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Cannizzaro reaction: characteristics, mechanisms, examples

This reaction was discovered by the Italian chemist Stanislao Cannizzaro, who in 1853 mixed benzaldehyde with potash, K 2 CO 3 , obtaining benzyl alcohol and potassium benzoate. Then the same reaction was repeated, but using even more basic substances, such as sodium and potassium hydroxides.

Equation and mechanism for the Cannizzaro reaction of benzaldehyde. Source: Krishnavedala / Public domain

The image above shows the Cannizzaro reaction for benzaldehyde, the simplest of all aromatic aldehydes. Two benzaldehyde molecules disproportion, in a strongly basic medium, to give rise to a mixture of benzoate ions and benzyl alcohol. That is, the reaction results in a mixture of a carboxylic acid salt and a primary alcohol.

Characteristics and conditions

Absence of alpha hydrogens

For the Cannizzaro reaction to be possible, the aldehyde in question must lack alpha hydrogen. This means that the carbon atom adjacent to the carbonyl group must not have any bond with a hydrogen atom: R 3 C-C = O.

In the case of benzaldehyde, if the C-CHO bond is closely observed, the total absence of this acidic hydrogen will be noted.

Basicity

The Cannizzaro reaction takes place in strongly basic media, generally provided by sodium and potassium hydroxides in aqueous or alcoholic solutions.

Temperature

The Cannizzaro reaction for formaldehyde occurs at room temperature. However, for all other aldehydes it is necessary to heat the reaction mixture. Thus, the temperature can range between 50-70 ° C, depending on the solution and the aldehyde.

Autoxidation-reduction

The aldehyde in the Cannizzaro reaction undergoes autoxidation-reduction. This means that an aldehyde molecule oxidizes itself while reducing another neighboring molecule. The result is that the aldehyde is disproportionate to give rise to a carboxylic acid salt (oxidized) and a primary alcohol (reduced).

General mechanism for the Cannizzaro reaction. Source: Roshan220195 at English Wikipedia / Public domain

In the first image for the Cannizzaro reaction of benzaldehyde its mechanism was shown. In this section, the two essential steps of this mechanism will be explained, starting from a reaction for all aldehydes in general (upper image).

Step 1

The OH  ions in the basic medium carry out a nucleophilic attack on an aldehyde molecule. In doing so, a tetrahedral intermediate is formed (right of the first row). This first step is reversible, so the intermediate can be disposed of back into the initial reagents.

Step 2

The second step of the mechanism involves the so-called autoxidation-reduction. In the intermediate of step 1, the C = O bond is formed at the cost of the migration of a hydride ion, H  , to another aldehyde molecule. Thus, the first molecule is oxidized, while the second one gains this H  , that is, it is reduced.

Here we already have the carboxylic acid and an alkoxide (right second row). An exchange of H + ions occurs between them , deprotonates the carboxylic acid and protonates the alcohol.

It is thus then that we have at the end a carboxylate anion, which interacts with the cations of the base to form a carboxylic acid salt. And we also have a primary spirit.

Crusade

Until now there has been talk of a Cannizzaro reaction between two molecules of the same aldehyde. This reaction can also take place between two different aldehyde molecules; especially if one of them consists of formaldehyde. We are talking about a crossed Cannizzaro reaction, whose general chemical equation is the one below:

ArCHO + HCHO → ArCH 2 OH + HCOO  Na +

The Cannizzaro cross reaction occurs between an aromatic aldehyde, ArCHO, and formaldehyde, to form a benzyl alcohol and formate ions.

Below is an example for the Cannizzaro cross reaction of anisaldehyde:

Equation for the cross Cannizzaro reaction of anisaldehyde. Source: Gabriel Bolívar via ChemSketch.

Note that the formate ion will always be formed, as it is derived from formic acid, HCOOH, the weakest acid and therefore the one that is predominantly produced. This reaction represents a synthetic route to synthesize aromatic alcohols from aromatic aldehydes without the need for reducing agents.

Examples

Next, and finally, several examples of Cannizzaro’s reaction will be shown.

Example 1

Chemical equation for the Cannizzaro reaction of formaldehyde. Source: Gabriel Bolívar via ChemSketch.

Two formaldehyde molecules disproportionate to produce formate ion and methanol. This reaction occurs at room temperature, so formaldehyde must not be mixed with a strongly basic solution if it is to be used for synthesis.

Example 2

Equation for the Cannizzaro reaction of m-chlorobenzaldehyde. Source: Gabriel Bolívar via ChemSketch.

In this reaction two molecules of m -chlorobenzaldehyde disproportionate in a mixture of m -chlorobenzoate, the carboxylic acid salt, and m -chlorobenzyl alcohol, the primary alcohol.

It is to be expected that this reaction requires a higher temperature and takes a little longer because it involves molecules that are larger than those of formaldehyde.

Example 3

Equation for the Cannizzaro reaction of 3,4-Dimethoxybenzaldehyde. Source: Gabriel Bolívar via ChemSketch.

A molecule of 3,4-Dimethoxybenzaldehyde undergoes a Cannizzaro cross reaction with formaldehyde to become 3,4-Dimethoxybenzyl alcohol. Note that this reaction requires a temperature of 65 ° C. Remember that aromatic alcohol will always be formed in the case of the Cannizzaro reaction crossed with formaldehyde.

Example 4

Equation for the Cannizzaro reaction of p-Nitrobenzaldehyde. Source: Gabriel Bolívar via ChemSketch.

Two molecules of p -Nitrobenzaldehyde disproportionate to form a mixture of p -nitrobenzyl alcohol and the sodium p -Nitrobenzoate salt . Again, this reaction represents a synthetic route to obtain said aromatic primary alcohol.

The first example, that of benzaldehyde, is the best known in organic chemistry teaching laboratories, as it is the one used to introduce the concept of the Cannizzaro reaction to students.

However, it can be seen that the possible examples can be innumerable if any aromatic aldehyde is considered. Or any other aldehyde without alpha hydrogens.

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