Tautomerism

Existence of two or more chemical compounds that have the same chemical composition but different structures (isomers) 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 covalent bond.

For example, in keto-enol tautomerism, the hydrogen atom bonded to the carbon atom in a carbonyl (keto) group (&singlehorzbond;CH&singlehorzbond;C&doublehorzbond;O; &see; functional group) moves to the oxygen atom, making it an enol group (&singlehorzbond;C&doublehorzbond;C&singlehorzbond;OH). The keto form predominates in many aldehydes and ketones, the enol form in phenols. Sugars (e.g., glucose) exhibit tautomerism between open (chain) forms and closed (ring) forms. &Seealso; isomerism.