Aldehydes And Ketones - Aldol Condensation

Aldol condensation occurs when a carbonyl compound bearing alpha-hydrogens is treated with dilute base, generating an enolate that attacks a second carbonyl molecule. With ethanal, the base removes an alpha-hydrogen to form the enolate, which adds to the carbonyl carbon of another ethanal to give 3-hydroxybutanal, the aldol. On heating, this beta-hydroxy aldehyde loses water across the alpha and beta carbons to form the conjugated alpha,beta-unsaturated aldehyde but-2-enal. Therefore 3-hydroxybutanal is only the intermediate, not the final heated product, so it is incorrect here. Butanal is wrong because no reduction of the double bond occurs under these conditions. Propanal cannot form because the carbon count from two ethanal units is four, not three. This sequence is the textbook NCERT aldol condensation and a classic JEE Advanced product question that tests whether students recognise dehydration on heating. The driving force for the final elimination is the formation of an extended conjugated system in which the new carbon-carbon double bond is in conjugation with the carbonyl, lowering the energy of the product. The term condensation specifically signals that water is eliminated, distinguishing it from a simple aldol addition that stops at the hydroxy aldehyde. As a sanity check, two two-carbon aldehydes combine to give a four-carbon product, and elimination of water yields the conjugated enal, consistent with but-2-enal.