tautomers

Tautomers are constitutional isomers that readily interconvert by intramolecular proton transfer accompanied by a shift in the arrangement of double bonds. In such systems, the two (or more) forms have the same molecular formula and connectivity, but differ in the placement of a proton and the associated pi electrons. The interconversion can be fast enough to establish an equilibrium in solution.

The most common form of tautomerism is prototropic tautomerism, where a proton transfer accompanies a rearrangement of pi bonds. Well-known examples include keto–enol tautomerism, imine–enamine tautomerism, and thione–thiol tautomerism. These equilibria are often catalyzed by acids or bases and are highly sensitive to the solvent, temperature, pH, and substituents on the molecule. The relative stability of the tautomers determines which form predominates under given conditions and can influence acidity, basicity, reactivity, and spectroscopic properties.

Another type is ring–chain tautomerism, common in carbohydrates and related compounds, where open-chain forms interconvert with cyclic hemiacetal forms. This interconversion can affect reactivity and stereochemical outcomes in transformations and biological processes. Tautomerism is also relevant in medicinal chemistry, where the tautomer distribution of a drug molecule can alter binding, solubility, and metabolism. Predicting and controlling tautomeric equilibria is therefore important in synthesis, characterization, and drug design.