diastereomeer

A diastereomer is a type of stereoisomer that is not the mirror image of another arrangement. It occurs when a molecule has two or more stereogenic centers, allowing several possible configurations. Diastereomers are distinct from enantiomers, which are non-superimposable mirror images of each other.

In molecules with multiple stereocenters, the total number of stereoisomers can be large. Enantiomeric pairs are mirror-image relationships, while diastereomeric relationships are any non-mirror-image pair. Thus, not all pairs of stereoisomers are enantiomeric; many pairs are diastereomeric. Diastereomers often have different physical properties, such as melting points and solubilities, and can show different reactivity patterns.

Examples help illustrate the concept. For 2,3-dichlorobutane, the RR and SS configurations are enantiomers, while the RS (meso) form is achiral and a diastereomer to RR and SS. In ring systems or molecules with cis/trans relationships, cis and trans forms are often diastereomers relative to each other when more than one stereocenter is present.

Significance and applications: diastereomers are important in organic synthesis and medicinal chemistry because they can behave very differently in biological systems and chemical reactions. Controlling diastereoselectivity—the preference for forming one diastereomer over others—is a central goal in stereoselective synthesis.