Name the reaction when a mixture of acetaldehyde and propanal is trea…

The correct answer is Option (1) → Cross Aldol Condensation

In aldol condensation, a dilute base removes an α-hydrogen from an aldehyde or ketone, and the carbanion formed adds to the C=O group of another molecule, giving a β-hydroxy aldehyde (aldol) that loses water on heating. When the two partners are two different aldehydes and/or ketones, the reaction is called cross aldol condensation. If both have α-hydrogens, each can act as the carbanion and as the carbonyl partner, so four products are formed.

Ethanal and propanal both have α-hydrogens. The four aldols are:

• ethanal + ethanal: \( \ce{CH3-CH(OH)-CH2-CHO} \) (3-hydroxybutanal), which dehydrates to but-2-enal;

• propanal + propanal: \( \ce{CH3CH2-CH(OH)-CH(CH3)-CHO} \) (3-hydroxy-2-methylpentanal), giving 2-methylpent-2-enal;

• carbanion of ethanal + propanal: \( \ce{CH3CH2-CH(OH)-CH2-CHO} \) (3-hydroxypentanal), giving pent-2-enal;

• carbanion of propanal + ethanal: \( \ce{CH3-CH(OH)-CH(CH3)-CHO} \) (3-hydroxy-2-methylbutanal), giving 2-methylbut-2-enal.

For example, the first cross product forms as:

    \( \displaystyle \ce{CH3CHO + CH3CH2CHO} \xrightarrow{\text{dil. }\ce{NaOH}} \ce{CH3CH2-CH(OH)-CH2-CHO} \xrightarrow[\ce{-H2O}]{\Delta} \ce{CH3CH2-CH=CH-CHO} \)

So the reaction is a cross aldol condensation.

Why the other options are incorrect

• Option (2) Self Aldol Condensation — Incorrect. This is between two molecules of the same carbonyl compound, such as two ethanal molecules; here two different aldehydes are mixed.

• Option (3) Intramolecular Aldol Condensation — Incorrect. This happens within one molecule that has two carbonyl groups, such as hexane-2,5-dione, and gives a ring.

• Option (4) Cannizzaro Reaction — Incorrect. This needs an aldehyde without α-hydrogen and concentrated alkali; both aldehydes here have α-hydrogens.

NCERT (2026-27 edition):
Cross Aldol Condensation

“… If both of them contain α-hydrogen atoms, it gives a mixture of four products. This is illustrated below by aldol reaction of a mixture of ethanal and propanal.”