Tautomers Structural isomers that differ

Tautomers Structural isomers that differ in the location of their hydrogen and double bonds. 

Tautomers (Section 11.6) Structural isomers that differ only in the position of a hydrogen and a pi bond. 

Tautomers are structural isomers that differ in the location of a proton and a double bond. The keto and enol forms of an aldehyde or ketone are tautomers. 

Tautomerism is a state of equilibrium between two structural isomers that differ in the position of a mobile group, which is usually a hydrogen atom. Under normal conditions, interconversion between different tautomeric forms is rapid, and it is not possible to separate the individual tautomers, though in some cases they have been isolated and characterized at low temperatures. The difference between tautomerism and structural isomerism is one of degree, not of kind, and the particular problem which tautomerism poses for chemical structure information systems is the decision as to when different tautomers should be regarded as the same compound and when as different compounds there is also the problem of how they may be related to each other automatically. 

Recall that the ketone and enol shown are tautomers—rapidly interconvertmg constimtional isomers that differ from each other in the placement of a proton and the position of a double bond. Do not confuse tautomers with resonance structures. The two structures above are not... 

This type of isomerization, occurring by the migration of a proton and the movement of a double bond, is called tautomerism, and the isomers that interconvert are called tautomers. Don t confuse tautomers with resonance forms. Tautomers are true isomers (different compounds) with their atoms arranged differently. Under the right circumstances, with no catalyst present, either individual tautomeric form may be isolated. Resonance forms are different representations of the same structure, with all the atoms in the same places, showing how the electrons are delocalized. 

Note the difference between tautomers and resonance forms Tautomers are different compounds (isomers) with different structures, while resonance forms are different representations of a single structure. Tautomers hatj their atoms arranged differently, while resonance forms differ only in the" position of their electrons. Note also that tautomers are rapidly interconvertible. Thus, keto and enol isomers are tautomers, but alkene isomers such as 1-butene and 2-butene are not, because they don t interconvert rapidly under normal circumstances. 

Tautomers are isomers. They have different cr bond networks, which clearly distinguishes them from resonance structures. The most important kinds of tautomers are carbonyl-enol tautomers, as in the preceding example. Tautomerization is a chemical equilibrium that occurs very rapidly in acidic or basic media it should not be confused with resonance, which is not an equilibrium at all. 

Isomers such as tautomers that can readily inter-convert represent another type of structural equivalence that is encountered frequently in databases of chemical structures. Although the different forms are distinct molecular species, techniques used for processing mesomers are equally applicable to tautomers. ... 

The enol and ketone are said to be tautomers, which are constitutional isomers that rapidly interconvert via the migration of a proton. The interconversion between an enol and a ketone is called keto-enol tautomerization. Tautomerization is an equilibrium process, which means that the equilibrium will establish specific concentrations for both the enol and the ketone. Generally, the ketone is highly favored, and the concentration of enol will be quite small. Be very careful not to confuse tautomers with resonance structures. Tautomers are constitutional isomers that exist in equilibrium with one another. Once the equilibrium has been reached, the concentrations of ketone and enol can be measured. In contrast, resonance structures are not different compounds and they are not in equilibrium with one another. Resonance structures simply represent different drawings of one compound. 

MP2/6-31-l-G calculations in the gas phase indicate that 2//-1,2,3-triazole is about -5 kcal/mol more stable than the H isomer [92JOC3698]. The energy differences between 1-hydroxy-l,2,3-triazole 56a and its 2H (56b) and 3// (56c) tautomers were investigated up to the MP4(SDTQ)/6-31 - -G level. The 1 -hydroxy form 56a is the preferred tautomer in the gas phase, but owing to the strong polarity of the V-oxide 3// tautomer 56c, this is the most stable structure in solution (Scheme 37) [92JOC3698]. 

Problem 1.10 a. The first structure is the same compound as the last the second is the same as the fourth. The first, second, and third structures meet the basic requirement for tautomers interconversion involves only movement of a double bond and one hydrogen atom. However, most chemists would not call them tautomers because the allylic proton that moves is not very acidic. Its can be estimated from the of the allylic proton of 1-propene 47.1-48.0 (from Appendix C). Thus, the intermediate anion necessary for the interconversion of these isomers would be formed only under extremely basic conditions. When the equilibration is this difficult to effect, the different isomers usually are not called tautomers. On the other hand, if another double bond were added to the structure, the compound would be veiy acidic (from Appendix C the related cyclopentadiene has a... 

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