The aldol reaction of a ketone under acidic conditions successfully forms an unsaturated carbonyl as the final product instead of an aldol. The acid-catalyzed aldol reaction is depicted in Figure 1.

Figure 1. The acid-catalyzed aldol addition reaction of ketones.

First, as shown in Figure 2, the acid protonates the ketone molecule to form the protonated ketone. The conjugate base of the acid deprotonates the α carbon of the protonated ketone to form the enol.

Figure 2. Formation of the enol.

Next, as shown in Figure 3, the enol functions as a nucleophile and attacks the second protonated ketone molecule to form a positively charged intermediate. Finally, the loss of a proton generates a β-hydroxy ketone as the aldol addition product, which dehydrates spontaneously under acidic conditions to form the condensation product.

Figure 3. Nucleophilic addition of the enol and dehydration of the β-hydroxy ketone.