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Organic Chemistry

23.4 Using Aldol Reactions in Synthesis

Organic Chemistry23.4 Using Aldol Reactions in Synthesis

23.4 • Using Aldol Reactions in Synthesis

The aldol reaction yields either a β-hydroxy aldehyde/ketone or an α,β-unsaturated aldehyde/ketone, depending on the experimental conditions. By learning how to work backward, it’s possible to predict when the aldol reaction might be useful in synthesis. Whenever the target molecule contains either a β-hydroxy aldehyde/ketone or a conjugated enone functional group, it might come from an aldol reaction.

The figure shows two aldol reactants giving two aldol products. The products are a beta-hydroxy carbonyl compound and a conjugated enone.

We can extend this kind of reasoning even further by imagining that subsequent transformations might be carried out on the aldol products. For example, a saturated ketone might be prepared by catalytic hydrogenation of the enone product. A good example can be found in the industrial preparation of 2-ethyl-1-hexanol, an alcohol used in the synthesis of plasticizers for polymers. Although 2-ethyl-1-hexanol bears little resemblance to an aldol product at first glance, it is in fact prepared commercially from butanal by an aldol reaction. Working backward, we can reason that 2-ethyl-1-hexanol might come from 2-ethylhexanal by a reduction. 2-Ethylhexanal, in turn, might be prepared by catalytic reduction of 2-ethyl-2-hexenal, which is the aldol condensation product of butanal. The reactions that follow show the sequence in reverse order.

The conversion of butanal to 2-ethyl-1-hexanol uses potassium hydroxide in ethanol, forming 2-ethyl-2-hexenal. The second step forms 2-ethylhexanal using hydrogen in platinum. The final step uses nascent hydrogen.
Problem 23-5
Which of the following compounds are aldol condensation products? What is the aldehyde or ketone precursor of each?
Problem 23-6
1-Butanol is prepared commercially by a route that begins with an aldol reaction. Show the steps that are likely to be involved.
Problem 23-7

Show how you would synthesize the following compound using an aldol reaction:

Ball-and-stick model of Z-2,4-dicyclopropyl-2-butenal.
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