Acid-catalyzed reactions keto-enol interconversion

Considerations of entropies of activation played a major role in a recent discussion of the acid-catalyzed interconversion of 1,2-cyclo-hexanedione and its enol (Bakule and Long, to be published). The reaction is more complex than the usual keto-enol interconversion because the ketone form, but not the enol, is found to be hydrated. 

Keto-enol interconversion can be catalyzed by acids or by bases. Generally, the keto tautomer is more stable. When an a-substitution reaction takes place under acidic conditions, an enol reacts with an electrophile when the reaction takes place under basic conditions, an enolate ion reacts with an electrophile. Whether C or O reacts with the electrophile depends on the electrophile and the reaction conditions. 

Any reaction that simply involves the intramolecular transfer of a proton is called a tautomerism. Keto-enol interconversion may happen in basic as well as acidic solution. The steps are reversed in the base-catalyzed and acid-catalyzed reactions. In the base-catalyzed reaction, the first step is removal of an a-proton and the... 

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. 

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. 

The rate of interconversion of keto and enol form is catalyzed by both specific acid (H+ or H30" ) and specific base (HO) catalysts. The formation of enol from ketone and ketone from enol involve the same intermediates and transition states as shown in Scheme 2.1 and Scheme 2.2, respectively. The transition states involved in the interconversion of Ke and Ij, Ij and En, Ke and I2, as well as I3 and En are TSj, TS2, TS3, and TS4, respectively. Thus, in these reactions, the... 

---