2,4-Pentanedione, enol tautomerization

Figure 1.11. NMR analysis of the keto-enol tautomerism of 2,4-pentanedione [CDCIa, 50% v/v, 25 °C, 60 MHz for H, 20 MHz for C]. (a) H NMR spectrum with integrais [resuit keto enoi = 13 87] (b) H broadband de-coupied C NMR spectrum (c) C NMR spectrum obtained by inverse gated H decoupiing with integrals [result keto enol = 15 85 ( 1)]...

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. 

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... 

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.

Pentanedione (acac) and related jS-dicarbonyl compounds are an extremely important class of hgands that have been studied widely for many years. In general, jS-dicarbonyl compounds exist as mixtures of tautomeric keto and enol forms (equation 15). These compounds are usually easily deprotonated to form monoanions, which form the basis for a large class of coordination compounds, encompassing vir-tuaUy every element. In addition, coordination compounds of the dianions and trianions of jS-diketones have been observed, as well as complexes of the neutral molecules. ... 

This second enolic form of 2,3-pentanedione is noted by Ho et al. (1993) in the volatile compounds of a roasted Columbian coffee (see D.44), with a concentration of 1.43 ppm. The other enolic form D.45B is also present (1.66 ppm) as well as the diketone D.45A (0.73 ppm). There is no comment about the presence of the three tautomeric forms. 

For example, chemical analyses and measurements of the absorption spectra of 2,4-pentanedione, CH3COCH2COCH8 (acetylacetone), by Grossman and by Acly and French, indicate that the enol form is present in water at 25° to the extent of 15 to 20 per cent and in 95 per cent alcohol to the extent of 55.7 per cent. The tautomerism has also been studied by means of the Raman effect, by exchange reactions with deuterium, and by polarographic oxidation-reduction measurements. Insofar as 2,4-pentanedione and other 1,3-diketones exist in the enol form, they probably... 

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. 

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. 

---