Diastereomers

Diastereomers are a type of stereoisomer. They share the same molecular formula and connectivity as the corresponding enantiomeric pair but are not mirror images of each other. Diastereomerism arises when a molecule contains two or more stereocenters (or axes of chirality). Each stereocenter can have different configurations (R or S). Because not all configurations are inverted, some pairs of stereoisomers are non-mirror images of one another and are diastereomers.

In contrast to enantiomers, diastereomers typically have different physical properties such as melting points, boiling points, and solubilities. This makes diastereomers separable by ordinary physical methods, whereas enantiomers often require chiral environments or reagents to separate. Diastereomers can also differ in reactivity and spectroscopic behavior, and they may interact differently in biological contexts.

A classic example is 2,3-dichlorobutane, which exists as three stereoisomers: the enantiomeric pair RR and SS, and the meso form RS (which is identical to SR). RR and SS are mirror images, while RS is a diastereomer to both. Another well-known case is tartaric acid: the D- and L-tartaric acids are enantiomers, while the meso form is a diastereomer relative to them. The terms erythro and threo describe the relative arrangement of substituents in vicinal diols or 1,2-disubstituted compounds in Fischer projections.

Nomenclature often uses R/S designations for individual stereocenters, with the overall relationship determining whether a pair is enantiomeric or diastereomeric. Interconversion between diastereomers requires bond breaking and reforming, unlike simple rotation within a molecule.