Practice Isomer Relationships Study guide
Two molecules with one formula are structural isomers if their connectivity differs, stereoisomers if only the 3D arrangement differs (enantiomers when mirror images, diastereomers when not), or simply the same compound; the study guide maps the decision tree.
Classify the relationship between the two molecules below; they always share a molecular formula.
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.