Q.Which of the following compounds do not undergo aldol condensation? (Two or more options may be correct.)

Aldol Condensation – From Intuition to Precision

Imagine you have two identical aldehyde molecules. Each has a carbon–oxygen double bond (the carbonyl) that is electron-hungry — the oxygen pulls electron density toward itself, leaving the carbonyl carbon slightly positive. Now look at the carbon next to the carbonyl (the $\alpha$-carbon). The hydrogens attached to it are unusually acidic, because if you remove one, the negative charge that forms can be stabilised by resonance with the carbonyl group.

What if you could make one molecule act as an "electrophile" (electron-poor, at its carbonyl carbon) and the other as a "nucleophile" (electron-rich, at its $\alpha$-carbon)? That is exactly what aldol condensation does. The two molecules join together, forming a new carbon–carbon bond.

The Mechanism in Two Stages

Stage 1: The Aldol Addition (the "aldol" part)

Under base catalysis (typically dilute NaOH or KOH), the base abstracts an $\alpha$-hydrogen from one molecule of the aldehyde or ketone. This generates an enolate ion — a carbanion that is resonance-stabilised.

The enolate then attacks the carbonyl carbon of a second, unreacted molecule. The result is a $\beta$-hydroxy carbonyl compound — an "aldol" (if starting from an aldehyde) or a "ketol" (if starting from a ketone).

Stage 2: Dehydration (the "condensation" part)

The $\beta$-hydroxy carbonyl compound now has an $\alpha$-hydrogen and a $\beta$-hydroxyl group. Under the reaction conditions (often mild heat or slightly stronger base), a molecule of water is eliminated. This creates a conjugated $\alpha,\beta$-unsaturated carbonyl compound — a much more stable product because the double bond is in conjugation with the carbonyl.

The overall process — addition followed by dehydration — is called aldol condensation.

The Precise Statement

Aldol condensation is a base-catalysed reaction in which two molecules of an aldehyde or ketone, each possessing at least one $\alpha$-hydrogen, combine to form a $\beta$-hydroxy carbonyl compound (the aldol addition product), which then undergoes dehydration to yield an $\alpha,\beta$-unsaturated carbonyl compound.

Key Conditions and Limitations …