In organic chemistry, isomerism is mainly divided into two broad types:
1. Structural (or Constitutional) Isomerism
This happens when compounds have the same molecular formula but different structures (different connectivity of atoms).
Types include:
- Chain isomerism → different arrangements of the carbon chain (straight vs branched).
- Position isomerism → same chain, but a functional group or substituent is at different positions.
- Functional group isomerism → same formula, but atoms form different functional groups (e.g., alcohols and ethers).
- Metamerism → due to different alkyl groups attached to the same functional group (common in ethers, amines, esters).
- Tautomerism → dynamic equilibrium between two structures differing in the position of a proton and a double bond (e.g., keto–enol forms).
2. Stereoisomerism
Here the connectivity is the same, but the spatial arrangement of atoms differs.
Types include:
- Geometrical (cis–trans or E–Z) isomerism → occurs due to restricted rotation around a double bond or in cyclic structures.
- Optical isomerism → compounds that are mirror images of each other but not superimposable (chiral molecules, enantiomers).
In short:
- Structural isomerism = different connections of atoms.
- Stereoisomerism = same connections, different 3D arrangement.