Question

An aldol addition can be catalyzed by acids as well as by bases. Propose a mechanism for the acid-catalyzedaldol addition of propanal.

Short answer

In an aldol addition reaction, the enolate ion of an aldehyde or a ketone reacts with the carbonyl carbon of another molecule of aldehyde or ketone, which results in the formation of a -hydroxy aldehyde or a -hydroxy ketone. The new C ¬ C bond forms between the -carbon of one molecule (the nucleophile) and the carbonyl carbon. Therefore, the carbon attached to OH is adjacent to the carbon attached to the carbonyl group.

Step-by-step solution

Aldol addition reaction

In an aldol addition reaction, the enolate ion of an aldehyde or a ketone reacts with the carbonyl carbon of another molecule of aldehyde or ketone, which results in the formation of a -hydroxy aldehyde or a -hydroxy ketone. The new C ¬ C bond forms between the -carbon of one molecule (the nucleophile) and the carbonyl carbon. Therefore, the carbon attached to OH is adjacent to the carbon attached to the carbonyl group.

Mechanism of the reaction

Kinetic enolate formation

When the temperature is low, LDA will remove the-H which is less substituted.

The reaction and the products are given below: