Optical isomerism 2.
Enantiomer, diastereomer, epimer, meso
If there are two or more stereocentres (chiral centres) in a molecule, each one doubles the number of possible optical isomers. Determine which of the isomers are mirror images of each other?
Diastereomers are not mirror images of each other and non-superimposable. They must have at least one identical and one different stereocentres.
Diastereomers that contain more than one chiral center but differ from each other in the configuration at only one chiral center are called epimers. The carbohydrates D-glucose and D-mannose are good examples of epimer compounds:
A special case when a molecule has two identical chiral centres. Determine which of the molecules are enantiomers, which are mirror images and which can be superimposed?
In the meso compounds there is an element of symmetry between two chiral centres, in this example a centre of inversion. If the two halves of the molecule are rotated into eclipsed conformation, the molecule is bisected by an internal plane of symmetry:
Therefore the molecule can be superimposed with its mirror image and is optically not active.
Excercise
Of the four possible optical isomers of tartaric acid which are the meso compounds?
A B C D