tautomerene

Tautomerene is a term used in chemistry to describe the phenomenon whereby a compound exists in two (or more) interconvertible structural forms called tautomers. The interconversion, tautomerization, typically involves relocation of a proton and a shift of a double bond, so that the two forms are constitutional isomers in rapid equilibrium. The best-known example is keto–enol tautomerism, in which a carbonyl compound (the keto form) is in equilibrium with its enol form.

Beyond keto–enol tautomerism, several other types are recognized, including imine–enamine tautomerism, thione–thiol tautomerism, lactam–lactim tautomerism, and ring–chain tautomerism. The relative stability of the tautomers depends on substituents, solvent, temperature, and intramolecular hydrogen bonding.

Mechanistically, tautomerization can be catalyzed by acids or bases and often proceeds via intramolecular or solvent-assisted proton transfer and electron rearrangement. In solution, the observed distribution of tautomers is described by a tautomeric equilibrium constant, which can differ markedly with solvent polarity and pH.

Implications of tautomerism are widespread: it influences acidity and basicity, molecular spectroscopy, reactivity, and, in pharmacology and biochemistry, the biological activity and binding properties of compounds. Nucleobases in DNA and RNA, for example, can exist in rare tautomeric forms that affect hydrogen bonding.

The word tautomer derives from Greek roots meaning “the same,” reflecting the relationship between tautomers. Tautomerism is a dynamic phenomenon and is distinguished from static isomerism by its rapid interconversion under many conditions.