Tautomers are organic compounds that are interconvertible by a chemical reaction called tautomerization. This reaction results in the formal migration of a hydrogen atom accompanied by a switch of adjacent conjugated double bonds. In solutions where tautomerization is possible, a chemical equilibrium of the tautomers will be reached. The exact ratio of the tautomers depends on several factors, including temperature, solvent, and pH. The concept of tautomers that are interconvertable by tautomerizations is called tautomerism.

Tautomerizations are catalyzed by:

• base (1. deprotonation; 2. formation of a delocalized anion (e.g. an enolate); 3. protonation at a different position of the anion).
• acids (1. protonation; 2. formation of a delocalized cation; 3. deprotonation at a different position adjacent to the cation).

Common tautomeric pairs are:

• ketone - enol, e.g. for acetone (see: keto-enol tautomerism).
• 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.
• amine - enamine and
• enamine - enamine, e.g. during pyridoxalphosphate catalyzed enzymatic reactions.