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Tautomers are constitutional isomers of organic compounds that readily interconvert with each other. The chemical reaction interconverting the two is called tautomerization. This reaction commonly results in the formal migration of a hydrogen atom or proton, accompanied by a switch of a single bond and adjacent double bond. The concept of tautomerizations is called tautomerism. Because of the rapid interconversion, tautomers are generally considered to be the same chemical compound. Tautomerism should be distinguished from resonance, where two structures differing only in the position of electrons coexist in quantum superposition, rather than as a rapidly interconverting mixture. Tautomerism is a special case of structural isomerism and can play an important role in non-canonical base pairing in DNA and especially RNA molecules.
In solutions in which 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.[non-primary source needed][better source needed]
Common tautomeric pairs include:
- ketone - enol, e.g., for acetone (see: keto–enol tautomerism);
- enamine - imine;
- ketene - ynol, e.g., for ethenone;
- nitroso - oxime;
- amide - imidic acid, e.g., during nitrile hydrolysis reactions;
- lactam - lactim, an amide-imidic acid tautomerism in heterocyclic rings, e.g., in the nucleobases guanine, thymine, and cytosine;
- enamine - enamine, e.g., during pyridoxalphosphate-catalyzed enzymatic reactions; and
- anomers of reducing sugars in solution, which interconvert through an intermediate open chain form.
Prototropy is the most common form of tautomerism and refers to the relocation of a proton. Prototropic tautomerism may be considered as a subset of acid-base behavior. Prototropic tautomers are sets of isomeric protonation states with the same empirical formula and total charge. Tautomerizations are catalyzed by:
- bases, involving a series of steps: deprotonation, formation of a delocalized anion (e.g., an enolate), and protonation at a different position of the anion; and
- acids, involving a series of steps: protonation, formation of a delocalized cation, and deprotonation at a different position adjacent to the cation).
There are two specific further subcategories of tautomerizations:
- Annular tautomerism is a type of prototropic tautomerism wherein 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.[non-primary source needed][better source needed]
- Ring–chain tautomers occur when the movement of the proton is accompanied by a change from an open structure to a ring, such as the open chain and pyran forms of glucose and furan form of fructose.
Valence tautomerism is a type of tautomerism in which single and/or double bonds are rapidly formed and ruptured, without migration of atoms or groups. It 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 heterocycles, such as azide - tetrazole or mesoionic münchnone-acylamino ketene. Valence tautomerism requires a change in molecular geometry and should not be confused with canonical resonance structures or mesomers.
- Antonov L (2013). Tautomerism: Methods and Theories (1st ed.). Weinheim: Wiley-VCH. ISBN 978-3-527-33294-6.
- Smith MB, March J (2001). Advanced Organic Chemistry (5th ed.). New York: Wiley Interscience. pp. 1218–1223. ISBN 0-471-58589-0.
- Katritzky AR, Elguero J, et al. (1976). The Tautomerism of heterocycles. New York: Academic Press. ISBN 0-12-020651-X.
- Roman M. Balabin (2009). "Tautomeric equilibrium and hydrogen shifts in tetrazole and triazoles: Focal-point analysis and ab initio limit". J. Chem. Phys. 131 (15): 154307. Bibcode:2009JChPh.131o4307B. doi:10.1063/1.3249968.[non-primary source needed][better source needed]
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Tautomerism".
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Valence tautomerization".