Question

If you use a strong base in high concentration to run the benzoin condensation, you will probably find that the product is not benzoin, but rather benzoic acid and benzyl alcohol. What has happened is that the Cannizzaro reaction has occured. How is this similar to the benzoin condensation? (research the mechanism and note what is similar/different)

Answer 1

For both benzoin condensation and Cannizzaro reaction, the starting material is benzaldehyde.

The Benzoin Condensation is a coupling reaction between two aldehydes that allows the preparation of α-hydroxyketones.

Addition of the cyanide ion to create a cyanohydrin effects an umpolung of the normal carbonyl charge affinity, and the electrophilic aldehyde carbon becomes nucleophilic after deprotonation: A thiazolium salt may also be used as the catalyst in this reaction.

A strong base is now able to deprotonate at the former carbonyl C-atom:

A second equivalent of aldehyde reacts with this carbanion; elimination of the catalyst regenerates the carbonyl compound at the end of the reaction:

Cannizzaro Reaction is the redox disproportionation of non-enolizable aldehydes to carboxylic acids and alcohols is conducted in concentrated base.

Thus, in presence of strong base, OH^- ions acts as a nucleophile rather than CN^-, and prefers to go via Cannizzaro reaction under these reaction conditions.