isomeri

Isomerism refers to the phenomenon whereby two or more chemical compounds have the same molecular formula but differ in the arrangement of atoms, leading to distinct properties. The term derives from Greek roots meaning equal parts.

Isomers are classified by how the atoms are arranged. Structural (constitutional) isomers have the same formula but different connectivity between atoms, resulting in different skeletal frameworks. Examples include the C4H10 pair n‑butane and isobutane (2‑methylpropane). Stereoisomers have the same connectivity but differ in spatial arrangement.

Stereoisomerism splits into enantiomers and diastereomers. Enantiomers are non-superimposable mirror images, typically arising from chiral centers, and often exhibit opposite behavior in biological systems. Diastereomers are not mirror images and can have multiple stereocenters. Within diastereomers, geometric isomers such as cis/trans (or E/Z) arise when restricted rotation around a bond creates different spatial arrangements, as seen in cis‑2‑butene versus trans‑2‑butene.

Common examples illustrate the concept: optical isomerism is demonstrated by lactic acid, which exists as two enantiomers with different optical activity; geometric isomerism is observed in alkenes like cis‑ and trans‑2‑butene.

Isomerism has practical significance because isomers can differ markedly in physical properties (melting and boiling points, solubility) and chemical reactivity, influencing fields from pharmaceuticals to materials science. Distinguishing and separating isomers often require methods such as chromatography, spectroscopy, or chiral discrimination techniques.