Tautomerie

Tautomerism, also known as tautomerie in German, is a chemical phenomenon in which two or more readily interconvertible constitutional isomers, called tautomers, differ mainly in the position of a proton and the location of a double bond. The interconversion is usually rapid and establishes an equilibrium between the forms.

The most common type is keto–enol tautomerism, where a carbonyl compound (keto form) is in equilibrium with its enol form that contains a hydroxyl group adjacent to a carbon–carbon double bond. Other common forms include imine–enamine tautomerism in nitrogen-containing compounds, and ring–chain tautomerism observed in sugars and heterocycles. Thio-tautomerism between thione and thiol forms is another well-known variant, occurring for sulfur-containing analogs of carbonyl compounds.

Mechanistically, tautomerization is typically acid- or base-catalyzed and involves intramolecular proton transfer and a rearrangement of bonding, often with solvent participation. The equilibrium position is influenced by solvent polarity, pH, temperature, and substituents that stabilize one form over the other. Stabilization can arise from hydrogen bonding, conjugation, or chelation effects.

Significance lies in the impact on chemical properties such as acidity and basicity, reactivity, color, and spectroscopic behavior. Tautomeric distributions can affect reaction outcomes, stability of intermediates, and the behavior of molecules in biological systems. In pharmacology and drug design, the dominant tautomer can influence target binding, metabolism, and efficacy, making tautomerism a relevant consideration in synthesis and analysis.