Tauteners

Tautomers are constitutional isomers of chemical compounds that readily interconvert. This interconversion, known as tautomerization, typically involves the migration of a proton and the relocation of a double bond. The most common type of tautomerism is keto-enol tautomerism, where a ketone or aldehyde interconverts with its corresponding enol form. In keto-enol tautomerism, a hydrogen atom from the alpha-carbon (the carbon adjacent to the carbonyl group) migrates to the oxygen atom of the carbonyl group, and the double bond shifts to form a carbon-carbon double bond between the alpha-carbon and the carbonyl carbon.

Another important type of tautomerism is imine-enamine tautomerism, involving the interconversion of an imine and an enamine. Here, a proton migrates from the carbon adjacent to the imine nitrogen to the nitrogen atom, with a concomitant shift of the double bond. Aromatic compounds can also exhibit tautomerism, though it is less common and often involves significant changes in aromaticity. For example, certain hydroxypyridines can exist in equilibrium with their pyridone tautomers.

The position of the tautomeric equilibrium depends on various factors, including the solvent, temperature, and the specific structure of the molecule. Often, one tautomer is more stable and predominates at equilibrium. Tautomerization is an important concept in organic chemistry and plays a role in various reactions, including electrophilic substitution, nucleophilic addition, and biological processes.