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20-September-2008 09:55:59 - Diastereomer Redirected from Diastereoisomer Erythro redirects here. For the fictional planet, see Erythro Asimov. Diastereomers or diastereoisomers are stereoisomers that are not enantiomers nonsuperimposable mirror images of each other. Diastereomers can have different physical properties and different reactivity. In another definition diastereomers are pairs of isomers that have opposite configurations at one or more of the chiral centers but are not mirror images of each other 1. In simpler terms two stereoisomers are said to be diastereoisomers if they are not mirror images of each other and one or more stereogenic centres differ between the two stereoisomers. According to this same definition, cis-trans isomerism is a form of diastereomerism. 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. In general, the number of diastereomers of a molecule can be determined by calculating 2n, where n=the number of chiral centers in the molecule. This holds true except in cases where the molecule has meso forms. Tartaric acid contains two asymmetric centers, but two of the isomers are equivalent and together 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. Image:L-tartaric acid.png Image:D-tartaric acid.png Image:DL-tartaric acid.png natural tartaric acid L-+-tartaric acid dextrotartaric acid D---tartaric acid levotartaric acid mesotartaric acid 1:1 DL-tartaric acid racemic acid 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. Applications As stated, two enantiomers will have identical physical properties, while diastereomers will not. This knowledge is harnessed in chiral synthesis to separate a mixture of enantiomers. This is the principle behind chiral resolution. After preparing the diastereomers, they are separated by chromatography or recrystallization. References ^ Garrett, Grisham Biochemistry 2nd ed., 1999, p. 213 v d e Concepts in asymmetric synthesis Nomenclature Chirality, Stereocenter, Stereoisomer, Enantiomer, Diastereomer, Meso compound, Planar chirality, Chiral ligand, Axial chirality Analysis Optical rotation, Enantiomeric excess, Diastereomeric excess, Chiral derivitizing agents Chiral resolution Crystallization, Kinetic resolution, Chiral column chromatography Reactions Asymmetric induction, Chiral pool synthesis, Chiral auxiliaries, Asymmetric catalytic reduction, Asymmetric catalytic oxidation, Organocatalysis, Biocatalysis Retrieved from http://en..org/wiki/Diastereomer Categories: Stereochemistry Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages العربية Deutsch Español Français Italiano Bahasa Melayu Nederlands 日本語 Polski СрпÑ?ки / Srpski Suomi Svenska This page was last modified on 9 May 2008, at 05:31
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