### Isomerism - **Definition:** Compounds with the same molecular formula but different structural or spatial arrangements of atoms. #### Structural Isomers (Constitutional Isomers) - Different connectivity of atoms. - **Chain Isomers:** Different carbon chain arrangements (e.g., n-butane vs. isobutane). - **Positional Isomers:** Same carbon skeleton, different position of functional group or substituent (e.g., 1-propanol vs. 2-propanol). - **Functional Group Isomers:** Different functional groups (e.g., ethanol vs. dimethyl ether). #### Stereoisomers - Same connectivity, different spatial arrangement. - **Enantiomers:** Non-superimposable mirror images. Have chiral centers. Rotate plane-polarized light in opposite directions (optically active). - **Diastereomers:** Stereoisomers that are not mirror images. - **Cis-Trans Isomers (Geometric Isomers):** Restricted rotation (e.g., double bonds, rings). Cis (same side), Trans (opposite sides). - **Meso Compounds:** Contain chiral centers but are achiral due to an internal plane of symmetry. - **Conformational Isomers (Conformers):** Can be interconverted by rotation around single bonds (e.g., staggered vs. eclipsed ethane, chair vs. boat cyclohexane).
