= Tautomerism
\b[Tautomerism] is the relationship between readily interconvertible <constitutional isomers>, called <tautomers>. In a common case, a proton changes its attached atom while associated bond orders change. The atoms are conserved, but their bonding arrangement is different. The IUPAC definition also allows transfer of groups other than a proton, so tautomerism is broader than one hydrogen-transfer pattern.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry>[Entry T06252, tautomerism; IUPAC1994 glossary, tautomerism/prototropy]{ref}
For a <keto-enol tautomerism> example, <acetaldehyde> and its enol form have the structures
$$
\mathrm{CH_3-CH=O\rightleftharpoons CH_2=CH-OH}.
$$
Both have formula $\mathrm{C_2H_4O}$. A hydrogen changes attachment from carbon to oxygen, the carbon–carbon bond becomes double, and the carbon–oxygen bond becomes single. Drawing the net change does not specify a complete reaction mechanism or imply that the two forms occur in equal amounts.
<Tautomerism> differs from <resonance (chemistry)>: <tautomers> have different atom attachments, whereas resonance drawings describe different electronic representations without moving the nuclei to new bonding positions. Nor is a pair of protonation states automatically a tautomer pair: losing a proton changes the hydrogen inventory and net charge. These distinctions matter when interpreting a chemical drawing or comparing a database's specific tautomer record with a broader compound name, as for <xanthine>.
Back to article page
