Diastereomer
Diastereomers (or diastereoisomers) are stereoisomers that are not enantiomers (mirror images of each other). Diastereomers can have different physical properties and different reactivity.
In simple terms two stereoisomers are said to be diastereoisomers of each other if only one stereogenic centre differs between the two stereoisomers. According to this same definition, geometric isomers (eg E- and Z-, and cis- and trans-) are also diastereoisomers.
If a molecule contains a single asymmetric carbon atom or stereocenter, it will have two mirror image forms. If a molecule contains two asymmetric carbons, there are up to 4 possible configurations, and they cannot all be mirror images of each other. The possibilities continue to multiply as there are more asymmetric centers in a molecule.
Tartaric acid contains two asymmetric centers, but two of the "isomers" are equivalent and are called a meso compound. This configuration is not optically active, while the remaining two isomers are D- and L- mirror images, i.e. enantiomers. The meso form is a diastereomer of the other forms.
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(natural) tartaric acid |
D-(-)-tartaric acid |
mesotartaric acid |
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The families of 4, 5 and 6 carbon carbohydrates contain many diastereomers because of the large numbers of asymmetric centres in these molecules. Two common prefixes used to distinguish diastereomers are threo and erythro. When drawn in the Fischer projection the erythro isomer has two identical substituents on the same side and the threo isomer has them on opposite sites. Cis-trans isomerism and conformational isomerism are also forms of diastereomerism.
Diastereoselectivity is the preference for the formation of one or more than one diastereomer over the other in an organic reaction.
Main article: Stereochemistry