Keto-enol tautomerism carbanions

Note that the final product in Figure 15-2 is stabilized as illustrated in Figure 15-3. This is an example of stabilization by keto-enol tautomerization (see Chapter 11 to review). The driving force is the result of the stability of the carbanion as shown in Figure 15-1. In this example of condensation, two ester molecules join, and alcohol, a smaller molecule, splits out. 

Keto-acids, decarboxylation, 286 Keto-enol tautomerism, 201, 219, 225 carbanions in, 278 catalysis of, 277... 

We shall take up first the behavior of ketones toward the halogens, and see evidence that carbanions do indeed exist at the same time, we shall see an elegant example of the application of kinetics, stereochemistry, and isotopic tracers to the understanding of reaction mechanisms. And while we are at it, we shall see something of the role that keto-enol tautomerism plays in the chemistry of carbonyl compounds. 

Alpha hydrogens are hydrogens on carbons directly attached to a carbonyl group. They are weakly acidic and can be abstracted by base to form a carbanion. The carbanion is called an enolate ion and is resonance stabilized. Neutralization of the enolate ion results in an enol, a compound in which an alcohol group is directly bonded to a carbon involved in a carbon-carbon double bond. The enol is in equilibrium with the original aldehyde or ketone in an equilibrium referred to as keto-enol tautomerism. The equilibrium usually favors the keto form. 

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