15.14:
C–C Bond Formation: Aldol Condensation Overview
Aldol condensation involves an aldol addition reaction followed by dehydration.
In the aldol addition reaction, two identical aldehydes interact in acidic or basic conditions to form a β-hydroxy aldehyde as the initial product. Here, one aldehyde acts as a nucleophile and the other as an electrophile, creating a new carbon–carbon bond.
The name aldol comes from the presence of both alcohol and aldehyde functional groups — 'ald' representing aldehyde and 'ol' corresponding to alcohol.
Like aldehydes, two identical ketones also undergo an aldol addition reaction generating a β-hydroxy ketone.
The formation of the β-hydroxy carbonyl compound is reversible.
However, under suitable reaction conditions, the β-hydroxy aldehyde and ketone dehydrate to yield an enal and an enone, respectively.
An aldol condensation between two different carbonyl compounds is called a crossed aldol reaction. Such a reaction yields a mixture of products.
15.14:
C–C Bond Formation: Aldol Condensation Overview
Aldol condensation is an important route in synthetic organic chemistry used to generate a new carbon–carbon bond under basic or acidic conditions. The aldol condensation reaction presented in Figure 1 constitutes an aldol addition reaction followed by the dehydration process.
Figure 1. The general aldol addition reaction of aldehydes.
Aldol addition reactions are reversible and are of two types: self-addition and crossed-addition. Combining two identical carbonyl compounds is called self-addition. As shown in Figure 2, the reaction between two different carbonyl compounds is called crossed-addition. Of the two carbonyl compounds involved in the reaction, one functions as a nucleophile and the other as an electrophile.
Figure 2. The crossed aldol addition reaction of aldehydes.
The two types of aldol addition reactions produce a β-hydroxy carbonyl as the aldol addition product. While a self-addition reaction yields a single aldol product, a crossed-addition results in a mixture of products, decreasing the reaction's usefulness in organic chemistry. Accordingly, the choice of reactants is paramount in defining the efficacy of the reaction.
Figure 3 depicts the subsequent dehydration of a β-hydroxy carbonyl compound under suitable reaction conditions to form the corresponding condensation product.
Figure 3. The dehydration reaction of aldols.