diastereomer

Suppose a molecule has two separate spots where handedness can be chosen, like a glove that can be sized at both the wrist and the fingers. Some combinations of those choices give mirror images of each other; others give shapes that are clearly different but not mirror images at all. Those non-mirror-image stereoisomers are diastereomers.

Formally, diastereomers are stereoisomers that are not enantiomers — that is, they have the same connectivity but a different spatial arrangement, and they are not mirror images of one another. In a molecule with two stereocenters, the (R,R) and (S,S) forms are enantiomers (every center flipped), and so are (R,S) and (S,R); but (R,R) compared with (R,S) is a diastereomer pair, because only one center differs. Cis and trans alkene isomers are diastereomers too, as are cis and trans ring substituents. The defining feature is that they differ at some but not all stereocenters, leaving no overall mirror relationship.

Unlike enantiomers, diastereomers have genuinely different physical properties — different melting points, boiling points, solubilities, and even different chemical reactivity — because they are differently shaped molecules, not mirror twins. This is wonderfully useful: it means diastereomers can be separated by ordinary methods like distillation, crystallization, or chromatography, and it is the trick chemists exploit to resolve enantiomers by temporarily turning them into diastereomers.