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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. |
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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:
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•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.
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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. |
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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.
