Carbonyl-enol tautomerizations tautomers

Reprotonation occurs on the nitrogen to produce the amide. As was the case with the carbonyl—enol tautomerization, the stability of the carbon—oxygen double bond causes the amide tautomer to be favored 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. 

Carbonyl compounds are in a rapid equilibrium with called keto-enol tautomerism. Although enol tautomers to only a small extent at equilibrium and can t usually be they nevertheless contain a highly nucleophilic double electrophiles. For example, aldehydes and ketones are at the a position by reaction with Cl2, Br2, or I2 in Alpha bromination of carboxylic acids can be similarly... 

If a carbonyl or carboxyl compound that is capable of enolization, is to undergo complete reaction with an electrophile via its enol tautomer, it must be ensured that the latter is constantly resupplied from the carbonyl or carboxyl tautomer under the reaction conditions. If the tautomerization, which this process requires, occurs faster than the further reaction of the enol, the enol will be continuously available for the electrophile to react to completion—and always at the respective equilibrium concentration. But if the tautomerization is the slower reaction of the two, the enol will be depleted to less than its equilibrium concentration in the extreme case, it may even be totally consumed, and it will take some time before new enol will have formed. 

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. 

This type of compound is called an enol, and the reorganisation of the carbon/ oxygen double bond in the carbonyl group to the carbon/carbon double bond in the enol compound is called the keto/enol tautomerism. Notice, also, that this is accompanied by a hydrogen atom changing place, from the carbon to the oxygen. Tautomers are different from canonical structures in that the former... 

Notice that the steps in the enol — acetaldehyde reaction are simply the reverse of the acetaldehyde — enol reaction (Fig. 19.15). Note also that in acid, as in base, aldehydes and ketones that have a hydrogens are in equilibrium with their enol forms. We will soon see that although enols are in equilibrium with the related keto forms, it is usually the keto forms that are favored. This equilibrium is called the keto-enol tautomerization.The carbonyl compound and its associated enol are called tautomers. 

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. 

Keto-enol tautomerism encountered in ethyl acetoacetate (Figure 3.40) is another slow exchange process. It results in a small contribution of the enol tautomer of ethyl acetoacetate to the spectrum, the enol form being partly stabilized due to conjugation of the double bond of the enol with the a-carbonyl group of the ester moiety. 

Carbons involved in keto-enol tautomerism can experience large cbemical shift differences according to the tautomer present. For example, for acetylace-tone the carbonyl carbon resonates at 201.1 ppm in the keto form but is at 190.5 ppm in the enol form. Similarly, the carbon which is a methylene group in the keto form with a shift of 56.6 ppm moves to 99.0 ppm as an olefinic carbon in the enol form. 

Figure 3.6 Top Keto-enol tautomerization in acetone. The equilibrium for this molecule strongly favors the keto-tautomer. Bottom The tautomeric equilibrium for 2,4-pentanedione (acetylacetone) favors the enol form because the double bond is conjugated with the remaining carbonyl group and the hydrogen of the enol can form a hydrogen bond with the carbonyl oxygen that creates a very stable six-membered ring.

Although the conversion of an aldehyde or a ketone to its enol tautomer is not generally a preparative procedure, the reactions do have their preparative aspects. If a full mole of base per mole of ketone is used, the enolate ion (10) is formed and can be isolated (see, e.g., 10-105). When enol ethers or esters are hydrolyzed, the enols initially formed immediately tautomerize to the aldehydes or ketones. In addition, the overall processes (forward plus reverse reactions) are often used for equilibration purposes. When an optically active compound in which the chirality is due to an asymmetric carbon a to a carbonyl group (as in 11) is treated with acid or base, racemization results. If there is another asymmetric center in the molecule. 

This enediol can be regarded as a common enol tautomer for two different keto structures. In other words, there are two ways in which this enediol can tautomerize back to a keto form, and the reaction thus appears to shift the position of the carbonyl group. The reaction is enzyme catalysed, which allows the normal equilibrium processes to be disturbed. 

The pyridine ring system may carry snbstituents, jnst as we have seen with benzene rings. We have enconn-tered a nnmber of snch derivatives in the previons section. Hydroxy or amino heterocycles, however, may sometimes exist in tautomeric forms. We have met the concept of tantomerism primarily with carbonyl componnds, and have seen the isomerization of keto and enol tantomers (see Section 10.1). In certain cases, e.g. 1,3-dicarbonyl componnds, the enol form is a major component of the eqnilibrinm mixtnre. In the example shown, liquid acetylacetone contains about 76% of the enol tautomer. 

Although usually less stable than the oxo (keto) form, the enol is present in a small amount. It is formed readily from the oxo tautomer by virtue of the fact that hydrogen atoms attached to carbon atoms that are immediately adjacent to carbonyl (C=0) groups are remarkably acidic. Easy dissociation of a proton is a prerequisite for tautomerism. Since most hydrogen atoms bound to carbon atoms do not dissociate readily, tautomerism is unusual unless a carbonyl or other "activating group" is present. 

Due to the properties of the cx-hydrogen and carbonyl ketones and aldehydes exist at room temperature as enol tautomers. Tautomerization involves a proton shift, in this case from the a-carbon position to the carbonyl oxygen position. Both tautomers exist at room temperature, but the ketone or aldehyde tautomer is usually favored. Tautomerization is a reaction at equilibrium, not a resonance. (Remember, in resonance structures atoms don move and neither resonance structure actually exists.)... 

Ketones that have hydrogen atoms on their a-carbon (the carbon next to the carbonyl group) are in rapid equilibrium with an isomeric structure known as an enol in which the a-hydrogen ends up on the oxygen instead of the carbon. The two isomeric forms are known as tautomers and the process of equilibrium is known as tautomerism (Following fig.). Generally the equilibrium favours the keto tautomer and the enol tautomer may only be present in very small quantities. 

This compound is a tautomer of an amide. Tautomerization occurs in the same manner as was the case for the conversion of an enol to its carbonyl tautomer. 

Numerous applications of IR spectral data have been reported for derivatives of (5,5)-fused ring systems, especially in connection with the problem of tautomeric equilibria. For example, the IR spectra of 5,6,7,8-tetrahydro-l//,3H-pyrrolo[l,2-c][l,3]oxazole-1,3-diones (24 X=0) and corresponding thiazole-l,3-diones (24 X = S) show no carbonyl absorption, but broad bands at 3300 cm-1 due to the associated OH group, indicating that the fused structures (24) exist in the enolic forms rather than their diketonic tautomers (81BJC1844). 

Carbonyl-containing molecules, such as 1, with an a-carbon-hydrogen C(25p )-H(li) hybridized bond can exist as an enol tautomer 2, and their relative proportions depend on the relative stability of each tautomeric component (Scheme i)"4-i20 p j. saturated carbonyl-containing molecules, like 1, the amount of enol content 2 is quite low (<1% ... 

The naphthylamines may be prepared by reduction of the corresponding nitro compound, but they are readily accessible from naphthois by the Bucherer reaction The naphthol is heated, preferably under pressure in an autoclave, with ammonia and aqueous sodium hydrogen sulfite solution, when an addition-elimination sequence occurs. The detailed mechanism is not completely elucidated, but the Bucherer reaction is restricted to those phenols that show a tendency to tautomerize to the keto form, such as the naphthois and 1,3-dihydroxybenzene (resorcinol). Using 1-naphthol for illustration, the first step is addition of the hydrosulfite across the 3,4-double bond of either the enol or keto tautomer (Scheme 12.9). Nucleophilic attack by ammonia at the carbonyl group... 

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