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[[Image:tautomers.gif|right|Tautomers]]
'''Tautomers''' are [[organic compound]]s that are interconvertible by a [[chemical reaction]] called '''tautomerization'''. As most commonly encountered, this reaction results in the formal migration of a [[hydrogen]] [[atom]] or [[proton]], accompanied by a switch of a [[single bond]] and adjacent [[double bond]]. In [[solution]]s where tautomerization is possible, a [[chemical equilibrium]] of the tautomers will be reached. The exact ratio of the tautomers depends on several factors, including [[temperature]], [[solvent]], and [[pH]].  The concept of tautomers that are interconvertible by tautomerizations is called '''tautomerism'''. Tautomerism is a special case of [[structural isomerism]] and can play an important role in non-canonical [[base pair]]ing in [[DNA]] and especially [[RNA]] molecules.

Tautomerizations are [[Catalysis|catalyzed]] by:
* [[Base (chemistry)|base]] (1. deprotonation; 2. formation of a [[delocalized]] [[anion]] (e.g. an [[enolate]]); 3. [[protonation]] at a different position of the anion).
* [[acid]]s (1. protonation; 2. formation of a delocalized cation; 3. deprotonation at a different position adjacent to the cation).

Common tautomeric pairs are:

* [[ketone]] - [[enol]], e.g. for [[acetone]] (see: [[keto-enol tautomerism]]).
* [[amide]] - imidic acid, e.g. during [[nitrile]] [[hydrolysis]] reactions.
* [[lactam]] - [[lactim]], an amide - imidic acid tautomerism in [[heterocyclic]] rings, e.g. in the [[nucleobase]]s [[guanine]], [[thymine]], and [[cytosine]].
* [[enamine]] - [[imine]]
* enamine - enamine, e.g. during [[pyridoxal-phosphate|pyridoxalphosphate]] catalyzed [[Enzyme|enzymatic]] reactions.

'''Prototropic tautomerism''' refers to the relocation of a proton, as in the above examples, and may be considered a subset of [[acid-base]] behavior. Prototropic tautomers are sets of isomeric [[protonation]] states with the same [[empirical formula]] and total [[charge]].

'''Annular tautomerism''' is a type of prototropic tautomerism where a proton can occupy two or more positions of a [[heterocyclic]] system. for example, 1H- and 3H-[[imidazole]]; 1H-, 2H- and 4H- [[1,2,4-triazole]]; 1H- and 2H- [[isoindole]].

'''Ring-chain tautomerism''' occurs when the movement of the proton is accompanied by a change from an open structure to a ring, such as the [[aldehyde]] and [[pyran]] forms of [[glucose]].  

'''Valence tautomerism''' is distinct from prototropic tautomerism, and involves processes with rapid reorganisation of bonding electrons. An example of this type of tautomerism can be found in [[bullvalene]]. Another example is open and closed forms of certain [[heterocycle]]s, such as [[azide]] - [[tetrazole]]. Valence tautomerism requires a change in molecular geometry and should not be confused with canonical [[resonance structures]] or [[mesomer]]s.

==References==
# Smith, M. B.; March, J. ''Advanced Organic Chemistry'', 5th ed., Wiley Interscience, New York, 2001; pp 1218-1223. ISBN 0471585890
# Katritzky, A.R.; Elguero, J. ''et al.'' ''The Tautomerism of heterocycles'', Academic Press, New York, 1976, ISBN 012020651X

[[Category:organic chemistry]]

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