The structural feature in carbonyl compound for Aldol condensation

The correct answer is Option (2) → presence of at least one \(α\)-hydrogen

In aldol condensation the base removes a proton from the α-carbon (the carbon next to C=O). The α-hydrogens are acidic because the carbanion formed (an enolate) is stabilised by resonance with the carbonyl group. This carbanion then adds to the C=O group of a second molecule, giving a β-hydroxy aldehyde (aldol) or β-hydroxy ketone (ketol), which loses water on heating:

    \( \displaystyle \ce{2CH3CHO} \xrightarrow{\text{dil. }\ce{NaOH}} \ce{CH3-CH(OH)-CH2-CHO} \xrightarrow[\ce{-H2O}]{\Delta} \ce{CH3-CH=CH-CHO} \)

So the structural feature a carbonyl compound needs is at least one α-hydrogen.

Why the other options are incorrect

• Option (1) presence of at least one \(β\)-hydrogen — Incorrect. A β-hydrogen is not acidic, because the carbanion it would leave is not stabilised by the C=O group.

• Option (3) Concentrated base — Incorrect. This is a reaction condition, not a structural feature; in fact dilute alkali is used for aldol condensation.

• Option (4) lack of \(α\) hydrogen — Incorrect. Aldehydes with no α-hydrogen, such as HCHO and \( \ce{C6H5CHO} \), cannot form the carbanion; with concentrated alkali they undergo the Cannizzaro reaction instead.

NCERT (2026-27 edition):
Aldol Condensation