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.