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The reversible interconversion of structural isomers of organic chemical compounds; such interconversions usually involve transfer of a proton.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



rapid, reversible structural isomerization. At equilibrium, substances that can tautomerize are mixtures of two or more interconverting isomers, or tautomers. The most common type of tautomerism is triadic prototropy, which involves the migration of a proton between the end atoms of a triad, or system of three atoms; two of the three atoms are linked by a double bond, which is shifted when the migration occurs. Examples of triadic prototropy are given below. The types shown are the keto-enol (a), imino-enamino (b), and amido (c), including lactim-lac-tam (d), triple-carbon (e), and ring-chain (f) tautomerism:

Dyadic prototropy, that is, tautomerism where proton migration occurs in a system of two atoms, describes the isomerization of hydrocyanic acid and the acids and thio acids of phosphorus. This type of tautomerism is associated with a change in the valence of one of the dyad atoms:

Anionotropy, or tautomerism involving migration of a negatively charged atom or group of atoms, is observed in many allyl halides and allyl alcohols and in their respective derivatives:

Tautomerism can also be encountered in valence isomers—that is, isomers that interconvert through a redistribution of valences between the atoms of the molecule’s skeleton rather than through a migration of substituents. An example is provided by cyclooctatriene, which exists in equilibrium with a bicyclic isomer:

Individual tautomers can sometimes be isolated. In most cases (HCN and others), however, the presence of tautomers is established spectrally.

Many substances form two series of derivatives corresponding to two possible tautomeric forms. As a rule, however, this phenomenon is explained not by tautomerism but by duality in the reactivity (seeRESONANCE). Tautomerism presupposes the existence of tautomers; the existence must be proved in each case.


Ingold, C. Teoreticheskie osnovy organicheskoi khimii. Moscow, 1973. Page 658. (Translated from English.)


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
Civcir, "A theoretical study of tautomerism of 6-thiopurine in the gas and aqueous phases using AM1 and PM3," Journal of Molecular Structure: THEOCHEM, vol.
Ruiz-Molina, "Valence tautomerism: more actors than just electroactive ligands and metal ions," Comptes Rendus Chimie, vol.
(47) This charge has a basis in the nature of resonance theory itself, but Chelintsev put forward a second charge based on his misapprehension of resonance as a form of valence tautomerism in which bonds were continually interchanging.
Theoretical studies of the tautomerism in 3-(2-R-Phenylhydrazono)-naphthalene-1,2,4-triones: synthesis of copper(II) complexes and studies of antibacterial arid antitumor activities.
Compound 13 exists in a lactam-lactim tautomerism, the existence of the lactim form was proved by its reaction with PO[Cl.sub.3] to give the chloro derivative (16).
Mizuno et al., "Switching redox-active sites by valence tautomerism in prussian blue analogues [A.sub.x][Mn.sub.y][Fe[(CN).sub.6]] x n[H.sub.2]0(A = K, Rb): robust frameworks for reversible Li storage," Journal of Physical Chemistry Letters, vol.
Pihlaja, "A comparison of ring-chain tautomerism in heterocycles derived from 2-aminobenzenesulfonamide and anthranilamide," Tetrahedron, vol.
(18,51) Third, hydrogen-bonded unnatural base pairs sometimes suffer from the instability of the hydrogen-bonding geometry, due to tautomerism. Especially, isoG undergoes keto-enol tautomerization, depending on the solvent polarity, and the enolform of isoG mispairs with T (Fig.
Particularly, the enol-keto tautomerism in the investigated structures can be expressed by a first-order rate equation: In [A.sub.0]/[A.sub.t], = kt, where [A.sub.0] and [A.sub.t] are the values of the absorbance at time [t.sub.0] and t, respectively, and k is the rate constant.
Ketoopioids are subject to tautomerism depending upon matrix conditions so they must undergo derivatization to avoid recovery issues.
The alkylation of carbon in the ortho and para substitution occurs, however substitution in meta is exclusively not obvious, the phenol reacts with the methanal in the reaction of hydroxymethylation (figure 3) where the condensation aldol takes place but here the balance of tautomerism is in favour of the form phenol which is most stable due to its aromatic character.
It shows keto-enol and N9H-N7H tautomerism. However, under aqueous environment the keto-N9H form dominates.