Keto-enol interconversion base-catalyzed

MECHANISM FOR BASE-CATALYZED KETO-ENOL INTERCONVERSION... 

Now that we know that a hydrogen on a carbon adjacent to a carbonyl carbon is somewhat acidic, we can understand why keto and enol tautomers interconvert as we first saw in Chapter 6. Keto-enol interconversion is also called keto-enol tautomer-ization or enolization. The interconversion of the tautomers can be catalyzed by either acids or bases. 

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... 

Keto-enol interconversion can be catalyzed by acids or by bases. Generally, the keto tautomer is more stable. 

If it is possible to measure the actual rate of formation of an enol from its keto isomer (or vice versa), then of course two successive proton transfers are involved in catalysis by both acids and bases. It is rarely feasible to study this interconversion directly, but it is sometimes possible to obtain indirect evidence about its kinetics. The typical scheme for the base-catalyzed conversion of keto to enol is... 

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 interconversion between an enol and a ketone is called keto-enol tautomerization and is catalyzed by trace amounts of acid or base. 

The base-catalyzed interconversion of aldo and keto sugars. The key intermediate is the double enol formed by protonation of an enolate. 

We have already encountered several base-catalyzed reactions, such as the interconversion of keto and enol tautomers (Section 18.3), the Claisen condensation (Section 18.13), and the enediol rearrangement (Section 21.5). A base catalyst increases the rate of a reaction by removing a proton from the reactant. For example, the dehydration of a hydrate in the presence of hydroxide ion is a base-catalyzed reaction. Hydroxide ion (the base) increases the rate of the reaction by removing a proton from the neutral hydrate. 

Both the acid- and base-catalyzed enol-keto interconversions occur rapidly in solntion whenever there are traces of the reqnired catalysts. Remember that although the keto form (usually) predominates, the enol-to-keto conversion is reversible and the mechanisms by which the keto form equilibrates with its enol connterpart are the exact reverse of the preceding two schemes. 

Interconversion of the enol and keto forms of ethyl 3-oxobutanoate is powerfully catalyzed by bases through the anion, 15, and less so by acids ... 

The interconversion of keto and enol isomers takes place spontaneously, but slowly. It is efficiently catalyzed by acids and bases. Mechanism 20.4 shows the mechanism that operates in aqueous base. Mechanism 20.5 shows the mechanism in aqueous acid. 

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... 

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