By Anne Mullenniex – Updated August 30, 2022
Isomers share a molecular formula but differ in structure or spatial arrangement. While many texts mention three broad categories, chemists actually divide them into two main groups—constitutional (structural) isomers and stereoisomers—with several subtypes within each.
Constitutional isomers have identical atoms but distinct bonding patterns. The connectivity creates different functional groups or skeletons. For example, n‑butane (a straight‑chain C4H10) and isobutane (a branched C4H10) share the same formula yet exhibit different chain arrangements.
Stereoisomers share the same atoms and bonds but differ in three‑dimensional orientation. Geometric (cis/trans) isomers are a subset of configurational stereoisomers, arising when rotation around a bond is restricted.
Double bonds or cyclic structures prevent free rotation, giving rise to geometric isomers. These display distinct cis–trans relationships across the restricted bond, with functional groups positioned on the same or opposite sides.
A tetrahedral carbon bonded to four different groups is a stereogenic center. Molecules with such centers fall under optical isomers, which can be classified as enantiomers—non‑superimposable mirror images—or diastereomers—non‑superimposable, non‑mirror images.
When modeling isomers, remember that if you can rotate a single bond to align two structures, they are not distinct isomers. Enantiomers and diastereomers can involve one or multiple chiral centers; the key difference is whether they are mirror images of each other.
