Tautomeri

Tautomeri is a chemical phenomenon in which two or more interconvertible structures called tautomers exist in rapid equilibrium, typically involving relocation of a proton and a shift of a π bond within the molecule. The process is known as prototropic tautomerism when the transfer occurs through a proton, and the two forms differ in the position of the proton and the location of a double bond. The equilibrium position depends on temperature, solvent, pH, and substituents, and the interconversion can be catalyzed by acids or bases.

One of the most familiar examples is keto–enol tautomerism: a carbonyl-containing compound (keto form) exists in equilibrium with its enol form (an alcohol with a neighboring carbon–carbon double bond). In many carbonyl compounds, the enol form is stabilized by intramolecular hydrogen bonding or conjugation. Acetylacetone shows a well-studied keto–enol tautomerism.

Other common forms include imine–enamine tautomerism (nitrogen-centered), lactam–lactim tautomerism in amide–imidic acid systems, and ring–chain tautomerism where a ring opens or closes during the tautomerization, as seen in certain sugars and nucleobases.

The tautomeric mixture is described by a tautomeric ratio or the free-energy difference ΔG between forms; this distribution can be probed by NMR, IR, UV spectroscopy and computational methods. Tautomerism can affect reactivity, acidity/basicity, polarity, and biological properties; in biology, it can influence base pairing and mutagenicity of nucleobases, and in chemistry it can influence reaction pathways and drug activity.

Note: the term tautomeri is used in some languages to refer to tautomerism; in English the standard term is tautomerism.