Tautomerism

Tautomerism is the reversible interconversion of constitutional isomers that differ in the position of a proton and a double bond. The resulting forms, called tautomers, coexist in a condition-dependent equilibrium.

Prototropic tautomerism: Tautomerism in which a proton shifts between atoms as a double bond changes position. This common mechanism captures the proton migration at the heart of tautomerism.
Constitutional isomerism: Isomerism in which compounds differ in the connectivity of their atoms. Tautomers are constitutional isomers distinguished by a mobile proton and shifted multiple bond.
Nucleobase tautomerism: Tautomeric interconversion among nucleobases such as adenine, cytosine, guanine, and thymine. Rare nucleobase tautomers can alter hydrogen-bonding patterns during DNA replication.
Conformational isomerism: Isomerism arising from different spatial arrangements produced by rotation around single bonds. Conformers differ by bond rotation, whereas tautomers differ in proton and double-bond placement.
Keto–enol tautomerism: Interconversion between a carbonyl compound and an enol through proton transfer and double-bond rearrangement. It is the best-known case for tracing how proton and π-bond positions change.
Resonance (chemistry): A representation of one electronic structure using multiple contributing Lewis structures without atom movement. Resonance shifts electrons only; tautomerism changes proton positions and produces distinct structures.
DNA replication: The cellular process that copies DNA before cell division. Tautomer-dependent base pairing has been proposed as one source of replication errors.
Prototropy: A type of tautomerism involving migration of a proton between atoms. This term names the proton-shifting subset, not every form of tautomerism.
Acid–base catalysis: Acceleration of a reaction by proton donation, proton acceptance, or both. Acids and bases often provide lower-energy pathways for tautomer interconversion.
Enol: A compound containing a hydroxyl group directly attached to a carbon–carbon double bond. Enols are one structural form in keto–enol tautomerism.

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