Enolate ions of acetaldehyde

The enolate ion of acetaldehyde attacks the carbonyl group of another acetaldehyde molecule. Protonation gives the aldol product. 

The aldol condensation is versatile in that the enolate anion of one carbonyl compound can be made to add to the carbonyl carbon of another, provided that the reaction partners are carefully selected. Consider, for example, the reaction between acetaldehyde and benzaldehyde, when treated with base. Only acetaldehyde can form an enolate anion (benzaldehyde has no a-hydrogen). If the enolate ion of acetaldehyde adds to the benzaldehyde carbonyl group, a mixed aldol condensation occurs. 

Although the carbonyl condensation reaction appears different from the three processes already discussed, it s actually quite similar. A carbonyl condensation reaction is simply a combination of a nucleophilic addition step and an -substitution step. The initially formed enolate ion of one acetaldehyde molecule acts as a nucleophile and adds to the carbonyl group of another acetaldehyde molecule, as shown in Figure 5. 

A further evidence for the extreme reactivity of acetaldehyde with its enolate ion is provided by the fact that, when the aldol condensation is run in D2O, there is no substitution of D atoms on the a carbon positions. This is interpreted as indicating a much more rapid reaction of the enolate ion with acetaldehyde than with D2O, as might have been already anticipated from the relative slowness of the reaction with BH+. In contrast, the base-catalyzed exchange of acetone with D20 is much faster than the rate of dimerization of acetone. 

Macromolecular transformations are also of scientific and commercial interest. They can be used for the manufacture of new compounds, particularly in cases where no monomer exists (vinyl alcohol as the enolic form of acetaldehyde) or where the monomer polymerizes with difficulty or not at all (e.g., vinyl hydroquinone). In these cases, derivatives such as vinyl acetate or vinyl hydroquinone ester are polymerized and the polymers are then saponified to poly(vinyl alcohol) and poly(vinyl hydroquinone), respectively. Other processes of industrial importance are conversions of inexpensive macromolecular compounds such as cellulose into new materials (cellulose acetate, cellulose nitrate, etc.), manufacture of ion exchange resins, and dyeing with reactive dyestuffs. All of these reactions lead to a definite product. If the degree of polymerization is retained, they are called polymer analog reactions. 

The enolate ion adds as a nucleophile to the carbonyl group of a second acetaldehyde, producing a tetrahedral aikoxide ion. 

In general, any molecule capable of producing an enolate ion and also possessing two ligands for chelation of a metal ion will exhibit such a catalysis. For example, it has been reported (47) that magnesium ion catalyzes the aldol condensation of pyruvate with acetaldehyde, presumably through a mechanism such as ... 

Enolate ions can also react with aldehydes and ketones by nucleophilic addition. The enolate ion acts as the nucleophile while the aldehyde or ketone acts as an electrophile. As the enolate ion is formed from a carbonyl compound itself, and can then react with a carbonyl compound, it is possible for an aldehyde or ketone to react with itself. To illustrate this we look at the reaction of acetaldehyde with sodium hydroxide. Under these conditions,... 

Ring closure reactions can be induced electrochemically in liquid ammonia. For instance, the reaction of o-iodoaniline with acetaldehyde enolate ion gives 95% yield of indole, and with acetone enolate ion 87% of 2-methylindole321. 

When the enolate of one aldehyde (or ketone) adds to the carbonyl group of a different aldehyde or ketone, the result is called a crossed aldol condensation. The compounds used Crossed Aldol in the reaction must be selected carefully, or a mixture of several products will be formed. Condensations Consider the aldol condensation between ethanal (acetaldehyde) and propanal shown below. Either of these reagents can form an enolate ion. Attack by the enolate of ethanal on propanal gives a product different from the one formed by attack of the enolate of propanal on ethanal. Also, self-condensations of ethanal and propanal continue to take place. Depending on the reaction conditions, various proportions of the four possible products result. 

The simplest enolizable aldehyde is acetaldehyde (ethanal, CH3CHO). What happens if we add a small amount of base, say NaOH, to this aldehyde Some of it will form the enolate ion. 

Dissociation of protonated dimers of acetaldehyde/ketone (homogenous or heterogenous clusters) leads to water loss [144,145], which is also the result of the corresponding ion molecule reactions [130]. Rearrangement of at least one of the reaction partners to the (protonated) enol seems likely. No comparable rates or barriers can be inferred from these data. 

Whereas the allyl anion, with a plane of symmetry through the central atom, has a node at that atom in j/ j, amides, esters, enamines, enol ethers and enolate ions do not have a node precisely on the central atom. Taking planar A V-dimethylviny-lamine and the enolate of acetaldehyde as examples, simple Hiickel calculations give the n orbitals in Fig. 2.15, which includes the allyl anion for comparison. 

Thus the gross differences in the rate laws for acetone and acetaldehyde condensation arise not from differences in reaction mechanism, but rather from differences in the relative rates of attack by enolate ion on reactant. In principle, at sufficiently low acetaldehyde concentrations, the rate law for the acetaldehyde should approach that for acetone. 

The abstraction of a-hydrogen in carbonyl compounds such as acetaldehyde by sodium hydroxide is a reversible reaction and forms an enolate ion that undergoes addition to the carbonyl carbon of another acetaldehyde molecule to give the aldol 3.13. This is called the aldol condensation " and its mechanism is shown in Scheme 3.6. 

Hydroxide ion abstracts an acidic alpha proton from one molecule of acetaldehyde, yielding an enolate ion. 

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