformations to a standard compound. It is only used by biologists.

The latter (R/S) is based on sequence rules that are the same as those for E/Z in double bonds.

a) Put the lowest priority towards the back (by rotating, not remaking, the molecule)

b) Look at the direction of highest to lowest: if clockwise, then R, else, it's S.

What if there is more than one chiral center? The molecule is usually chiral, and the possibility for new isomers has expanded. The number of stereoisomers in a molecule with N chiral centers is usually 2N (that is, with 2 centers, there can be 4 stereoisomers; with 4 centers, 16 stereoisomers, and so forth). This introduces stereoisomers that are not mirror images, called diastereomers.

If, by coincidence, the molecule has a plane of symmetry, it is, of course, not chiral. Molecules with chiral centers which are achiral by reason of having a plane of symmetry are called meso compounds.