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Aldol Reactions and Aldol-like Reactions

Knoevenagel Condensation

In a variation of the aldol reaction, other CH-acidic compounds are applied instead of the classical enol. Theoretically, every carbanion is capable of nucleophilically attacking the electrophilic carbonyl group of aldehydes and ketones.

Knoevenagel condensation
The condensation of aldehydes or ketones with a variety of CH-acidic compounds in the presence of a base, such as a tertiary amine or KOH, is called Knoevenagel condensation.

The reaction is named after Emil Knoevenagel (1865-1921). In comparison to aldehydes and ketones (pKa values: 14-20), 1,3-diketones or β-keto carbonyl compounds are much stronger acids (pKa values: 5-13). Thus, they can relatively easily be deprotonated to the corresponding carbanion or enolate. Nitroalkanes, depicted in the second example in the illustration below, are considerably CH-acidic compounds (pKa values about 10). As a result, they may be successfully applied in the Knoevenagel condensation. This reaction is also often called the Henry reaction. Due to the relatively high acidity of the CH-acidic compounds applied in the Knoevenagel condensation, only a relatively weak base, such as an amine or KOH, is required. Much like in aldol reactions, the addition of the carbanion to the carbonyl compound is frequently followed by dehydration.

Knoevenagel condensation.
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