What distinguishes enantiomers from diastereomers?

• A. Enantiomers are identical to their mirror images.
• B. Enantiomers are non-superimposable mirror images; diastereomers are non-mirror image stereoisomers.
• C. Enantiomers are a subset of diastereomers.
• D. Diastereomers are non-superimposable mirror images.

Answer: (B)

The key idea is chirality and how molecules relate to their mirror images. Enantiomers are non-superimposable mirror images, meaning you cannot perfectly align one with the other even if you rotate, and they have opposite configurations at all chiral centers. Diastereomers, on the other hand, are stereoisomers that are not mirror images of each other; they may share some chiral-center configurations and differ at others, and they usually have different physical properties.

So the distinguishing feature is whether the pair is a true mirror image that cannot be superimposed (enantiomers) or simply a different arrangement of atoms that is not a mirror image (diastereomers). This makes the given statement the best description: enantiomers are non-superimposable mirror images; diastereomers are non-mirror image stereoisomers.

A quick note on why the alternatives don’t fit: claiming enantiomers are identical to their mirror images contradicts the whole idea of chirality, and saying diastereomers are non-superimposable mirror images would describe enantiomers, not diastereomers.