Relative rates of acid-catalyzed enolization for some ketones

The amount of enol present in equilibrium with a carbonyl group is a function of the substituent groups. In the case of compounds containing a single ketone, aldehyde, or ester function, there is almost none of the enol present at equilibrium. 

When two such groups are close to one another in a molecule, however, particularly if they are separated by a single carbon atom, a major amount of enol may be present. 

The enol forms of -diketones and jS-ketoesters are stabilized by intramolecular hydrogen bonding and conjugation of the carbon-carbon double bond with the other carbonyl group. Electron diffraction studies of the enol form of acetylacetone in the gas phase are consistent with a symmetrical structure having a linear O-H-O 

The accessibility of enolates and enols in, respectively, basic and acidic solutions of carbonyl compounds makes possible a wide range of reactions that depend on the nucleophilicity of such species. The reactions of enolates as nucleophiles in SN2-type processes will be discussed in Part B, Chapter 1. Enols and enolates can also act as nucleophiles toward carbonyl centers. These reactions are a major topic in Part B, Chapter 2. Both these classes of reactions are of fundamental importance in the construction of carbon frameworks in organic synthesis. 

Amino substituents on a carbon-carbon double bond also enhance the nucleophilicity at the -carbon—even more than enols. Such compounds are called enamines. Their preparation and reactions will be discussed in Chapter 8 and in Part B, Chapter 1. 

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Table 7.11. Relative Rates of Acid-Catalyzed Enolization for Some Ketones"...

Relative rates of acid-catalyzed enolization for some ketones... 

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