15.40:
Nonenolizable Aldehydes to Acids and Alcohols: The Cannizzaro Reaction
The Cannizzaro reaction is a redox process producing an alcohol and a carboxylic acid from two aldehyde molecules in the presence of a base.
The reaction is specific with nonenolizable aliphatic or aromatic aldehydes that contain no α hydrogens.
Aldehydes bearing α hydrogens, under basic conditions, often undergo deprotonation to give the enolate ion, followed by an aldol reaction.
Nonenolizable aldehydes endure the Cannizaro reaction pathway wherein the anion of the base attacks the carbonyl carbon, forming a tetrahedral alkoxide intermediate.
The base further deprotonates the intermediate resulting in an unstable dianionic species.
The strong electron-donating ability of O– facilitates the transfer of the aldehydic hydrogen to another aldehyde molecule.
This intermolecular hydride transfer generates an alkoxide and a carboxylate ion.
The more basic alkoxide ion is protonated by water to produce an alcohol, while the less basic carboxylate ion requires an acid workup to generate the carboxylic acid.
The Cannizzaro reaction is a disproportionation reaction where one aldehyde molecule is oxidized while the other is reduced.
15.40:
Nonenolizable Aldehydes to Acids and Alcohols: The Cannizzaro Reaction
The Cannizzaro reaction is a base-promoted redox reaction producing a primary alcohol and a carboxylic acid from two molecules of a nonenolizable aldehyde. The reaction commences when the anionic counterpart of the base attacks the carbonyl carbon, resulting in a tetrahedral alkoxide intermediate. The base then abstracts a proton from the intermediate to generate an unstable dianionic species. This intermediate enables the release of the aldehydic hydrogen as a hydride ion. An intermolecular hydride shift to another aldehyde molecule produces an alkoxide and a carboxylate ion. The more basic alkoxide undergoes protonation by the water molecule to form an alcohol, while the carboxylate is protonated via an acid work-up to yield a carboxylic acid. A disproportionation reaction involving two different aldehydes is called a Crossed-Cannizzaro reaction.