Practice Isomer Relationships Study guide
Two molecules with the same formula are structural (constitutional) isomers if their atom connectivity differs. If their connectivity is the same but the 3D arrangement of atoms differs, then the structures are either conformational isomers or stereoisomers. If the structures can be made equivalent by bond rotations, then they are different conformational isomers of the same molecule. If they cannot be made equivalent by bond rotations, then the two molecules are stereoisomers. If the two structures are mirror images, then they are enantiomers. If they are not mirror images, then they are diastereomers. The study guide maps the decision tree.
The molecules below have the same molecular formula. Classify 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. Note that there are other sources of chirality that can be inverted (e.g. P/M helical chirality)
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.