tautomerism

Tautomerism is a class of isomerism in which two or more constitutional isomers, called tautomers, interconvert rapidly, typically by a proton transfer accompanied by a rearrangement of bonding. The most familiar example is keto–enol tautomerism, where a carbonyl compound equilibrates with its enol.

The interconversion is usually base- or acid-catalyzed and involves the migration of a proton and a shift of a pi bond or lone-pair. Tautomeric equilibria are influenced by solvent, temperature, pH, and catalysts, and the relative stability of the forms depends on factors such as conjugation, hydrogen bonding, and steric effects. In many carbonyl compounds, the keto form is thermodynamically favored, but the enol form can be stabilized by intramolecular hydrogen bonds, extended conjugation, or aromaticity.

Common forms of tautomerism include keto–enol tautomerism, lactam–lactim tautomerism (amidic to imidic forms), and imine–enamine tautomerism, as well as amide–imidic acid tautomerism. Ring–chain tautomerism also occurs in certain aldehydes and sugars. Valence tautomerism, a less common type, involves relocation of bonding without a simple proton transfer.

Implications of tautomerism are widespread. It affects acidity and basicity (pKa), reactivity, spectroscopy, and the interpretation of reaction mechanisms. In biology and pharmacology, tautomeric shifts can influence base pairing in nucleic acids and the binding, stability, and metabolism of drugs. Examples include preferential formation of enol or iminium forms under specific conditions and in specialized environments.