Keto-enol interconversion acid-catalyzed

MECHANISM FOR ACID-CATALYZED 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. 

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. 

Both an acidic and an alkaline environment will catalyze a shift of tiie equilibrium to the right, making the portion of the molecule actually present in the enol form higher. This equilibrium runs through a number of steps that are different depending on whether the solution is acid or alkaline. If the solution is alkaline, the keto-enol interconversion occurs via the enolate ion. In this enolate ion a proton is abstracted from either the alcohol group or the a-carbon of the enol form, and the two forms are stabilized by resonance (Figure 3.17.3). 

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. 

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. 

Step 1 Keto-enol tautomerism. The acid catalyzes interconversion of the... 

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. 

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 ... 

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... 

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. 

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. 

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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