Tautomerism

TAUTOMERISM, a special type of isomerism (see), characterized by the fact that the compound reacts simultaneously in two forms, giving two series of derivatives. For tautomers, a transition into each other is characteristic, caused by the migration of a hydrogen atom and the shifting of double bonds in the molecule (so-called intramolecular rearrangement). Tautomers exist in a state of mobile equilibrium (see Chemical equilibrium), which explains the possibility of a tautomeric substance reacting in two forms. The rate of conversion of one tautomer into another, as well as the position of equilibrium (the amount of one form relative to the other at the moment of equilibrium) varies in different substances, which gave rise to the subdivision of the phenomenon of T. into: a) desmotropy, when significant amounts of tautomers are in equilibrium, so that both forms can be determined, and at times even isolated, and their structure is known; b) pseudomerism, when the position of equilibrium is strongly shifted toward one tautomeric form, so that the other cannot be isolated and proven analytically—the presence of the other is indicated only by the formation of derivatives.

In biochemical processes, keto-enol tautomerism is of great importance, occurring in acetoacetic acid, in uric acid (see) and other compounds that react simultaneously in both ketone and enol forms: acetoacetic acid

in its ketone form

in its enol form