As described previously, aldehydes and ketones enolize in base to produce a small equilibrium concentration of the corresponding enolate anion. If the concentration of carbonyl compound is sufficiently high, this enolate anion can function as a nucleophile towards the carbonyl carbon of other aldehydes in ketones in the solution. The result is the formation of a bond between the a-carbon of one mole of carbonyl compound and the carbonyl carbonyl of a second, to give a b-hydroxy aldehyde or ketone. This condensation reaction between two moles of an aldehyde or ketone is called the aldol condensation.

The reaction is general for both aldehydes and ketones, bearing at least one a-hydrogen.

The b-hydroxy carbonyl compounds which are formed are stable in base, but readily dehydrate in acid solution to give a-b-unsaturated carbonyl compounds.

As an example of the use of an aldol reaction in synthesis, consider the preparation of 2-ethyl-2-hexenal. This molecule is an a-b-unsaturated aldehyde. We can make these by the dehydration of b-hydroxy aldehydes in aqueous acid. The required b-hydroxy aldehyde, in turn can be prepared by the aldol condensation of two moles of butanal in aqueous base. To determine the structure of the required aldehyde, simply split the compound at the bond between the hydroxyl group and the a-carbon of the carbonyl, as shown on the right. As you can see, splitting like this breaks the molecule into two, four-carbon parts, both with an oxygen bonded to carbon #1.

Drawing the structure of the reacting enol-aldehyde pair, as shown above, the mechanism of the condensation becomes clear.

It is also possible to utilize two different aldehydes or ketones in an aldol-type condensation reaction. In order to minimize self-condensations, generally one reactant is chosen which has no a-hydrogens, and that reactant is maintained in large excess over the second reactant (which will have a-hydrogens and can form the enolate anion). An example of this is the synthesis of 4-phenyl-3-buten-2-one, shown on the right. This a-b-unsaturated ketone can be prepared by dehydration of the b-hydroxy ketone, as shown. Splitting this between the a-carbon and the hydroxyl group, it is evident that it can be prepared by the condensation of the enol of 2-propanone (acetone) and benzaldehyde, as shown below.

This molecule can also be prepared by the condensation of ethyl acetoacetate with benzaldehyde, followed by dehydration and decarboxylation, as shown below. This type of mixed aldol does not generate side-products due to self-condensation of the acetoacetate since the a-hydrogen of this molecule is acidic and it is converted essentially entirely to the enolate anion in the basic solution.