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The two tautomers of an amino acid: (1) neutral and (2) zwitterionic forms.
Tautomers (/ˈtɔːtəmə/) are structural isomers (constitutional isomers) of chemical compounds that readily interconvert. This reaction commonly results in the relocation of a proton. Tautomerism is for example 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.
Care should be taken not to confuse tautomers with depictions of 'contributing structures' in chemical resonance. Tautomers are distinct chemical species and can be identified as such by their differing spectroscopic data, whereas resonance structures are merely convenient depictions and do not physically exist.
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 common 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:
ketene – ynol, 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
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.
Oxepin – benzene oxide equilibrium
A pair of valence tautomers with formula C6H6O are benzene oxide and oxepin.
^Smith, Kyle T.; Young, Sherri C.; DeBlasio, James W.; Hamann, Christian S. (27 January 2016). "Measuring Structural and Electronic Effects on Keto–Enol Equilibrium in 1,3-Dicarbonyl Compounds". Journal of Chemical Education. 93 (4): 790–794. doi:10.1021/acs.jchemed.5b00170.