Keto-enol tautomerization reactions acid-catalyzed ketonization

The initial product has a hydroxy group attached to a carbon-carbon double bond. Compounds such as this are called enols (ene + ol) and are very labile—they cannot usually be isolated. Enols such as this spontaneously rearrange to the more stable ketone isomer. The ketone and the enol are termed tautomers. This reaction, which simply involves the movement of a proton and a double bond, is called a keto—enol tautomerization and is usually very fast. In most cases the ketone is much more stable, and the amount of enol present at equilibrium is not detectable by most methods. The mechanism for this tautomerization in acid is shown in Figure 11.6. The mercury-catalyzed hydration of alkynes is a good method for the preparation of ketones, as shown in the following example ... 

The mechanism of the acid-catalyzed aldol reaction involves an initial acid-catalyzed keto-enol tautomerization to provide the enol form protonation of a second molecule on the carbonyl oxygen creates an electrophilic oxonium ion that is then attacked by the nucleophilic enol, followed by loss of a proton to give the j8-hydroxy aldehyde or ketone product. 

Tautomerizations involve the shift of a hydrogen atom across a tt system. The most typical tautomerization is a 1,3-shift, and the focus of this section is the interconversion of a ketone (or aldehyde) and an enol, often termed keto-enol tautomerization. The reaction can be catalyzed by acid or base, and it is technically an isomerization, a class of reactions we will cover later in this chapter. However, knowledge of the mechanism of keto-enol tautomerizations is crucial to understanding enol and enolate chemistry, and therefore we cover it here. 

Catalyzed by Acid and Mercuric Salts Hydration of a terminal alkyne is a convenient way of making methyl ketones. This reaction is catalyzed by a combination of sulfuric acid and mercuric ion. The initial product of Markovnikov hydration is an enol, which quickly tautomerizes to its keto form. Internal alkynes can be hydrated, but mixtures of ketones often result. 

When an alkyne undergoes the acid-catalyzed addition of water, the product of the reaction is an enol. The enol immediately rearranges to a ketone. A ketone is a compound that has two alkyl groups bonded to a carbonyl (C=0) group. An aldehyde is a compound that has at least one hydrogen bonded to a carbonyl group. The ketone and enol are called keto-enol tautomers they differ in the location of a double bond and a hydrogen. Interconversion of the tautomers is called tautomerization. The keto tautomer predominates at equilibrium. Terminal alkynes add water if mercuric ion is added to the acidic mixture. In hydroboration-oxidation, H is not the electrophile, H is the nucleophile. Consequently, mercuric-ion-catalyzed addition of water to a terminal alkyne produces a ketone, whereas hydroboration-oxidation of a terminal alkyne produces an aldehyde. 

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