The Cannizzaro reaction is a chemical reaction that involves the oxidation of an aldehyde to a carboxylic acid and the simultaneous reduction of another molecule of the same aldehyde to an alcohol. The reaction is named after its discoverer, Stanislao Cannizzaro, an Italian chemist who first described the reaction in 1853.
The Cannizzaro reaction is an essential synthetic route for preparing both carboxylic acids and alcohols from aldehydes. The reaction is widely used in organic synthesis and is particularly useful for converting aldehydes that are difficult to oxidize by other means.
The mechanism of the Cannizzaro reaction can be divided into two stages: the first stage involves the nucleophilic attack of hydroxide ion on the carbonyl group of the aldehyde, while the second stage involves the transfer of a hydride ion from the reduced aldehyde to the oxidized aldehyde.
The mechanism of the Cannizzaro reaction can be described in the following steps:
Step 1: Nucleophilic attack by hydroxide ion
In the first step of the reaction, the hydroxide ion attacks the carbonyl group of the aldehyde to form an unstable intermediate called a hemiacetal. This step is a nucleophilic addition reaction, in which the hydroxide ion acts as the nucleophile and the carbonyl carbon of the aldehyde acts as the electrophile.
The resulting hemiacetal is unstable and can undergo further reaction. The reaction is reversible, and the equilibrium lies towards the aldehyde.
Step 2: Hydride transfer
In the second step of the reaction, a molecule of the aldehyde acts as a reducing agent and transfers a hydride ion to the carbonyl group of another aldehyde molecule. The hydride ion is transferred from the alpha carbon (next to the carbonyl group) of the reduced aldehyde to the carbonyl carbon of the oxidized aldehyde.
This transfer of hydride ions reduces the aldehyde to an alcohol and the oxidation of the other aldehyde to a carboxylic acid. The aldehyde reduction occurs via a nucleophilic addition of the hydride ion to the aldehyde’s carbonyl group, followed by water’s protonation. The oxidation of the other aldehyde occurs via a nucleophilic attack of hydroxide ion on the carbonyl group, followed by deprotonation.
Cannizzaro reaction-mechanism-Hydride transfer
Overall Reaction
The overall reaction of the Cannizzaro reaction can be represented as follows:
Overall Cannizzaro Reaction
Where R represents an alkyl group or a hydrogen atom.
The reaction is carried out in an alkaline medium, and the role of the base is to generate a hydroxide ion, which acts as a nucleophile in the first step of the reaction. The base also helps to maintain the equilibrium of the reaction towards the aldehyde by removing the acid formed during the reaction.
The reaction is highly dependent on the structure of the aldehyde used. The aldehyde must have no alpha-hydrogens, as these would be preferentially oxidized to a carboxylic acid by the hydroxide ion. Aldehydes with electron-withdrawing groups or steric hindrance around the carbonyl group are also less reactive and may require higher temperatures or longer reaction times to undergo the reaction.
Applications of the Cannizzaro Reaction
The Cannizzaro reaction is essential in organic synthesis and has several applications. It is commonly used to prepare carboxylic acids and alcohols from aldehydes.
