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

The products formed in this reaction are the aldol addition product, which is a β-hydroxy ketone, and its subsequent dehydration product, an α,β-unsaturated carbonyl compound. The structures of these products are shown below: Aldol Addition Product:  Dehydration Product: 

Step-by-step solution

Identify the enolate donor and carbonyl acceptor

First, let's identify the enolate donor and carbonyl acceptor. In this crossed aldol reaction between benzaldehyde and 3-pentanone, 3-pentanone is the enolate donor, as it has an α-hydrogen that can be deprotonated by the base to produce an enolate ion. Benzaldehyde will act as the carbonyl acceptor as it has a carbonyl group that can accept the nucleophilic attack from the enolate ion.

Draw the enolate intermediate

Now let's draw the enolate intermediate produced by deprotonation of the enolate donor (3-pentanone) by a base. In the base-catalyzed crossed aldol reaction, a base such as hydroxide ion (OH⁻) or alkoxide ion (RO⁻) will abstract the α-hydrogen from 3-pentanone, generating a resonance-stabilized enolate ion, as shown below: 3-pentanone + Base → Enolate Intermediate Here's the enolate intermediate: 

Draw the aldol addition product

Now, let's draw the aldol addition product formed by the nucleophilic attack of the enolate intermediate on the carbonyl acceptor (benzaldehyde). The enolate ion reacts with the electrophilic carbonyl carbon in benzaldehyde, forming a new carbon-carbon bond and an alkoxide ion. The alkoxide ion will then abstract a proton from the solvent or another molecule of the enolate donor, yielding the aldol addition product. Enolate Intermediate + Benzaldehyde → Aldol Addition Product Here's the aldol addition product: 

Draw the dehydration product

Finally, let's draw the α,β-unsaturated carbonyl compound formed by eliminating water from the aldol addition product. In the presence of a base, the oxide ion (from the base) will abstract the α-hydrogen from the aldol product, leading to the formation of a new enolate ion. This enolate then loses a hydroxide ion, resulting in the formation of an α,β-unsaturated carbonyl compound. Aldol Addition Product + Base → Dehydration Product Here's the dehydration product:  The final products are the aldol addition product and its corresponding dehydration product.