Q3(d). Intramolecular Cannizzaro of furylglyoxal; furoin

Introduction. Both substrates are aldehydes without α-hydrogens. Hydroxide or cyanide adds to the CHO group and the adduct transfers a hydride or acts as an acyl-anion equivalent.

(i) Furylglyoxal and NaOH: intramolecular Cannizzaro reaction

Scheme: One carbonyl is oxidised (to CO₂⁻) and the neighbouring one reduced (to CHOH).

1. OH⁻ adds to the aldehyde carbon to give a tetrahedral alkoxide.
2. The alkoxide pushes its H, as hydride, to the adjacent ketone carbon (1,2-shift; the RDS); the aldehyde carbon becomes COOH and the ketone carbon CH-O⁻.
3. Proton transfer gives sodium 2-(furan-2-yl)-2-hydroxyacetate (furylglycolate); acidification gives the α-hydroxy acid. Name: intramolecular Cannizzaro reaction (glyoxal → mandelic-type acid).

(ii) Furfural and NaCN: benzoin condensation

1. CN⁻ adds to C=O; the cyanohydrin anion loses its formerly aldehydic H to give a carbanion stabilised by CN (polarity inversion, umpolung).
2. This acyl-anion equivalent adds to a second furfural; loss of CN⁻ regenerates the catalyst.
3. Product: furoin, 1,2-di(furan-2-yl)-2-hydroxyethan-1-one (C₁₀H₈O₄). Name: benzoin condensation.

Scheme: Cyanide is only a catalyst.

Conclusion. (i) 2-(furan-2-yl)-2-hydroxyacetic acid (as its sodium salt), intramolecular Cannizzaro reaction; (ii) furoin, benzoin condensation.

Application. Thiamine (vitamin B1) catalyses benzoin-type couplings in living cells (pyruvate decarboxylase), working by the same umpolung; the glyoxal-to-glycolic acid reaction is the laboratory route to mandelic acid from phenylglyoxal.