Organic Chemistry MCAT Practice Exam

What reaction occurs when an enolate reacts with a ketone or aldehyde in aldol condensation?

1. Nucleophilic substitution

2. Elimination

3. Hydrolysis

4. Rearrangement

The correct answer is: Nucleophilic substitution

In the aldol condensation, when an enolate reacts with a ketone or aldehyde, the primary reaction that occurs involves the nucleophilic attack of the enolate on the carbonyl carbon of the ketone or aldehyde. This forms a beta-hydroxy ketone or aldehyde, which is sometimes referred to as an aldol. The enolate serves as a nucleophile due to the presence of a negatively charged carbon atom (the alpha carbon) which is highly reactive toward the electrophilic carbonyl carbon of the aldehyde or ketone. This nucleophilic attack is essential because it establishes new carbon-carbon bonds, leading to the formation of a more complex molecule. Following the initial formation of the aldol product, further dehydration may occur, resulting in the elimination of a water molecule, which leads to the formation of an alpha, beta-unsaturated carbonyl compound. This step is crucial in aldol condensation reactions, but it follows from the initial nucleophilic attack. Thus, the first key step in the aldol condensation involves nucleophilic substitution, where the enolate acts as the nucleophile and forms a new C-C bond with the carbonyl carbon of the ketone or aldehyde.