Molecules with Several Chirality Centers and Meso Compounds
Some compounds with two or more chirality centers possess a special symmetry element that actually results in less than 2n stereoisomers than normally expected. The carbon atoms C2 and C3 of tartaric acid, for instance, are asymmetric carbons (chirality centers). Therefore, 22 = 4 different stereoisomers of tartaric acid could be expected. However, the perspective formulas of the four stereoisomers of tartaric acid with the configurations (S,S), (R,R), (S,R), and (R,S), that are to be expected, illustrate a specific characteristic of symmetry in the cases of (S,R) and (R,S).
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Try to discover the isomeric relationship between the four tartaric acid stereoisomers by manipulating the orientation of the interactive molecular models with your mouse or by using the "compare" and "put back" controls.
Molecules I, II, and III are different stereoisomers, but molecules III and IV are identical! Molecules I and II are a pair of enantiomers. Molecules III and IV have a mirror-image relationship. However, they are superimposable by rotating one of the interactive molecular models to 180° within the illustration plane (compare molecules III and IV). This specific characteristic may be explained by the presence of an internal mirror plane in molecules III and IV. Therefore, they are not enantiomeric but identical! The perspective formulas of III and IV represent the same compound. This compound is achiral, as it is superimposable on its mirror image.
- If a compound has a plane of symmetry, it will be achiral and not optically active, even though it contains several chirality centers. Such a compound is called meso compound.
As a result, tartaric acid consists of only three, and not four, stereoisomers: a pair of entantiomers, molecules I and II, and a meso compound that is diastereomeric to them.