Enantiomers have what relationship to their stereocenters?

• A. Enantiomers have identical configurations at all stereocenters
• B. Enantiomers have opposite configurations at all stereocenters
• C. Enantiomers have random configurations
• D. Enantiomers differ only in connectivity

Answer: (B)

Enantiomers are non-superimposable mirror images. For a molecule with stereocenters, each stereocenter can have the R or S arrangement. To create the enantiomer, every stereocenter is inverted, so the enantiomer has the opposite configuration at all stereocenters compared to the original. If only some centers were inverted, you’d get a diastereomer, not the enantiomer. The connectivity of the atoms stays the same; only the spatial arrangement changes. (A caveat: certain molecules with internal symmetry can be meso and not form a pair of enantiomers, but in general enantiomers are opposite at all stereocenters.)