Clarke Earley, Ph.D.
Department of Chemistry
Kent State University Stark Campus
North Canton, OH 44720
The aldol condensation is an important reaction in organic chemistry, primarily because this reaction is one of a limited number of reactions that result in the formation of a new carbon-carbon bond. This reaction is also relevant to biochemistry. For example, an aldol condensation reaction occurs in the synthesis of glucose, and the reverse of this reaction occurs in the catabolism (breakdown) of glucose.
The aldol reaction requires two molecules, both of which may or may not be the same. The first molecule must contain a C-H bond next to a carbonyl (C=O) group, which is referred to as an a-hydrogen. The second molecule must contain a carbonyl group.
In the first step of the aldol condensation, a strong base reacts with the a-hydrogen to produce a negatively-charged (and very reactive) carbon atom. This reactive molecule is referred to as a nucleophile ("nucleus loving").
In the second step of the aldol condensation, the carbanion produced in the first step does a nucleophilic addition to one of the carbonyl groups. This step is very similar to the first step in the reduction of aldehydes/ketones with NaBH4 and the reaction of aldehydes/ketones with alcohol.
The product of this second step is an alkoxide, which is a strong base. This base can react with a dilute acid (or solvent) to produce a neutral product. The final product contains exactly the same number of atoms as both of the two reactants.
The mechanism for this reaction explains how the aldol product is formed. The important points can be summarized as:
Examples of these reactions are shown below. While not a requirement, in all of these examples, two identical aldehydes or ketones are used as reactants.
Shown below are several aldol condensation reactions.