Pentane-2,4-dione keto/enol equilibrium

The carbonyl stretching frequency of both the keto and enol tautomers can be recognized in the vibrational spectrum of pentane-2,4-dione. The enol has v(C=0) at 1618cm" , generally the dominant peak in the spectrum and more intense than the in- and out-of-phase v(C=0) stretching modes of the keto form, which are found at 1727 and 1707 cm" , respectively. These are identified by their Raman counterparts at 1719 cm" (polarized) and 1697 cm" (depolarized) (Ernstbrunner, 1970). The ratio of absorbances of the enol and the out-of-phase keto bands in the ir was used as an early method of analysis of the keto/enol equilibrium in different solvents (Le Fevre and Welsh, 1949). 

Another approach used in the empirical characterization of liquid polarity is the study of the outcome of a chemical reaction. Earle et al. [216] report a preliminary study of the keto-enol tautomerization of pentane-2,4-dione, and create an empirical correlation between the degree of tautomerization and the dielectric constant of molecular liquids. They then predict dielectric constants for a series of ILs based on the observed keto-enol equilibrium the values range from 40 to 50, slightly higher than those of short-chain alcohols. A more detailed study by Angelini et al. [217] considers the tautomerization of a nitroketone complex in a series of five imidazolium-based ILs. The results, parameterized as a linear free energy analysis of the behavior of the equilibrium constant, indicates an overall polarity comparable to that of acetonitrile, consistent with the partitioning and spectroscopic studies referenced above. 

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-pentane-dione, Fig. 2.42). These yS-diketones often exhibit tautomerization, which yields an equilibrium mixture of enol and keto tautomers. Since many yS-diketones contain large amounts of the enol form, you may observe carbonyl peaks for both the enol and keto tautomers. 

NMR spectroscopy of enols has been a key tool in determining the relative proportions of keto and enol forms in highly enolized species. For example, in pentane-2,4-dione, the keto and enol forms (Figure 17.7) can be readily identified in the NMR spectrum of the material the keto-enol tautomerism is slow compared with the timescale of the NMR spectroscopy experiment. This experiment has been extended by changing the temperature, which allows the calculation of thermodynamic parameters for the equilibrium, and the use of a range of solvents to study the effect of polarity. The process has also been studied in the gas phase, where enols predominate for almost all p-dicarbonyl compounds. In solution, the keto forms are generally some 8-9 kj moH more stable than in the gas phase. 

Of special interest is also the equilibrium between the keto and enol tautomers of pentane-2,4-dione (see the scheme VI). 

Solvents and temperature also affect the proportions of enol. Thus pentane-2,4-dione contains 80 % enol as a neat liquid, but in water, it is only 15 % enolized. The equilibrium is shifted because water hydrogen bonds so strongly to the keto form. An increase in temperature also favors the keto form, but this is not a large effect at the temperatures at which ordinary organic reactions are carried out. The rate of ketone/enol interconversion depends strongly on pH, and the individual tautomers of pentane-2,4-dione have been isolated at low temperature, under strictly neutral conditions. Other carbonyl compounds, including esters and amides, can and do enolize, but with much more difficulty. For example, neat diethylmalonate (diethyl propanedioate, 17.13) contains only 0.01 % enol at equilibrium, in contrast to the 80 % for pentane-2,4-dione. 

---