2.4- Pentanedione enol content

The equilibrium position between a simple ketone and its enol usually lies far on the side of the ketone (see Table 17-2). However, there are some interesting and important exceptions to this generalization. For instance, the influence of two carbonyl groups on the enol content is very striking, as we can see from the fact that 85% of 2,4-pentanedione is the enol form at equilibrium ... 

On the other hand, the enol content is much higher in (B-dicarbonyl compounds than in simple aldehydes and ketones. As shown in Table 20.2, the enols of 2,4-pentanedione and ethyl acetoacetate are present in the amount of almost 20% and 7%, respectively. In these cases, structural features that stabilize the enol are important. The two most important ones are ... 

The far higher enol content in the P-dicarbonyl compounds of Table 20.1—almost 7% in ethyl acetoacetate and 20% in 2,4-pentanedione—reflects the stabilization of the enol by conjugation of the carbon-carbon double bond with the other carbonyl group plus intramolecular hydrogen bonding of the enolic OH with C=0. Both features are apparent in the structure of the enol of 2,4-pentanedione shown in Figure 20.1. Analogous structural features stabilize the enol of ethyl acetoacetate. 

Similarly, the electron-withdrawing substituent at the central atom also increases the enol content 3-chloro- and 3-ethoxycarbonyl-2,4-pentanedione are composed of 84 and 100% enol, respectively. On the other hand, the electron-releasing substituents decrease the enol content. Thus, 3-methyl-, 3-ethyl-, and 3-isopropyl-2,4-pentanedione contain 29, 28, and 0% enol at 25 °C, respectively. ... 

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