This reaction commonly results in the formal migration of a hydrogen atom or proton, accompanied by a switch of a single bond and adjacent double bond. The concept of tautomerizations is called tautomerism. Because of the rapid interconversion, tautomers are generally considered to be the same chemical compound.
Tautomerism is a special case of structural isomerism and can play an important role in non-canonical base pairing in DNA and especially RNA molecules.
Common tautomeric pairs are:
Ketone - enol, e.g., for acetone
Amide - imidic acid, e.g., during nitrile hydrolysis reactions
Lactam - lactim, an amide - imidic acid tautomerism in heterocyclic rings, e.g., in the nucleobases guanine, thymine, and cytosine
Enamine - imine
Enamine - enamine, e.g., during pyridoxalphosphate-catalyzed enzymatic reactions.
Anomers of reducing sugars in solution interconvert through an intermediate open chain form.
Prototropy:It is the most common form of tautomerism and refers to the relocation of a proton.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.
Valence tautomerism:It 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. An example of this type of tautomerism can be found in bullvalene.
Another example is open and closed forms of certain heterocycles, such as azide - tetrazole or mesoionic münchnone-acylamino ketene. Valence tautomerism requires a change in molecular geometry and should not be confused with canonical resonance structures or mesomers.
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