1. Reaction: When reacting with concentrated sodium hydroxide (50 wt%) or other strong bases (such as alkoxides), the aliphatic and aromatic aldehydes without -hydrogen undergo an intermolecular hydride transfer reaction, called cannizzaro reaction, which is a typical disproportionation reaction, in which one aldehyde molecule oxidizes another molecule into the corresponding carboxylic acid, and itself is reduced to the corresponding primary alcohol, with a maximum yield of 50%.
If the aldehyde contains -H, the aldol reaction will occur faster than the cannizzaro reaction. Alternatively, when the reaction is carried out in the presence of excess formaldehyde, a high yield of alcohol can be obtained from almost any aldehyde, which is known as the crossed cannizzaro reaction.
Carbonyl aldehydes can undergo intramolecular cannizzaro reaction. However, due to the advent of hydride reducing agents in 1946, this method became obsolete. In the presence of an appropriate Lewis acid catalyst, the intramolecular cannizzaro reaction can be stereo-controlled to synthesize useful -hydroxy esters directly from ready-made glyoxals under neutral conditions. The study also showed that the reaction rate was significantly increased when the cannizzaro reaction was performed under solvent-free conditions.
2. Mechanism. A variety of mechanisms have been proposed, but the generally accepted one involves hydride transfer. First, a hydroxyl group is added to the carbonyl group, and the resulting substance is deprotonated under the basic conditions of application to obtain the corresponding anion. This dianion promotes the departure of aldehyde hydrogen in the form of hydrogen anions. This allows the hydrogen anion to attack another aldehyde molecule in a rate-determining step (RDS) to obtain an alkoxide of a primary alcohol, which is subsequently protonated by a solvent (water) to obtain an alcohol. The results of the reaction by performing the reaction in the presence of D2O show that the reduced hydrogen anion comes from another aldehyde and not from the reaction medium, since the resulting primary alcohol does not contain deuterium. Using ESR spectroscopy, Ashby and colleagues showed that substituted benzaldehyde radical anions were formed in the reaction of substituted benzaldehyde with sodium hydroxide or KOT-BU. This observation suggests that the reaction is carried out by a single electron transfer (SET) mechanism.