Diastereomerie

Diastereomerie (diastereomerism) is a concept in stereochemistry describing stereoisomers that are not mirror images of each other. It contrasts with enantiomers, which are non-superimposable mirror images. Diastereomers arise when a molecule contains two or more stereogenic centers or other stereochemical elements. Among such molecules, some stereoisomers are enantiomeric pairs, while others are not mirror images and are diastereomeric pairs.

In a molecule with two stereocenters, for example, the configurations R,R and S,S are enantiomers, as are the configurations R,S and S,R. The cross-pairs (R,R) vs (R,S) are diastereomers. Geometric isomers at rings or about double bonds, such as cis and trans forms, are also considered diastereomeric relative to other stereoisomeric forms.

Diastereomers generally exhibit different physical properties, including melting points, boiling points, and solubilities, enabling their separation by standard purification methods. They can have different optical activities; some diastereomers are chiral and rotate plane-polarized light, while others are meso or achiral.

Interconversion between diastereomers typically requires breaking and reforming bonds (epimerization or other chemical transformations), not mere rotation about single bonds. The concept is important in synthesis, structure determination, and pharmacology, where the diastereomeric form of a compound can influence reactivity, properties, and biological activity.