1. Constitutional isomers have different connectivity
Constitutional (structural) isomers share a molecular formula but bond their atoms together in a different order.
2. Stereoisomers share connectivity but differ in space
Stereoisomers connect the same atoms in the same order yet arrange them differently in three dimensions.
3. Enantiomers are non-superimposable mirror images
Enantiomers are stereoisomers related as left and right hands — mirror images that cannot be overlaid.
4. Diastereomers are stereoisomers that are not mirror images
Diastereomers differ in 3-D arrangement without being mirror images — including cis/trans and E/Z alkenes.
5. Summary
Same formula · different connectivity = constitutional · same connectivity + 3-D difference = stereoisomers · mirror image = enantiomers · not a mirror image = diastereomers.
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Constitutional (structural) isomers — same molecular formula, different atom connectivity.
Enantiomers — a pair of stereoisomers that are mirror images of each other.
Diastereomers — they are stereoisomers, but they are not mirror images of one another.
Connectivity: stereoisomers keep the same atom-to-atom bonding and differ only in 3-D arrangement, while constitutional isomers bond the atoms in a different order.
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