Question

How will you convert ethanal into but-2-enal?

Answer 1

Aldol condensation:

• Aldol condensation is a condensation reaction that is a reaction in which two molecules are combined to form a single molecule.
• In this reaction, an enol(alkenes with a hydroxyl group attached to one of the double bonds' carbon atoms) or an enolate ion(deprotonated anions of enols) reacts with carbonyl compounds.
• It forms a β-hydroxy aldehyde or β-hydroxy ketone and is followed by dehydration to give a conjugated enone(a functional group in which a ketone is conjugated with an alkene).

Aldol Condensation in Aldehydes with Base as a Catalyst:

Step 1: The α-Hydrogen from the base is removed
Step 2: Then the carbanion is formed which undergoes a nucleophilic addition reaction with the carbonyl group present in the second molecule of the aldehyde Step 3: Then the formation of the condensation product.
Step 4: Protonation of alkoxide ions will occur due to a reaction with water.
Step 5: The heating of the aldol compound in the basic solution will help in dehydrating the product to form $\mathrm{\alpha },\mathrm{\beta }$‐ unsaturated aldehyde compound.

Ethanal to but-2-enal:

Step 1 Formation of But-2-enal:

• Here, the α-Hydrogen from the base is removed and carbanion is formed which undergoes a nucleophilic addition reaction with the carbonyl group present in the second molecule of the aldehyde
• Ethanal on treatment with dilute alkali undergoes Aldol condensation and the aldol compound in the basic solution will help in dehydrating the product to form the But-2-enal compound.