Enolate ion forming

Only the a hydrogens are replaced by deuterium m this reaction The key intermediate IS the enolate ion formed by proton abstraction from the a carbon atom of cyclopen tanone Transfer of deuterium from the solvent D2O to the enolate gives cyclopentanone containing a deuterium atom m place of one of the hydrogens at the a carbon... 

In contrast to 10-104 ordinary esters react quite well, that is, two Z groups are not needed. A lower degree of acidity is satisfactory because it is not necessary to convert the attacking ester entirely to its ion. Step 1 is an equilibrium that lies well to the left. Nevertheless, the small amount of enolate ion formed is sufficient to attack the readily approachable ester substrate. All the steps are equilibria. The reaction proceeds because the product is converted to its conjugate base by the base present (that is, a P-keto ester is a stronger acid than an alcohol) ... 

Enolization is an acid-base reaction (2-24) in which a proton is transferred from the a carbon to the Grignard reagent. The carbonyl compound is converted to its enolate ion form, which, on hydrolysis, gives the original ketone or aldehyde. Enolization is important not only for hindered ketones but also for those that have a relatively high percentage of enol form, e.g., p-keto esters, etc. In reduction, the carbonyl compound is reduced to an alcohol (6-25)... 

The mechanism is illustrated in the simple case of the self-condensation of an aldehyde in the presence of base. Here the nucleophilic, mesomerically stabilised a-carbanion (the enolate ion) formed by the action of base, attacks the electrophilic carbonyl carbon of a second aldehyde molecule to form, after proton exchange with the solvent, the / -hydroxyaldehyde. 

Enolate ions formed from, ketones or aldehydes are extremely important in the synthesis of more complex organic molecules. The ease with which an enolate ion is formed is related to the acidity of the a proton. The pKa of propane (acetone) is 19.3 that means that it is a stronger acid compared to ethane (pKa 60) and a much weaker acid than acetic acid (pKa 4.7), i.e. strong bases like sodium hydride, sodium amide, and lithium diisopropylamide LiN(i-C3H7)2 are needed to form an enolate ion. 

Figure 9 shows a proposed mechanism (24) to account for the product distribution data given in Fig. 7. In very dilute base, the homoannular enolate (XX) is formed first and, as it forms, is preferentially adsorbed on the catalyst in the cis configuration. Hydrogenation of this species with proton transfer from the solution will give predominantly the trans product. In these very dilute basic solutions, competitive hydrogenation of the neutral molecule also occurs. As the base concentration increases more of the homoannular enolate is formed, adsorbed, and hydrogenated. At the base concentration corresponding to the first breakpoint in Fig. 7, the number of enolate ions formed equals the number of active sites on the catalyst and, hence, there is a dependency on the quantity of catalyst at this point. In more concentrated base solutions, more homoan-...

Another base-catalyzed side reaction is the enediol rearrangement, which moves the carbonyl group up and down the chain, as shown in Mechanism 23-3. If the enolate ion (formed by removal of a proton on C2) reprotonates on the Cl oxygen, an enediol intermediate results. Removal of a proton from the C2 oxygen and reprotonation on Cl gives fructose, a ketose. 

Plot logA, against ApK, (pA", - pA/ ) for the reaction of acids (AH) with the enolate ion form of acetylacetone (SH) using data from Table 3 Comment on the origin of the curvature. 

Integration of the bioluminescence spectra for the three forms of oxyluciferin under the assumption that the bioluminescence quantum yields of all the three emitter forms are equal gave the relative content of each form at different pH values. As can be seen from Fig. 2, at pH > 7.0 the enolate-ion form prevails in the wild-type luciferase, whereas the ketone and enol forms prevail in the mutant luciferase. The relative content of the enol reaches its maximum at pH 7.0 for the wild-type luciferase and at pH 8.6 for the mutant one. Thus, it can be concluded that the observed changes in bioluminescence spectra of the mutant luciferase are due to shifts in the ketone enol enolate equilibria. 

The planar enolate ion formed from A can be protonated from either side, resulting in racemization. B does not racemize because the chiral center is not the a-carbon. 

However, the bromine on the a-carbon of carboxylate ions can be replaced by basic nucleophiles, because carboxylate ions do not form enolate ions. Forming an enolate ion would require putting a second negative charge on the compound. 

This enzyme catalyses the decarboxylation of the ) -ketoacid oxaloacetate, with the same stoichiometry as acetoacetate decarboxylase. The former however, requires a Mn ion for activity and is insensitive to the action of sodium borohydride. This duality of mechanism is not unlike the one observed for enzymatic aldol condensation, where enzymes of Class 1 react by forming Schiff-base intermediates, whereas enzymes of Class II show metal ion requirements [47]. Oxaloacetate decarboxylase from cod also catalyses the reduction by borohydride of the enzymatic reaction product pyruvate. This is evidenced by the accumulation of D-lactate in presence of enzyme, reducing agent, and manganous ions. It has been proposed that both reduction and decarboxylation occur by way of an enzyme-metal ion-substrate complex in which the metal ion acts as an electron sink, thereby stabilizing the enolate ion formed in the decarboxylation reaction [48] ... 

Why do carboxylic acids dissociate to a greater extent than do alcohols The difference is that the hydroxy snbstitnent of a carboxylic acid is attached to a carbonyl gronp, whose positively polarized and -hybridized carbonyl carbon exerts a powerfnl electron-withdrawing indnctive effect. In addition, the carboxylate ion is signihcantly stabilized by resonance, mnch as is the enolate ion formed by deprotonation of the a-carbon in aldehydes and ketones (Section 18-1). 

The stabilized enolate ion (formed in the first step) can function as a base, rather than a nucleophile, giving an E2 reaction ... 

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