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Concept
Stereoisomerism
Résumé
In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space. This contrasts with structural isomers, which share the same molecular formula, but the bond connections or their order differs. By definition, molecules that are stereoisomers of each other represent the same structural isomer.
Chirality (chemistry) and Enantiomers
Enantiomers, also known as optical isomers, are two stereoisomers that are related to each other by a reflection: they are s of each other that are non-superposable. Human hands are a macroscopic analog of this. Every stereogenic center in one has the opposite configuration in the other. Two compounds that are enantiomers of each other have the same physical properties, except for the direction in which they rotate polarized light and how they interact with different optical isomers of other compounds. As a result, different enantiomers of a compound may have substantially different biological effects. Pure enantiomers also exhibit the phenomenon of optical activity and can be separated only with the use of a chiral agent. In nature, only one enantiomer of most chiral biological compounds, such as amino acids (except glycine, which is achiral), is present. An optically active compound shows two forms: D-(+) form and L-(−) form.
Diastereomer
Diastereomers are stereoisomers not related through a reflection operation. They are not mirror images of each other. These include meso compounds, cis–trans isomers, E-Z isomers, and non-enantiomeric optical isomers. Diastereomers seldom have the same physical properties. In the example shown below, the meso form of tartaric acid forms a diastereomeric pair with both levo- and dextro-tartaric acids, which form an enantiomeric pair.
The D- and L- labeling of the isomers above is not the same as the d- and l- labeling more commonly seen, explaining why these may appear reversed to those familiar with only the latter naming convention.
Source officielle
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