text: 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 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: .  Common tautomeric pairs include:
  * ketone – enol: , see keto–enol tautomerism
  * enamine – imine: 
  **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:  (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 
  *nitro – aci-nitro (nitronic acid): 
  * nitroso – oxime: 
  * ketene – ynol, which involves a triple bond: 
  * 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: 
  *phosphite – phosphonate:  between trivalent and pentavalent phosphorus.