Question

Draw the product of the base-catalyzed crossed aldol reaction between benzaldehyde and 3-pentanone and the product formed by its dehydration.

Short answer

Answer: The product formed by the base-catalyzed crossed aldol reaction between benzaldehyde and 3-pentanone is a β-hydroxyketone. Upon dehydration, this β-hydroxyketone undergoes elimination to form an α,β-unsaturated carbonyl compound.

Step-by-step solution

Identifying the nucleophilic and electrophilic sites

To determine the nucleophilic and electrophilic parts of the molecules involved, benzaldehyde has no enolizable hydrogens, so it can only serve as an electrophilic part (carbonyl group). As for 3-pentanone, it has enolizable α-hydrogens, which can act as nucleophiles after being deprotonated. The base-catalyzed crossed aldol reaction proceeds with the enolate ion formed by deprotonating the α-hydrogen of 3-pentanone attacking the carbonyl carbon of benzaldehyde.

Formation of the intermediate β-hydroxyketone

The nucleophilic enolate ion attacks the electrophilic carbonyl carbon of benzaldehyde and forms a new C-C bond. In this process, an intermediate β-hydroxyketone is formed. The product structure should have a hydroxyl group attached to a carbon in the β position, relative to the initial carbonyl group in 3-pentanone. The structure can be drawn as follows: 

Dehydration to form the α,β-unsaturated carbonyl compound

The β-hydroxyketone intermediate can undergo dehydration in the presence of a base to form the α,β-unsaturated carbonyl compound. This takes place by the elimination of a molecule of water, with the α-hydrogen and the β-hydroxyl group leaving. The carbonyl group reforms, and a new C=C double bond is created. The final product structure can be drawn as follows:  In conclusion, the product of the base-catalyzed crossed aldol reaction between benzaldehyde and 3-pentanone is a β-hydroxyketone, which can undergo dehydration to form an α,β-unsaturated carbonyl compound.