Tautomer

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An amino acid, which shown in two ionization states. First, it is shown in the same arrangement as the lead image. This is the unionised form. It is also shown in the ionized form, after the carboxyl group has lost a hydrogen atom, which introduces a negative charge, and the amino group has gained a hydrogen, which introduces a positive charge.
Two tautomers of an amino acid: (1) neutral and (2) zwitterionic forms.

Tautomers are constitutional isomers of organic compounds that readily interconvert.[1][2][3] This reaction commonly results in the relocation of a proton. Tautomerism is relevant to the behavior of amino acids and nucleic acids, two of the fundamental building blocks of life.

The concept of tautomerizations is called tautomerism. The chemical reaction interconverting the two is called tautomerization.

Examples[edit]

Some examples of tautomers
Keto-enol tautomerization typically strongly favors the keto tautomer, but an important exception is the case of 1,3-diketones such as acetylacetone.

Tautomerization is pervasive in organic chemistry. It is typically associated with polar molecules and ions containing functional groups that are at least weakly acidic. Most commonly tautomers exist in pairs, which means that the proton is located at one of two positions, and even more specifically the most common form involves a hydrogen changing places with a double bond: H−X−Y=Z ⇌ X=Y−Z−H. Common tautomeric pairs include:[4]

  • ketoneenol: H−O−C=CH ⇌ O=C−CH2, see keto–enol tautomerism
  • enamineimine: H2N−C=N ⇌ HN=C−NH
    • guanidine – guanidine – guanidine: With a central carbon surrounded by three nitrogens, a guanidine group allows this transform in three possible orientations
  • amide – imidic acid: H−N−C=O ⇌ N=C−O−H (e.g., the latter is encountered during nitrile hydrolysis reactions)
  • enamine – enamine, e.g., during pyridoxal phosphate catalyzed enzymatic reactions
  • nitrosooxime: H−C−N=O ⇌ C=N−O−H
  • keteneynol, which involves a triple bond: R−CH=C=O ⇌ R−C≡C−OH
  • amino acid – ammonium carboxylate, which applies to the building blocks of the proteins. This shifts the proton more than two atoms away, producing a zwitterion rather than shifting a double bond: NH2−CH−COOH ⇌ NH+
    3
    −CH−CO
    2

Prototropy[edit]

Prototropy is the most common form of tautomerism and refers to the relocation of a proton.[5] 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:[citation needed]

  • 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).
Glucose can exist in both a straight-chain and ring form.

Two specific further subcategories of tautomerizations:

Valence tautomerism[edit]

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.[7] It is distinct from prototropic tautomerism, and involves processes with rapid reorganisation of bonding electrons.

Oxepin – benzene oxide equilibrium

A pair of valence tautomers with formula C6H6O are benzene oxide and oxepin.[7][8]

Other examples of this type of tautomerism can be found in bullvalene, and in open and closed forms of certain heterocycles, such as organic azides and tetrazoles,[9] or mesoionic münchnone and acylamino ketene.

Valence tautomerism requires a change in molecular geometry and should not be confused with canonical resonance structures or mesomers.

See also[edit]

References[edit]

  1. ^ Antonov L (2013). Tautomerism: Methods and Theories (1st ed.). Weinheim: Wiley-VCH. ISBN 978-3-527-33294-6. 
  2. ^ Smith MB, March J (2001). Advanced Organic Chemistry (5th ed.). New York: Wiley Interscience. pp. 1218–1223. ISBN 0-471-58589-0. 
  3. ^ Katritzky AR, Elguero J, et al. (1976). The Tautomerism of heterocycles. New York: Academic Press. ISBN 0-12-020651-X. 
  4. ^ a b Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, ISBN 0-471-72091-7 
  5. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "Tautomerism".
  6. ^ 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]
  7. ^ a b IUPAC Gold Book: Valence tautomerization
  8. ^ E. Vogel and H. Günther (1967). "Benzene Oxide-Oxepin Valence Tautomerism". Angewandte Chemie International Edition in English. 6 (5): 385–401. doi:10.1002/anie.196703851. 
  9. ^ Lakshman Mahesh K., Singh Manish K., Parrish Damon, Balachandran Raghavan, Day Billy W. (2010). "Azide−Tetrazole Equilibrium of C-6 Azidopurine Nucleosides and Their Ligation Reactions with Alkynes". The Journal of Organic Chemistry. 75 (8): 2461–2473. doi:10.1021/jo902342z.