Which statement defines a diastereomer?

Which statement defines a diastereomer?

• A. Mirror-image stereoisomers
• B. Stereoisomers that are not mirror images
• C. Stereoisomers that are not mirror images and differ at some but not all stereocenters
• D. Constitutional isomers

Answer: (C)

When a molecule has two or more stereocenters, multiple stereoisomers can exist. The key distinction is between mirror images and non‑mirror images. If two stereoisomers are mirror images and cannot be superimposed, they are enantiomers. If they are not mirror images of each other, they are diastereomers. The defining feature of diastereomers is that they differ at some but not all stereocenters. For example, with two stereocenters, changing just one center changes the configuration at only that center, giving a diastereomer rather than a mirror image. This contrasts with enantiomers, which differ at all stereocenters (in the two-center case, both inverted). Constitutional isomers, on the other hand, have different connectivity, not just different stereochemistry. So the statement that best defines a diastereomer is that they are stereoisomers that are not mirror images and differ at some but not all stereocenters.

When a molecule has two or more stereocenters, multiple stereoisomers can exist. The key distinction is between mirror images and non‑mirror images. If two stereoisomers are mirror images and cannot be superimposed, they are enantiomers. If they are not mirror images of each other, they are diastereomers. The defining feature of diastereomers is that they differ at some but not all stereocenters. For example, with two stereocenters, changing just one center changes the configuration at only that center, giving a diastereomer rather than a mirror image. This contrasts with enantiomers, which differ at all stereocenters (in the two-center case, both inverted). Constitutional isomers, on the other hand, have different connectivity, not just different stereochemistry. So the statement that best defines a diastereomer is that they are stereoisomers that are not mirror images and differ at some but not all stereocenters.