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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 alde…
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.
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Q1: What is the Cannizzaro reaction and what types of aldehydes undergo it?
The Cannizzaro reaction is a base-promoted redox process that converts two molecules of a nonenolizable aldehyde into a primary alcohol and a carboxylic acid. Nonenolizable aldehydes contain no alpha hydrogens, preventing enolate formation. These include aliphatic and aromatic aldehydes. In contrast, aldehydes with alpha hydrogens typically undergo aldol reactions under basic conditions instead.
Q2: How does the tetrahedral alkoxide intermediate form in the Cannizzaro reaction?
The reaction begins when the anionic counterpart of the base attacks the carbonyl carbon of the aldehyde, forming a tetrahedral alkoxide intermediate. The base then abstracts a proton from this intermediate, generating an unstable dianionic species. This intermediate is crucial for enabling the subsequent hydride transfer step.
Q3: What role does hydride transfer play in the Cannizzaro reaction mechanism?
The strong electron-donating ability of the oxygen anion facilitates the transfer of the aldehydic hydrogen as a hydride ion to another aldehyde molecule. This intermolecular hydride shift generates an alkoxide ion and a carboxylate ion from the two aldehyde molecules, making the Cannizzaro reaction a disproportionation process.
Q4: Why are different workup procedures needed for the alcohol and carboxylic acid products?
The more basic alkoxide ion is readily protonated by water to produce the primary alcohol. The less basic carboxylate ion requires an acid workup to generate the carboxylic acid product. This difference in basicity determines which product forms under aqueous versus acidic conditions.
Q5: What is a crossed Cannizzaro reaction?
A crossed Cannizzaro reaction occurs when two different nonenolizable aldehydes participate in the disproportionation process. Instead of one aldehyde molecule being oxidized and another reduced, the reaction involves two distinct aldehyde substrates undergoing the same redox transformation to yield different alcohol and carboxylic acid products.
Q6: How does the Cannizzaro reaction differ from aldol condensation?
The Cannizzaro reaction is specific to nonenolizable aldehydes lacking alpha hydrogens and produces an alcohol and carboxylic acid through hydride transfer. Aldol condensation occurs with aldehydes or ketones containing alpha hydrogens, forming enolate ions that attack carbonyl carbons to create C-C bonds and beta-hydroxy carbonyl products.
Q7: Why is the Cannizzaro reaction classified as a disproportionation reaction?
The Cannizzaro reaction is a disproportionation because one aldehyde molecule is oxidized to a carboxylate ion while another is simultaneously reduced to an alkoxide ion. Both transformations originate from the same aldehyde functional group, making it a self-redox process where the same substrate acts as both oxidizing and reducing agent.