Diastereomers are defined as?

• A. Stereoisomers that are mirror images
• B. Stereoisomers that are not mirror images
• C. Constitutional isomers
• D. Same molecule

Answer: (B)

Stereoisomers come in two main flavors: enantiomers and diastereomers. Enantiomers are pairs that are mirror images and non-superimposable. Diastereomers, on the other hand, are stereoisomers that are not mirror images of each other. This distinction matters because diastereomers can differ in configuration at one or more stereocenters while not being the exact opposite (not mirror images) of another form.

So the best description is that diastereomers are stereoisomers that are not mirror images. In a molecule with multiple stereocenters, you can have enantiomeric pairs (mirror images) and diastereomeric forms (not mirror images to each other). For example, with two stereocenters, RR and SS are enantiomers, while RR and RS are diastereomers. The other options describe enantiomers (mirror images), constitutional isomers (different connectivity), or the same molecule (identical), which do not define diastereomers.