Benzoin condensation template

Cyanide adds to benzaldehyde, a proton transfer makes the carbon nucleophilic, it adds to a second benzaldehyde and cyanide leaves to give benzoin.

Mechanism step by step

Cyanide reverses the polarity of an aldehyde carbon (umpolung)

1. Cyanide adds – Cyanohydrin anion
2. Protonation – The solvent protonates the alkoxide: a cyanohydrin
3. Acyl-anion equivalent – The C–H next to C≡N is acidic: hydroxide removes it
4. C–C bond – The carbanion adds to a second aldehyde
5. Proton transfer – O–H to alkoxide
6. Cyanide leaves – C=O re-forms; the catalyst is released

Key points

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

Drawn with 207 atoms, 19 arrows, 8 shapes – every one editable in MolDraw.

Sources

• Wöhler & Liebig, Ann. Pharm. 1832, 3, 249 Original paper
• Lapworth, J. Chem. Soc., Trans. 1903, 83, 995 Original paper
• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012 Textbook