Acetylacetone keto-enol tautomerization

The reaction of metal ion M"+ with the keto, enol tautomeric mixture of acetylacetone (acacH) in acidic aqueous solution has been treated by a similar approach to that outlined above (see Prob. 16). 

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. 

Watarai H, Suzuki N. Keto-enol tautomerization rates of acetylacetone in mixed aqueous media. J Inorg Nucl Chem 1974 36 1815-1829. Great Britain Pergamon Press. 

Dicarbonyl compounds are capable of exhibiting keto-enol tautomerism this is illustrated for the case of acetylacetone. 

Keto-enol tautomerism and the enthalpy of mixing between tautomers of acetylacetone... 

The keto-enol tautomerization of acetylacetone (CH3-CO-CH2-CO-CH3), a prototype /3-diketone, has been extensively studied experimentally, and attention has been paid to its solvent effect. Although the enol form is more stable than the keto in the gas phase owing to the intramolecular hydrogen bonding, the equilibrium is known to shift toward the keto in solution as the solvent polarity increases. The tautomerization in various types of solution, which includes H2O, dimethyl sulfoxide (DMSO), and carbon tetrachloride (CCI4), was examined by means of RISM-SCF method. [18]... 

The results from the calculation of the equilibrium constants of keto-enol tautomerism for some aliphatic ketones in CCl are given in Table 7. The data for acetylacetone and 2-naphthylmethylketone are not presented in the table because in the former case the rate of reaching the equilibrium is commensurable with the rate of ozone consumption and in the latter case the ozone reacts with the double bonds in the naphthyl ring. The equilibrium constants do not differ from those found within the temperature range of 21°C to 3°C and agrees with data from the literature [59-62],... 

Figure 1 Keto-enol tautomerism equilibrium and equalization of the keto-enol bond lengths upon ligandation for acetylacetone derivatives...

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... 

K Akao, Y Yoshimura. Keto-enol tautomeric equilibrium of acetylacetone in tri-fluoromethane near the critical temperature. J Chem Phys 94 5243, 1991. 

The keto-enol tautomeric equilibrium of acetylacetone is an intramolecular hydrogen exchange process. High-pressure NMR was used to study changes in this equilibrium over a pressure range to 2.5 kbar and temperatures to 145 C (51). With an increase in temperature at constant pressure, the equilibrium distribution shifted to the keto tautomer. An increase in pressure did not change the keto-enol distribution at any temperature. From the high-pressure experiments as a function of temperature the reaction enthalpy, A/J, and entropy, AS, were determined to be 2.80 0.02 kcal/mol and 7.2 0.3 cal/K mol, respectively. 

Figure 5.72. 2D-Exchange spectrum for the keto-enol tautomerism of acetylacetone measured by NOESY. The upper spectrum was obtained at 40° C while the lower one was recorded at 28 °C. The broad signal corresponds to the water protons. The cross lines represent directly connected protons.

Watarai, H. and Suzuki, N. (1976) Keto-enol tautomerization rates of acetylacetone in water-acetonitrile and water-dimethyl sulfoxide mixtures. J. Inorg. Nucl. Chem., 38, 1683-1686. 

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.

The tautomerism of slow keto-enol and fast enol-enol tautomeric equilibria of a number of l-(2-hydroxyphenyl)-3-naphthylpropane-l,3-diones can be easily monitored also by NMR spectroscopy, making this nucleus as suitable as and for this kind of studies [diketo form 5( 0) 469 and 548ppm, respectively enol forms 5( 0) 332 to 313 ppm (higher double bond character) and i5( 0) 156 to 135 ppm (lower double bond character)] [34]. Also computed chemical shifts were included in the analysis of the extremely fast tautomeric equilibrium of the two enol forms in asymmetric 1,3-diketones [35], for example, acetylacetone [5(C=0) 473.8 ppm 5(0-H) 191.2 ppm]. The equilibrium constants fCp thus obtained were compared with earlier experimental results based on 5( 0) in model or blocked tautomeric structures. The theoretical methodology could complement some inadequacies in experimental NMR techniques in evaluating equilibrium constants of compounds with rapid dynamic exchange [35]. 

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. 

In the above structure, if Ph3P is replaced by a bidentate ligand such as bipyridyl, there results a C-rj, -acetylacetonate complex in which palladium is tr-bonded to a terminal CH2 group. These presumably exist as tautomeric keto and enol forms (equation l).53... 

Enols The familiar tautomeric equilibrium of keto and enol forms of acetylacetone is described in Section 3.8.3.1 (see Figure 3.45). The enol form predominates over the keto form under the conditions described. 

Propose mechanisms for the reactions shown in Problems 22-62 parts (a) and (b) and 22-63 parts (a) and (b). Pentane-2,4-dione (acetylacetone) exists as a tautomeric mixture of 8% keto and 92% enol forms. Draw the stable enol tautomer, and explain its unusual stability. 

The proton NMR spectrum of acetylacetone is shown in Figure 6.6. The O—H proton of the enol form can be seen very far downfield, at 5= 15.5 ppm, as well as the vinyl C—H proton at 5= 5.5 ppm. Note also the strong CH3 peak from the enol form (2.0 ppm), and compare it with the much weaker CH3 peak from the keto form (2.2 ppm). Also note that the CH2 peak at 3.6 ppm is weak. Clearly, the enol form predominates in this equdibrium. The fact the we can see the spectra of both tautomeric forms, superimposed on each other, suggests that the rate of conversion of keto form to enol form, and vice versa, must be slow on the NMR time scale. 

Since the preparation of acetylacetone and similar /J-dicarbonyl compounds in the latter half of the nineteenth century, organic chemists have had considerable interest in their properties and reactions. The best known phenomenon of -dicarbonyls is their participation in tautomeric equilibria where an apparently pure compound is actually a mixture of two substances having keto and enol structures. Early studies with / -dicarbonyls revealed that these compounds exhibited chemical properties indicative of both the dicarbonyl and unsaturated hydroxy ketone structures. In an unsuccessful attempt to explain this anomalous behavior Laar (7) in 1885 coined the word tautomerism" which has survived to the present. By the turn of the century it was generally held that the dichotomous properties of -dicarbonyls resulted from the pure compound existing as a mixture of both keto and enol tautomers. Since... 

Pentane-2. (acetylacetone) exists as a tautomeric mixture of 8% keto and 92% enol forms. Draw the stable enol... 

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