Cannizzaro reaction

The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro, is a chemical reaction that involves the base-induced disproportionation of an aldehyde lacking a hydrogen atom in the alpha position.

Cannizzaro first accomplished this transformation in 1853, when he obtained benzyl alcohol and potassium benzoate from the treatment of benzaldehyde with potash (potassium carbonate). More typically, the reaction would be conducted with sodium or potassium hydroxide:
 * 2 C6H5CHO +  KOH   →   C6H5CH2OH  +  C6H5CO2K

The oxidation product is a salt of a carboxylic acid and the reduction product is an alcohol. For aldehydes with a hydrogen atom alpha to the carbonyl, i.e. R2CHCHO, the preferred reaction is an aldol condensation, originating from deprotonation of this hydrogen. This reaction restricts the scope of the Cannizzaro reaction.

Mechanism
The reaction begins with nucleophilic attack of hydroxide on the carbonyl centre. The resulting anion attacks another molecule of aldehyde, transferring hydride. In the final step of the reaction, the acid and alkoxide ions formed exchange a proton. In the presence of a very high concentration of base, the aldehyde first forms a doubly charged anion from which a hydride ion is transferred to the second molecule of aldehyde to form carboxylate and alkoxide ions. Subsequently, the alkoxide ion acquires a proton from the solvent.



Overall, the reaction follows third-order kinetics. It is second order in aldehyde and first order in base:
 * rate = k[RCHO]2[OH-]

At very high base a second path (k') becomes important that is second order in base:
 * rate = k[RCHO]2[OH-] +  k'[RCHO]2[OH-]2

The k' pathway implicates a reaction between the doubly charged anion (RCHO22-) and the aldehyde. The direct transfer of hydride ion is evident from the observation that the recovered alcohol does not contain any deuterium attached to the α-carbon when the reaction is performed in the presence of D2O.

Scope
The reaction is limited to aldehydes lacking alpha hydrogen centers. Under ideal conditions the reaction produces only 50% of the alcohol and the carboxylic acid. The latter is obtained only after acidification of the highly basic reaction mixture, typically 30% base. To avoid the low yields, it is more common to conduct the crossed Cannizzaro reaction with a sacrificial aldehyde. In this variation, the reductant is formaldehyde, which is oxidized to sodium formate and the corresponding alcohol is obtained in a high yield, although the atom economy is still low.

A solvent-free reaction has been reported involving grinding liquid 2-chlorobenzaldehyde with potassium hydroxide in a mortar and pestle: