Tautomeria

Tautomeria, or tautomerism, is a phenomenon in organic chemistry in which two or more interconvertible isomers, called tautomers, exist in equilibrium. The interconversion typically involves relocation of a proton and a shift of a double bond, so the tautomers differ in the position of the proton and the arrangement of bonding rather than in connectivity. Tautomerism is a form of prototropic isomerism and can occur in many molecular classes; other forms include ring–chain tautomerism and, in some cases, valence tautomerism.

The most common and well-studied type is prototropic tautomerism, especially keto–enol tautomerism. In carbonyl compounds with an alpha hydrogen, the keto form can equilibrate with its enol form. The enol form may be stabilized by intramolecular hydrogen bonding or conjugation, and the equilibrium is influenced by solvent, temperature, and pH. Other notable tautomers include imine–enamine tautomerism and lactam–lactim tautomerism in heterocycles.

Factors that influence tautomeric equilibria include solvent polarity, acidity or basicity of the medium, temperature, and substituent effects that stabilize one form over another. Acid or base catalysts often accelerate tautomerization by facilitating proton transfers. The relative stability of tautomers affects acidity and basicity, reactivity at carbonyl or imine centers, and the outcome of synthetic transformations.

Tautomerism has broad implications for spectroscopy, reactivity, and biological activity. Different tautomers can display distinct UV–Vis spectra, NMR signals, and chemical reactivity, impacting structure elucidation, drug design, and metabolic pathways. Examples range from simple 1,3-dicarbonyl compounds to various heterocyclic systems.