Isomerism

Isomerism is the phenomenon by which compounds share the same molecular formula but differ in the arrangement of atoms, giving distinct substances with different properties. The concept is central to chemistry because small changes in structure can cause large changes in reactivity, physical properties, and biological activity. Isomerism is generally divided into constitutional (structural) isomerism and stereoisomerism.

Constitutional isomers differ in the connectivity of atoms. Common types include chain (skeletal) isomerism, where different carbon skeletons exist (for example, n-butane and isobutane); position isomerism, where substituents occupy different positions on the same skeleton (such as 1-chloropropane vs 2-chloropropane); and functional group isomerism, where the same formula yields different functional groups (for example ethanol and dimethyl ether). Metamerism is a subtype defined by differing alkyl groups around a functional group, as in certain ether isomers.

Stereoisomerism involves the same connectivity but different spatial arrangement. Geometric isomerism arises in restricted rotation about double bonds or rings, and is described using cis/trans or E/Z nomenclature (for example cis-2-butene vs trans-2-butene). Optical isomerism involves chiral centers, yielding enantiomers that are non-superimposable mirror images (such as the D- and L- forms of lactic acid). Some stereoisomers are diastereomers, which have multiple stereocenters and differing properties. Conformational isomerism due to rotation about single bonds also exists; these conformers interconvert readily and may have different energies but are often in rapid equilibrium.

In practice, isomerism helps predict properties and behaviors, including interactions with enzymes and receptors, and plays a crucial role in drug development and materials science.