Practice Isomer Relationships Study guide
The two molecules below always share a molecular formula. What is their relationship?
Molecule A
Molecule B
Common Questions
How do you determine the relationship between two molecules with the same formula?
First compare connectivity: if the atoms are bonded differently, they are structural (constitutional) isomers. If the connectivity is the same, assign R/S and E/Z at every stereocenter: all the same means identical, all opposite means enantiomers, and a mix means diastereomers.
What are structural isomers?
Structural (constitutional) isomers share a molecular formula but connect their atoms differently — for example, acetone and methyl vinyl ether are both C3H6O.
What is the difference between enantiomers and diastereomers?
Enantiomers are non-superimposable mirror images — every stereocenter is inverted. Diastereomers are stereoisomers that are not mirror images — at least one stereocenter is the same and at least one is different. E/Z (cis/trans) isomers are a subclass of diastereomers.
Can mirror images be the same compound?
Yes — a meso compound contains stereocenters yet has an internal mirror plane, so its mirror image is superimposable on the original: they are identical, not enantiomers.