Keto-enol equilibria base catalyzed

In Summary Aldehydes and ketones are in equilibrium with their enol forms, which are roughly 10 kcal mol less stable. Keto-enol equilibration is catalyzed by acid or base. Enolization allows for easy H-D exchange in D2O and canses isomerization at stereocenters next to the carbonyl group. 

In this way, base catalyzes an equilibrium between the isomeric keto and enol forms of a carbonyl compound. For simple ketones and aldehydes, the keto form predominates. Therefore, a vinyl alcohol (an enol) is best described as an alternative isomeric form of a ketone or aldehyde. In Section 9-9F, we saw that an enol intermediate, formed by hydrolysis of an alkyne, quickly isomerizes to its keto form. 

Tautomerization is the shift of an H from a carbon adjacent to a carbon-heteroatom double bond to the heteroatom itself (and the reverse). It is an acid- or base-catalyzed equilibrium. Two examples are the keto/enol pair (Z = oxygen) and the imine/enamine pair (Z = nitrogen). Base catalysis goes via the enolate anion. 

Enolization can be either acid- or base-catalyzed, and general catalysis mechanisms have been observed. With general-base catalysis, deprotonation of the a-carbon in the first step is rate-determining. Protonation of the enolate oxygen gives the enol (Eq. 11.1). Both reactions in the second equilibrium of Eq. 11.1 are faster than the initial deprotonation of the a-carbon of the keto form. 

Keto-enol tautomerization is an equilibrium process that is catalyzed by even trace amounts of acid (or base). Glassware that is scrupulously cleaned will still have trace amounts of acid or base adsorbed to its surface. As a result, it is extremely difficult to prevent a keto-enol tautomerization from reaching equilibrium. 

Ketones and aldehydes bearing a hydrogens are in equilibrium with their enol forms, although for simple ketones and aldehydes the carbonyl forms are greatly favored. This equilibrium is the keto—enol tautomerization. Equilibration with the enol form can be either acid- or base-catalyzed. The enol form can be favored in special cases. Esters and other acid derivatives also have acidic a hydrogens. LDA is a strong base that can be used to drive ketones, aldehydes, or esters completely to their corresponding enolates. 

The aldol condensation and related reactions are among the most important, and ubiquitous, construction reactions in organic synthesis. In these condensations, the carbonyl compound acts as both nucleophile and electrophile—the enol or enolate is the nucleophile, and the keto form is the electrophile. The reaction works with enolizable aldehydes (Figure 17.24) or ketones (Figure 17.25) and may be catalyzed by either acid or base. Almost all of the steps we write are equilibria—how can we persuade the reaction to go to completion In the base-catalyzed reaction, a catalyst such as barium hydroxide is placed inside a Soxhlet thimble, as in Figure 17.26. The reaction mixture is heated so that the acetone refluxes, but the product does not. Thus only the SM, and not the product, comes into contact with the catalyst, ensuring that the reverse reaction does not occur. Note that in the acid-catalyzed processes, it is common for the product to be dehydrated under the reaction conditions—this usually pulls the equilibrium over to the product. The mechanism of the elimination reaction may be El or E2 depending on the... 

Equilibrium favors the keto form largely because a C=0 is much stronger than a C=C. Tautomerization, the process of converting one tautomer into another, is catalyzed by both acid and base. Under the strongly acidic conditions of hydration, tautomerization of the enol to the keto form occurs rapidly by a two-step process protonation, followed by deprotonation as shown in Mechanism 11.3. 

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