Keto-enol isomers interconversion

A ketone and an enol differ only in the location of a double bond and a hydrogen. Such isomers are called tautomers ( taw-toe-mers ). The ketone and enol are called keto-enol tautomers. Interconversion of the tautomers is called tautomerization. We will examine the mechanism of this reaction in Chapter 19. For now, the important thing to remember is that the keto and enol tautomers come to equilibrium in solution, and the keto tautomer, because it is usually much more stable than the enol tautomer, predominates at equilibrium. 

A carbonyl compound with a hydrogen atom on its a carbon rapidly equilibrates with its corresponding enol (Section 8.4). This rapid interconversion between two substances is a special kind of isomerism known as keto-enol tautomerism, from the Greek Canto, meaning "the same," and meros, meaning "part." The individual isomers are called tautomers. 

Many simple organic compounds exist as mixtures of two or more rapidly interconvertible isomers or tautomeric forms. Tautomers can sometimes be separated one from the other at low temperatures where the rate of interconversion is low. The classic example is the oxo-enol (or keto-enol) equilibrium (Eq. 2-1). 

Carbonyl (or keto) compounds are interconvertible with their corresponding enols. This rapid interconversion of structural isomers under ordinary conditions is known as tautomerism. Keto-enol tautomerism is catalysed by acids or bases. 

The interconversion of keto and enol isomers takes place spontaneously, but slowly. It is efficiently catalyzed by acids and bases. Mechanism 20.4 shows the mechanism that operates in aqueous base. Mechanism 20.5 shows the mechanism in aqueous acid. 

Interconversion of constitutional isomers by keto-enol tautomerism (Reactions 2 and 5). 

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. 

Tautomers are constitutional isomers related by switching the locations of a multiple bond and an atom or group. In keto-enol tautomerism these are the double bond of the carbonyl group and a proton on the carbon adjacent to C=0 (the a carbon). Tautomerism describes the relationship between a keto and enol form, tautomerization refers to their interconversion. 

Interestingly, the product actually isolated from alkyne hydration is not the vinylic alcohol, or enol (ene + ol), but is instead a ketone. Although the enol is an intermediate in the reaction, it immediately rearranges to a ketone by a process called keto-enol tautomerism. The individual keto and enol forms are said to be tautomers, a word used to describe two isomers that under spontaneous interconversion accompanied by the change in position of a hydrogen. With few exceptions, the keto-enol tautomeric equilibrium lies on the side of the ketone enols are almost never isolated. We ll look more closely at this equilibrium in Section 22.1. 

It is not necessary to protonate an enolate to obtain these two isomers. They exist in equUibrium with each other by a proton transfer reaction known as keto-enol tautoffleristn. Tautomerization describes the interconversion of two isomeric structures that differ in the location of a hydrogen atom. Tautomerization requires a change in the kinds of bonds between at least two other sets of atoms in the structures. We encountered this phenomenon in the isomerization reaction of the enol formed in the hydration of an alkyne (Chapter 7). We know that the keto form is more stable than the enol form. As we saw in the last section, this order of stabilities results primarily from the difference between the bond strengths of a carbon—oxygen double bond and a carbon—carbon double bond. 

Nudeic acid bases exist as mbctures of two or more rapidty interconvertible isomers of tautomeric forms. Tautomers, at least in prin< le, can be separated at low temperatures where the rate of interconversion is low. The keto-enol equilibrium is the classic example of tautomerism. Hie enol is present in small amount, since it is usually less stable than the keto form. Uracil (1) is one of the nucleic add bases, that exists as mixture of the tautomeric keto and enol forms (Scheme 1). The ratio of the enol tautomer to the keto tautomer of uradl, however, is small. The existence of the enol form of Sdieme 1 is the basis for referring to uracil as dihydroitypyrimidine. 

N. Blechta et al. [63] used LC-NMR experiments with H- Si indirect detection to analyze mixtures of siloxan polymers. Other studies take advantage of the unique ability of NMR to study dynamic processes like isomerization, for example, the interconversion of rotational isomers or enol-keto tautomers [64,65]. 

The keto and enol forms seen in Figure 11-2 are readily interconvertable isomers called tautomers. The interconversion of these two forms is tautomerization. Normally the keto form predominates because its carbon-oxygen double bond is more stable than the enol form s carbon-carbon double bond. 

Nonetheless, if contact with acidic and basic substances is rigidly excluded to the extent of using quartz equipment in place of glass (glass normally has a slightly alkaline surface), then interconversion is slow enough that it is possible to separate the lower-boiling enol from the keto form by fractional distillation under reduced pressure. The separated isomers are indefinitely stable when stored at —80° in quartz vessels. 

If it is possible to measure the actual rate of formation of an enol from its keto isomer (or vice versa), then of course two successive proton transfers are involved in catalysis by both acids and bases. It is rarely feasible to study this interconversion directly, but it is sometimes possible to obtain indirect evidence about its kinetics. The typical scheme for the base-catalyzed conversion of keto to enol is... 

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