Tautomerization

Tautomerization is a chemical process in which two constitutional isomers, called tautomers, rapidly interconvert. The interconversion involves relocation of a proton and a rearrangement of bonding, most often accompanied by a shift of a double bond. The equilibrium between tautomers can depend on solvent, temperature, and catalysts, and the process is typically described as keto–enol tautomerization in many carbonyl compounds.

The mechanism is usually mediated by acid or base catalysis. In keto–enol tautomerism, a proton is removed from the alpha carbon of a carbonyl compound and rearranged to form an enol, while the carbonyl oxygen shifts its lone pair to produce a hydroxyl group. The reverse process regenerates the carbonyl. Imine–enamine tautomerization involves proton transfer between nitrogen and adjacent carbon, converting a C=N bond to a C–N single bond with a neighboring C=C or C–N bond rearrangement. Amide–imidic acid tautomerism is another well-known form, where an amide can convert to its corresponding imidic acid form under certain conditions. Ring–chain tautomerism describes reversals between cyclic and open-chain forms in some compounds.

Tautomeric equilibria influence physical properties, acidity (pKa), reactivity, and spectroscopic behavior. They are important in organic synthesis, medicinal chemistry, and biochemistry, where the preferred tautomer can affect binding, catalysis, or metabolism. Detecting and quantifying tautomers often relies on NMR, IR spectroscopy, and computational methods, together with consideration of solvent and temperature effects.