Izomeria

Isomerism, or izomeria in Italian, is a phenomenon in chemistry where compounds share the same molecular formula but differ in the arrangement of atoms or in the three-dimensional orientation of their atoms. These differences give rise to distinct physical and chemical properties, including boiling points, melting points, reactivity, and biological activity.

The main divisions are constitutional isomers, which differ in how atoms are connected, and stereoisomers, which have the same connectivity but differ in geometry or arrangement in space. Conformational isomers arise from rotation about single bonds and can interconvert rapidly at most temperatures, such as staggered and eclipsed conformers of ethane. Configurational isomers require breaking bonds to interconvert and include enantiomers (non-superimposable mirror images) and diastereomers (not mirror images). Geometric isomerism, a subset of diastereomerism, has cis/trans forms (also described by E/Z nomenclature) due to restricted rotation around double bonds or within rings; example: cis-2-butene vs trans-2-butene.

Enantiomers often occur in chiral molecules with at least one stereogenic center; they rotate plane-polarized light differently and can have different biological activities. An example is lactic acid, which has R and S enantiomers. Diastereomers include compounds like certain tartaric acid isomers, including meso-tartaric acid as a special diastereomer.

Tautomerism is another form, such as keto–enol tautomerism, where proton shifts yield constitutional isomers in rapid equilibrium.