Q.Assertion: p-N,N-dimethylaminobenzaldehyde undergoes benzoin condensation. Reason: The aldehydic (-CHO) group is meta directing. a) if both assertion and reason are true and reason is the correct explanation of assertion. b) if both assertion and reason are true but reason is not the correct explanation of assertion. c) assertion is true but reason is false d) both assertion and reason are false.

Benzoin Condensation: From Intuition to Mechanism

Imagine you have two identical aromatic aldehyde molecules — say, benzaldehyde. Each has a carbonyl group ($\ce{C=O}$) that is normally electrophilic at the carbon. You want to join them together so that one aldehyde becomes a ketone and the other becomes an alcohol, all while keeping the aromatic rings intact. That is exactly what the benzoin condensation does: it takes two aromatic aldehydes and produces an $\alpha$-hydroxy ketone, commonly called a benzoin.

The reaction is unusual because it is catalysed by cyanide ion ($\ce{CN-}$), not by acid or base in the usual sense. Cyanide is a nucleophile, but here it plays a dual role — it attacks the carbonyl carbon and then, after a proton transfer, becomes a powerful leaving group later. The reaction is also reversible, so conditions must be chosen to drive it forward.

where Ar is an aromatic ring (typically phenyl). The product is an $\alpha$-hydroxy ketone: the carbonyl and the hydroxyl are on adjacent carbons.

The Mechanism — Step by Step

The mechanism is the real beauty. It explains why only aromatic aldehydes work well and why cyanide is essential.

Step 1: Nucleophilic attack by cyanide.

The cyanide ion attacks the electrophilic carbonyl carbon of one aldehyde molecule. This forms a tetrahedral intermediate — a cyanohydrin anion.

Step 2: Proton transfer.

The negatively charged oxygen picks up a proton from the solvent (or from another molecule), giving a neutral cyanohydrin.

Step 3: Deprotonation at the $\alpha$-carbon.

The carbon next to the cyano group (the $\alpha$-carbon) is now slightly acidic because the cyano group is electron-withdrawing. A base (another cyanide ion or the solvent) removes a proton from this carbon, generating a carbanion that is resonance-stabilised by the cyano group.

The carbanion is the key intermediate — it is nucleophilic enough to attack a second aldehyde molecule.

Step 4: Attack on the second aldehyde.

This carbanion attacks the carbonyl carbon of a second benzaldehyde molecule. A new carbon–carbon bond forms, and the oxygen becomes negatively charged.

Step 5: Proton transfer and elimination of cyanide.

The alkoxide picks up a proton. Then the cyanide ion is expelled (the reverse of step 1), regenerating the catalyst and forming the $\alpha$-hydroxy ketone.

Why Only Aromatic Aldehydes?

Aliphatic aldehydes (like acetaldehyde) do not undergo this reaction cleanly. The reason lies in the stability of the carbanion intermediate. In the aromatic case, the cyano-stabilised carbanion is relatively stable and selective. With aliphatic aldehydes, the intermediate is too reactive and undergoes side reactions like self-condensation (aldol) or polymerisation. The aromatic ring also prevents enolisation, which would compete with the desired pathway. …