Existence of two or more chemical compound s that have the same chemical composition but different structures ( isomer s) and that convert easily from one to another.
A major class of tautomeric reactions involves exchange of a hydrogen atom between two other atoms in the same molecule, in both cases forming a functional group ) moves to the oxygen atom, making it an enol group (―C=C―OH). The keto form predominates in many aldehyde s and ketone s, the enol form in phenol s. Sugar s (e.g., {{link=glucose">glucose ) exhibit tautomerism between open (chain) forms and closed (ring) forms. See also isomerism .