page_title: Tautomer
section_title: Examples
hierarchical_section_title: Tautomer / Examples
context_page_description: Tautomers are structural 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. Tautomerism is also called desmotropism. 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.
context_section_description: 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:
  ketone – enol:  H−O−C=C ⇌ O=C−C−H, see keto–enol tautomerism
  enamine – imine: H−N−C=C ⇌ N=C−C−H
  cyanamide – carbodiimide
  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)
  lactam – lactim, a cyclic form of amide-imidic acid tautomerism in 2-pyridone and derived structures such as the nucleobases guanine, thymine, and cytosine
  imine – imine, e.g., during pyridoxal phosphate catalyzed enzymatic reactions
  R¹R²C(=NCHR³R⁴) ⇌ (R¹R²CHN=)CR³R⁴
  nitro – aci-nitro (nitronic acid): RR'HC–N⁺(=O)(O–) ⇌ RR'C=N⁺(O–)(OH)
  nitroso – oxime: H−C−N=O ⇌ C=N−O−H
  ketene – ynol, which involves a triple bond: H−C=C=O ⇌ C≡C−O−H
  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: H₂N−CH₂−COOH ⇌ H₃N⁺
  −CH₂−CO⁻
  ₂
  phosphite – phosphonate: P(OR)₂(OH) ⇌ HP(OR)₂(=O) between trivalent and pentavalent phosphorus.
caption: Some examples of tautomers