diastereoisomers

Diastereoisomers are stereoisomers that are not mirror images of one another. They arise when a molecule contains two or more stereogenic centers, and the configurations at some but not all of these centers differ. In other words, diastereomers are non-mirror-image stereoisomers, whereas enantiomers are non-superimposable mirror images.

In sets of stereoisomers with multiple stereocenters, enantiomeric pairs and diastereomeric members coexist. Enantiomers have identical physical properties in achiral environments and rotate plane-polarized light in opposite directions. Diastereomers, by contrast, typically exhibit different physical properties such as melting points, boiling points, and solubilities, and can often be separated by standard methods like chromatography or crystallization.

Common examples illustrate the concept. Tartaric acid has three stereoisomers: the enantiomeric pair D- and L-tartaric acid, and the meso form, which is a diastereomer relative to the enantiomers. Another example is 1,2-dichloro-1,2-dimethylcyclohexane, which exists as cis and trans diastereomers; each diastereomer may or may not be chiral depending on its substituent arrangement.

Notation and identification rely on assigning configurations at stereocenters (R or S) and comparing the complete set of configurations. Diastereoisomerism is a key concept in chemical synthesis, pharmacology, and crystallography because diastereomers often differ in biological activity and practical handling, even though they share the same molecular formula.