Cannizzaro Reaction - Mechanism, Examples

Cannizzaro Reaction for Formaldehyde is written as

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Mechanism of Cannizzaro Reaction

A general Cannizzaro reaction is as follows (here, shown using an aldehyde): (‘R’ can be alkyl or aryl group and has no ɑ-H atom)

The Cannizzaro reaction mechanism can be better understood by taking the example of Benzaldehyde which lacks an ɑ-H atom. The Cannizzaro reaction for benzaldehyde is as follows:

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Mechanism:

Step1: Due to the absence of ɑ-H atom (which is usually acidic in nature), the alkali attacks the carbonyl ‘C’ only. Therefore, we can say that the alkali acts as a nucleophile instead of a base and attacks the electrophilic centre which is the carbon atom of the carbonyl group. Carbon atom of the carbonyl group is attached to a very electronegative atom i.e. ‘O’ and therefore possess a partial positive charge thereby acting as an electrophile. OH- of the alkali acts as a strong nucleophile and attacks the carbon atom of the carbonyl carbon. The first few steps of the reaction are reversible in nature therefore the alkali taken should be concentrated so that the reaction proceeds in the forward direction only, as per Le-Chatelier Principle.

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Step 2: The negative charge on the oxygen reverts back to reform the C=O bond pushing out the hydrogen as a hydride ion. Now the hydride ion acts as a nucleophile and attacks the carbonyl carbon of another molecule of an aldehyde. This is also known as hydride transfer. The hydride ion attacks the carbonyl carbon and the hybridisation of the sp2 carbonyl carbon changes to sp3 with 2 H atoms, one O- and a benzene group attached to it. The resulting ion is an alkoxide ion.

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Since the concentration of the alkali taken is very high so the reaction is not allowed to revert in the backward direction, instead the reaction is forced in the forward direction.

Step 3: In the final step, H+ is taken up by the alkoxide ion from the carboxylic acid (a Lewis acid base reaction) and we obtain a carboxylate ion/salt and an alcohol as the final product.

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Carboxylic acid can be obtained from carboxylate ion by the work up process (H3O+ work up) later if required.

The above steps showed that the Cannizzaro’s reaction and the Cannizzaro reaction mechanism for a single aldehyde which underwent a disproportionation reaction to give an alcohol and an acid. However, if two different aldehydes are used in the reaction mixtures, one of them undergoes oxidation to yield carboxylic acid whereas the other one gets reduced to form an alcohol. Such type of Cannizzaro reaction where two different aldehydes are involved is termed as Cross Cannizzaro Reaction.

Cross Cannizzaro Reaction

Cross Cannizzaro reaction is a type of Cannizzaro reaction in which the aldehydes used are of two different types. So, instead of using 2 equivalents of the same aldehyde, one equivalent of one aldehyde and one equivalent of another aldehyde are used. Both the aldehydes lack ɑ-H atom.

In such types of reactions, the one which is easily oxidisable gets oxidised and the other gets reduced. In other words, the one which forms a more stable carboxylate ion gets oxidised and the other one gets reduced.

The mechanism for Cross Cannizzaro remains the same as in Cannizzaro reaction. An example for cross cannizzaro is as follows:

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In the above reaction the two aldehydes that we have are benzaldehyde and formaldehyde. Their respective carboxylates are benzoate and formate. Both are resonance stabilised but in benzoate, cross-conjugation occurs making it less stable than formate ion. Therefore, formaldehyde undergoes the oxidation to formic acid and benzaldehyde reduces to alcohol.

It is important to note that in a Cannizzaro reaction, the reaction is carried forward forcefully by keeping the concentration of the alkali high i.e. by using a concentrated alkali. But still since the reaction is reversible and some of its steps exists in an equilibrium state therefore the yield is not very high. However, the yield is somewhat better in case of a Cross-Cannizzaro reaction.

Now, let’s come to the point i.e. why we use an aldehyde which has no ɑ-H atom. The answer goes this way: The ɑ-H atom is acidic in nature. Since we are using a base/alkali, we may end up with an acid base reaction instead of Cannizzaro reaction. The ɑ-H atom is acidic in nature because the conjugate base formed after removal of the ɑ-H atom is stabilised by resonance. So, if there is ɑ-H atom present in the aldehyde, the alkali would behave as a base instead of a nucleophile and instead of attacking the carbon of the carbonyl group it would rather take up the ɑ-H atom and end up forming a salt and water.

The second question i.e. why we use the concentrated form of alkali instead of using an aqueous or dilute form has already been explained during the mechanism.

Some of the pointers regarding this reaction are:

• Cannizzaro reaction is a name reaction i.e. it was discovered by a scientist and the name of the reaction is kept after him. He was Stanislao Cannizzaro who discovered this reaction in 1853 and after which the name of the reaction came to be Cannizzaro’s reaction.

• The reaction follows 3rd order kinetics as its rate determining step i.e. the attack of OH- and removal H- involves 2 equivalents of aldehyde and one equivalent of base. Therefore, it follows as 3rd order kinetics. It is 2nd order with respect to aldehyde and 1st order with respect to base.