Diastéréoisomères

Diastéréoisomères (diastereoisomers) are stereoisomers that are not mirror images of each other. They arise when a molecule contains two or more stereogenic centers or restricted rotation about a bond, producing several distinct configurations. Among stereoisomers, enantiomers are non-superimposable mirror images; diastereoisomers are the remaining non-mirror-image relationships.

Diastereoisomers share some, but not all, stereochemical configurations. Because they are not related by symmetry as mirror images, they often exhibit different physical properties such as melting points, boiling points, solubilities, and densities, and they can display different chemical reactivities. Optical activity may differ between diastereomers, with some being chiral and others, such as meso forms, being achiral.

Interconversion between diastereoisomers generally requires breaking and reforming bonds, rather than simple rotation about a bond. As a result, diastereomers are typically separable by standard methods such as crystallization or chromatography due to their differing properties.

Common examples include 2,3-dibromobutane: the RR and SS forms are enantiomers, while the RS form is the meso diastereomer; RR and RS are diastereomers, as are SS and RS. A related concept is epimers, diastereomers that differ at only one stereocenter.

Diastereoisomerism is a central concept in stereochemistry, with implications for synthesis, crystallography, pharmacology, and materials science.