Diastereomer

A diastereomer is a type of stereoisomer that is not a mirror image of another stereoisomer of the same compound. Like enantiomers, diastereomers have the same connectivity and the same atoms, but their spatial arrangement differs. However, unlike enantiomers, diastereomers are not related by reflection and therefore do not have opposite configurations at all stereocenters.

In molecules with two or more stereogenic centers, each center can be R or S. The set of possible configurations yields multiple stereoisomers. For example, 2,3-dichlorobutane has four stereoisomers: the enantiomeric pair (2R,3R) and (2S,3S), and a meso form (2R,3S). The relationships among these are: (2R,3R) and (2S,3S) are enantiomers; each of them is a diastereomer of the (2R,3S) meso form, and the two enantiomers are diastereomers of the meso form as well. In many molecules, diastereomers can also arise from cis/trans (geometric) isomerism around a double bond or ring.

Diastereomers generally have different physical properties, such as melting points, boiling points, and solubilities, and may show different reactivities. Because their properties differ, diastereomers are often separable by crystallization or chromatography.

Nomenclature uses the combinations of configurations at all stereocenters (e.g., 2R,3S). A diastereomer is any stereoisomer that is not an enantiomer of a given stereoisomer.

Applications include stereochemical control in synthesis and pharmaceuticals, where diastereomers can have different biological activity.