Isomers are different compounds that share the same molecular formula but differ in how their atoms are arranged. This matters because small changes in arrangement can cause large changes in boiling point, reactivity, odor, taste, or biological activity. A decision tree helps you classify isomers by first asking whether the atoms are connected in the same order.
From there, you can decide whether the difference is in connectivity or in three-dimensional shape.
Constitutional isomers have different atom-to-atom connections, so their structures can often be distinguished by tracing bonds. Stereoisomers have the same connectivity but different spatial arrangement, so wedges, dashes, double-bond geometry, and chiral centers become important. Enantiomers are stereoisomers that are non-superimposable mirror images, while diastereomers are stereoisomers that are not mirror images.
This classification is especially important in organic chemistry, medicine, and biochemistry because molecular shape controls how molecules fit into enzymes, receptors, and catalysts.
Key Facts
- Isomers have the same molecular formula but different structures.
- Constitutional isomers differ in connectivity, meaning the atoms are bonded in a different order.
- Stereoisomers have the same connectivity but different three-dimensional arrangement.
- Enantiomers are non-superimposable mirror images of each other.
- Diastereomers are stereoisomers that are not mirror images.
- For a molecule with n independent chiral centers and no internal symmetry, maximum stereoisomers = 2^n.
Vocabulary
- Isomer
- An isomer is one of two or more compounds with the same molecular formula but different atom arrangement.
- Constitutional isomer
- A constitutional isomer has the same molecular formula as another compound but a different pattern of bonds between atoms.
- Stereoisomer
- A stereoisomer has the same molecular formula and connectivity as another compound but a different three-dimensional arrangement.
- Enantiomer
- An enantiomer is one of a pair of stereoisomers that are non-superimposable mirror images.
- Diastereomer
- A diastereomer is a stereoisomer that is not the mirror image of another stereoisomer.
Common Mistakes to Avoid
- Calling all isomers stereoisomers. This is wrong because constitutional isomers have different connectivity, while stereoisomers must have the same connectivity.
- Ignoring atom connectivity before checking 3D shape. This leads to misclassification because the first decision should be whether the bonding pattern is identical.
- Assuming mirror images are always enantiomers. This is wrong because mirror images that can be superimposed are the same molecule, not enantiomers.
- Treating every molecule with chiral centers as having exactly 2^n stereoisomers. This can be wrong when symmetry creates meso compounds or when chiral centers are not independent.
Practice Questions
- 1 Two compounds both have formula C4H10, but one is CH3CH2CH2CH3 and the other is (CH3)3CH. Are they constitutional isomers or stereoisomers?
- 2 A molecule has 3 independent chiral centers and no internal plane of symmetry. What is the maximum number of stereoisomers possible?
- 3 Two molecules have the same connectivity and differ at one chiral center but are not mirror images of each other. Classify the relationship and explain the key evidence.