Question

Why are two equivalents of benzaldehyde and one equivalent of acetone used in a aldol condensation? Why not 1 to 1? What would the products be if 1:1 used?

Answer 1

Benzaldehyde reacts with Acetone in presence of base to form Aldol.

First part:

In presence of base, an an ion is generated on the -CH3 group of acetone and this an ion adds across carbonyl group of Benzaldehyde  by a nucleophilic addition which results in the formation of Aldol and it subsequently eliminates one molecule of water to form an unsaturated ketone.

Second part:

The other -CH3 group in the above unsaturated ketone again ionizes to form a -CH2 anion which reacts with a second molecule of Benzaldehyde by a nucleophilic addition way to form Aldol and it again eliminates a molecule of water to form final product.

Therefore, two molecules of Benzaldehyde with one molecule of Acetone are required for completion of reaction.

A mechanistic scheme is attached herewith