tautomerism

https://doi.org/10.1351/goldbook.T06252
isomerism of the general form:
T06252-1.png
where the isomers (called tautomers) are readily interconvertible; the atoms connecting the groups X, Y, Z are typically any of C, H, O or S, and G is a group which becomes an electrofuge or nucleofuge during isomerization. The commonest case, when the electrofuge is H+, is also known as 'prototropy'. Examples, written so as to illustrate the general pattern given above, include: keto-enol tautomerism, such as:
T06252-2.png
T06252-3.png
The grouping Y may itself be a three-atom (or five-atom) chain extending the conjugation, as in:
T06252-4.png
The double bond between Y and Z may be replaced by a ring, when the phenomenon is called ring-chain tautomerism, as in:
T06252-5.png
See also: ambident, sigmatropic rearrangement, tautomerization, valence tautomerization
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1171 (https://doi.org/10.1351/pac199466051077)