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tautomerism

 
American Heritage Dictionary:

tau·tom·er·ism

(tô-tŏm'ə-rĭz'əm) pronunciation
n.
Chemical isomerism characterized by relatively easy interconversion of isomeric forms in equilibrium.

[TAUTO- + (ISO)MERISM.]

tautomer tau'to·mer ('tə-mər) n.
tautomeric tau'to·mer'ic ('tə-mĕr'ĭk) adj.

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Existence of two or more chemical compounds that have the same chemical composition but different structures (isomers) and that convert easily from one to another. A major class of tautomeric reactions involves exchange of a hydrogen atom between two other atoms in the same molecule, in both cases forming a covalent bond. For example, in keto-enol tautomerism, the hydrogen atom bonded to the carbon atom in a carbonyl (keto) group (-CH-C=O; see functional group) moves to the oxygen atom, making it an enol group (-C=C-OH). The keto form predominates in many aldehydes and ketones, the enol form in phenols. Sugars (e.g., glucose) exhibit tautomerism between open (chain) forms and closed (ring) forms. See also isomerism.

For more information on tautomerism, visit Britannica.com.

The reversible interconversion of structural isomers of organic chemical compounds. Such interconversions usually involve transfer of a proton, but anionotropic rearrangements may be reversible and so be classed as tautomeric interconversions. A cyclic system containing the grouping —CONH— is called a lactam, and the isomeric form, —COH&dbnd;N—, a lactim. These terms have been extended to include the same structures in open-chain compounds when considering the shift of the hydrogen from nitrogen to oxygen.

Molecular grouping (1) may in certain substances exist partly or wholly as (2). The former constitutes the keto form and the latter the enol form. The existence of an enol in an acyclic system requires that a second carbonyl group (or its equivalent, for example, (3) be attached to the same (4) as an aldehyde or ketone

carbonyl. Thus, ethyl acetoacetate tautomerizes demonstrably, but ethyl malonate does not. Where the enol form includes an aromatic ring such as phenol, the existence of the keto form is often not demonstrable, although in some substances there may be either chemical or spectroscopic evidence for both forms. Closely related to keto-enol tautomerism is the prototropic interconversion of nitro and aci forms of aliphatic nitro compounds such as nitromethane.

In general, tautomeric forms exist in substances possessing functional groups which can interact additively and which are so placed that intramolecular reaction leads to a stable cyclic system. The cyclic form usually predominates (especially if it contains five or six members). See also Molecular isomerism.


Columbia Encyclopedia:

tautomer

Top
tautomer ('təmər), one of two or more structural isomers that exist in equilibrium and are readily converted from one isomeric form to another. Of the various types of tautomerism that are possible, two are commonly observed. In keto-enol tautomerism a simultaneous shift of electrons and a hydrogen atom occurs; it was first observed by K. Meyer in the ethyl ester of 3-oxobutanoic acid (ethyl acetoacetate), which occurs naturally as a mixture of the two forms. Ring-chain tautomerism, first recognized by Emil Fischer, is exhibited by glucose. It arises as a result of the aldehyde group (-CHO) in a sugar chain molecule reacting with one of the hydroxy groups (-OH) in the same molecule to give it a cyclic (ring-shaped) form.



a form of constitutional isomerism in which a structure may exist in two or more constitutional arrangements, particularly with respect to the position of hydrogens bonded to oxygen. The isomers, or tautomers, are readily interconvertible and exist in equilibrium. The best-known example is keto-enol tautomerism, in which the structures X−C(=O)−CH2−X (keto) and X−C(OH)=CH−X (enol) exist, in proportions determined by the equilibrium position for the conditions pertaining (normally strongly favouring the keto form). Other examples include quinone-quinol and amino-imino tautomerism.
tautomeric adj.

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Stereoisomerism in which the compounds are mutually interconvertible, under normal conditions, forming a mixture that is in dynamic equilibration.


n

Structural isomers that differ only in the position of a hydrogen atom, or proton.

Tautomers

Tautomers are isomers (structural isomers) of organic compounds that readily interconvert by a chemical reaction called tautomerization.[1][2] 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.

Contents

Chemistry

Aromaticity provides some stability to the triple lactim form of these triple lactam-lactim tautomers.

In solutions in which 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.[3]

Common tautomeric pairs are:

Prototropy

Prototropy is the most common form of tautomerism and refers to the relocation of a proton.[4] 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.

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).
Annular tautomerism
a type of prototropic tautomerism wherein a proton can occupy two or more positions of a heterocyclic system, for example, 1H- and 3H-imidazole; 1H-, 2H- and 4H- 1,2,4-triazole; 1H- and 2H- isoindole.[3]
Ring-chain tautomerism
occurring when the movement of the proton is accompanied by a change from an open structure to a ring, such as the open chain and pyran forms of glucose and furan form of fructose.

Valence tautomerism

Valence tautomerism is a type of tautomerism in which single and/or double bonds are rapidly formed and ruptured, without migration of atoms or groups.[5] 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.

References

  1. ^ *Smith MB, March J (2001). Advanced Organic Chemistry (5th ed. ed.). New York: Wiley Interscience. pp. 1218–1223. ISBN 0-4715-8589-0. 
  2. ^ Katritzky AR, Elguero J, et al. (1976). The Tautomerism of heterocycles. New York: Academic Press. ISBN 0-1202-0651-X. 
  3. ^ a b Roman M. Balabin (2009). "Tautomeric equilibrium and hydrogen shifts in tetrazole and triazoles: Focal-point analysis and ab initio limit". J. Chem. Phys. 131 (15): 154307. doi:10.1063/1.3249968. 
  4. ^ IUPAC Gold Book tautomerism
  5. ^ IUPAC Gold Book valence tautomerization

 
 
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