2,4-Pentanedione keto-enol equilibrium

Alcohols do not normally behave as acids in water, but the presence of an double bond adjacent to the OH group can substantially decrease the pATa by the mechanism of keto-enol tautomerism. Ascorbic acid is an example of this effect. The diketone 2,4-pentanedione (acetylacetone) is also a weak acid because of the keto-enol equilibrium. In... 

The potential of carbon-13 NMR in the analysis of keto-enol tautomerism has been demonstrated for 2,4-pentanedione (acetylacetone) and dimedone [293]. Quantitative evaluation of equilibrium concentrations is possible by application of the inverse gated decoupling technique illustrated in Fig. 2.23. 

Some carbonyl compounds exist predominantly in the enol form due to favorable interactions between the tautomeric end s OH group and an adjacent lone pair via a intermolecular hydrogen bond. For example, the equilibrium constant for 2,4-pentanedione in water lies in favor of the enol form and is about 4. Often, the equilibrium constant for the keto-enol tautomerism is very solvent dependent, and the keto form is more favored in less polar solvents, such as cyclohexane. 

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.

If a 3-diketone is used rather than a 3-keto ester, the result is a 4-alkylquinoline in what is known as the Combes quinoline synthesis. Reaction of aniline with acetyl acetonate (2,5-pentanedione), for example, generated enamine 258, which is in equilibrium with the imine (259). Enolization to 260 in the presence of HE was followed by cyclization to give the quinoline (261) in 96% yield.l57,l56a... 

Solution (a) Like most carbonyl compounds, 1,2 cyclo-pentanedione undergoes enolization and is found in equilibrium with its corresponding enol form. The question arises which is the predominating form, the keto or the enol ... 

Hydrates lose water to form an enol, which tautomerizes back to the carbonyl compound. It is known that most ketones exist in >99% as the keto form. It is also known that 2,4-pentanedione has a large percentage of the enol form in equilibrium with the keto form, depending on the solvent. Suggest a reason why this dione has such a high percentage of enol. 

Most monocarbonyl compounds exist almost entirely in their keto form at equilibrium, and it s usually difficult to isolate the pure enol. Cyclohexanone, for example, contains only about 0.0001% of its enol tautomer at room temperature. The percentage of enol tautomer is even less for carboxylic acids, esters, and amides. Only when the enol can be stabilized by conjugation or by intramolecular hydrogen bond formation does the enol sometimes predominate. Thus, 2,4-pentanedione is about 76% enol tautomer. Although enols are present only to a small extent at equilibrium, they are nevertheless responsible for much of the chemistry of carbonyl compounds because they are so reactive. 

P-Diketones (1,3-Diketones). Diketones with carbonyl groups located 1,3 with respect to each other may yield a more complicated pattern than those observed for most ketones (2,4-pentanedione, Fig. 2.42). These 8-diketones often exhibit tautomerization, which yields an equilibrium mixture of enol and keto tautomers. Since many 8-diketones contain large amounts of the enol form, you may observe carbonyl peaks for both the enol and keto tautomers. 

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