Aldol Reactions and Aldol-like Reactions
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 ( values: 14-20), 1,3-diketones or β-keto carbonyl compounds are much stronger acids ( 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 ( 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.