Name reaction · mechanism

Benzoin Condensation

In the benzoin condensation two molecules of an aromatic aldehyde join to form an α-hydroxy ketone (benzoin) under catalysis by cyanide. Cyanide adds to one aldehyde; the C–H next to C≡N becomes acidic, and the resulting carbanion — a masked acyl anion — attacks a second aldehyde before cyanide is expelled.

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Benzoin Condensation mechanism: Cyanide adds to benzaldehyde, a proton transfer makes the carbon nucleophilic, it adds to a second benzaldehyde and cyanide leaves to give benzoin.
Benzoin condensation: overall reaction, every curly-arrow step and key points. Click to open and edit it in MolDraw.

Overall reaction

2 C₆H₅CHO → (KCN, aqueous ethanol) C₆H₅CO–CH(OH)C₆H₅

Cyanide reverses the polarity of an aldehyde carbon (umpolung).

Mechanism step by step

  1. Cyanide adds. Cyanohydrin anion
  2. Proton transfer. The C–H is now acidic (next to CN): an acyl-anion equivalent
  3. C–C bond. The carbanion adds to a second aldehyde
  4. Proton transfer. O–H to alkoxide
  5. Cyanide leaves. C=O re-forms; the catalyst is released

Each step is checked electron by electron: the curly arrows give exactly the structures drawn.

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Key points

  • Umpolung: the electrophilic C=O carbon becomes a nucleophile.
  • Cyanide is a catalyst: added in step 1, released in step 5.
  • Thiamine (vitamin B₁) and N-heterocyclic carbenes do the same job.
  • Works with aromatic aldehydes (no α-H to give aldol reactions).

For JEE and NEET

  • Umpolung: the electrophilic carbonyl carbon is made nucleophilic.
  • Cyanide is a catalyst: it adds in the first step and leaves in the last.
  • Works with aromatic aldehydes (no α-H).
  • Benzoin oxidises to benzil, which undergoes the benzilic acid rearrangement.

FAQ

Why is cyanide a good catalyst?

It is a good nucleophile, it acidifies the neighbouring C–H, and it is a good leaving group at the end.

What does umpolung mean here?

Polarity reversal: the normally electrophilic aldehyde carbon acts as a nucleophile while bonded to CN.

Can aliphatic aldehydes be used?

Not easily with cyanide; enolisable aldehydes undergo aldol reactions. Thiazolium/NHC catalysts extend the scope.

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