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15.15 : Base-Catalyzed Aldol Addition Reaction

As depicted in Figure 1, base-catalyzed aldol addition involves adding two carbonyl compounds in aqueous sodium hydroxide to form a β-hydroxy carbonyl compound.

Cannizzaro reaction diagram; glyoxal, NaOH, formaldehyde; aldol condensation product at 5°C.

Figure 1: The base-catalyzed aldol addition reaction of aldehydes.

The reaction preferentially occurs with simple aldehydes, where the α carbon is monosubstituted. The equilibrium of the reaction involving disubstituted aldehydes and ketones shifts backward to the reactants due to the steric interactions at the α carbon. The trisubstituted aldehydes with no α hydrogen atom are unsuitable for the aldol addition reaction.

The mechanism occurs in three distinct steps: enolization, nucleophilic addition, and protonation. Firstly, as pictured in Figure 2, the base deprotonates the α carbon of the aldehyde to generate an enolate ion.

Organic chemistry mechanism diagram; arrow-pushing; electron movement; acetal formation.

Figure 2: The enolization step in the mechanism of the base-catalyzed aldol addition reaction

Figure 3 captures the subsequent nucleophilic addition step. Here, the enolate ion functions as a nucleophile and attacks the carbonyl group of the unreacted aldehyde to form an alkoxide ion intermediate.

Organic reaction mechanism diagram; keto-enol tautomerism; arrow-pushing; hydrogen transfer.

Figure 3: The nucleophilic addition step in the base-catalyzed aldol addition reaction mechanism

Finally, as illustrated in Figure 4, the alkoxide ion is protonated to form a β-hydroxy aldehyde as the aldol product. The aldehyde and alcohol functional groups present in the product give the name ‘aldol’ to the reaction.

Organic reaction mechanism, oxidation of alcohols to aldehydes, diagram with molecular structures.

Figure 4: The protonation step in the mechanism of the base-catalyzed aldol addition reaction

Tags

Base catalyzed Aldol AdditionCarbonyl Compoundshydroxy Carbonyl CompoundEnolizationNucleophilic AdditionProtonationEnolate IonAlkoxide IonAldol Product

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