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tautomerism

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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 (−CH−C=O; see functional group) moves to the oxygen atom, making it an enol group (−C=C−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. See also isomerism.



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Because of tautomerism (hydrogen shifts within the molecule), xanthines such as caffeine appear to have phenolic structure, but the preferred chemical structure in xanthines is the ketone.
 
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