Tautomer

Source: Wikipedia, the free encyclopedia.
An amino acid, illustrated in two different ionization states. First, it has a neutral amine and neutral carboxylic acid. Second, it has a protonated ammonium cation and deprotonated carboxylate anion.
The two tautomers of an amino acid: (1) neutral and (2) zwitterionic forms.

Tautomers (/ˈtɔːtəmər/)[1] are structural isomers (constitutional isomers) of chemical compounds that readily interconvert.[2][3][4][5] The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the relocation of a hydrogen atom within the compound. The phenomenon of tautomerization is called tautomerism, also called desmotropism. Tautomerism is for example relevant to the behavior of amino acids and nucleic acids, two of the fundamental building blocks of life.

Care should be taken not to confuse tautomers with depictions of "contributing structures" in chemical resonance. Tautomers are distinct chemical species that can be distinguished by their differing atomic connectivities, molecular geometries, and physicochemical and spectroscopic properties,[6] whereas resonance forms are merely alternative Lewis structure (valence bond theory) depictions of a single chemical species, whose true structure is a quantum superposition, essentially the "average" of the idealized, hypothetical geometries implied by these resonance forms.

Examples

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.[6]

Tautomerization is pervasive in organic chemistry.[2][7] It is typically associated with polar molecules and ions containing functional groups that are at least weakly acidic. Most common tautomers exist in pairs, which means that the hydrogen 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:[3][4]

Prototropy

Prototropy is the most common form of tautomerism and refers to the relocation of a hydrogen atom.

acid-base behavior. Prototropic tautomers are sets of isomeric protonation states with the same empirical formula and total charge. Tautomerizations are catalyzed by:[4]

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

Two specific further subcategories of tautomerizations:

Valence tautomerism

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.[9] 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.[9][10]

Other examples of this type of tautomerism can be found in

mesoionic münchnone
and acylamino ketene.

Valence tautomerism requires a change in molecular geometry and should not be confused with canonical

resonance structures
or mesomers.

Inorganic materials

In inorganic extended solids, valence tautomerism can manifest itself in the change of oxidation states its spatial distribution upon the change of macroscopic thermodynamic conditions. Such effects have been called charge ordering or valence mixing to describe the behavior in inorganic oxides.[12]

Consequences for chemical databases

The existence of multiple possible tautomers for individual

2-hydroxypyridine do not exist as separate isolatable materials: the two tautomeric forms are interconvertible and the proportion of each depends on factors such as temperature, solvent, and additional substituents attached to the main ring.[8][13]

2-pyridone-chemical-tautomer.svg

Historically, each form of the substance was entered into databases such as those maintained by the Chemical Abstracts Service and given separate CAS Registry Numbers.[14] 2-Pyridone was assigned [142-08-5][15] and 2-hydroxypyridine [109-10-4].[16] The latter is now a "replaced" registry number so that look-up by either identifier reaches the same entry. The facility to automatically recognise such potential tautomerism and ensure that all tautomers are indexed together has been greatly facilitated by the creation of the International Chemical Identifier (InChI) and associated software.[17][18][19] Thus the standard InChI for either tautomer is InChI=1S/C5H5NO/c7-5-3-1-2-4-6-5/h1-4H,(H,6,7).[20]

See also

References

  1. ^ "tautomer". Oxford Dictionaries - English. Archived from the original on 2018-02-19.
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  11. PMID 20297785.{{cite journal}}: CS1 maint: multiple names: authors list (link
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  14. ^ "CAS REGISTRY and CAS Registry Number FAQs". CAS, a division of the American Chemical Society. Retrieved 2022-08-10.
  15. ^ "2-Pyridone". CAS Common Chemistry. Retrieved 2022-08-10.
  16. ^ "2-Pyridone ("other name")". CAS Common Chemistry. Retrieved 2022-08-10.
  17. PMID 23343401
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  20. ^ "2-pyridone". ChemSpider. Retrieved 2022-08-10.

External links