. What is cannizaro reaction and its mechanism


Best Answer

Those aldehydes (aliphatic or aromatic), which do not have alpha-hydrogen atom on treatment with strong base undergoes a reaction involving its 2 moles, one getting oxidised to yield acid salt and the other getting reduced to primary alcohol. The important condition is that there should not be a good leaving group attached to the carbonyl group.

Mechanism:

The first step of the mechanism involve reversible attack of OH- on an aldehyde molecule to give hydroxy alkoxide. This hydroxy alkoxide in the subsequent step transfer hydride ion to second molecule of either same aldehyde (simple Cannizaro) or different aldehyde (crossed Cannizaro). The hydroxy alkoxide on transferring hydride becomes carboxylic acid molecule while second aldehyde molecule becomes alkoxide. The carboxylic acid and alkoxide then undergoes proton exchange to form carboxylate and alcohol respectively.

It is evident from the mechanism that the species acting as hydride donor finally forms acid salt while the one, which accepts hydride will form primary alcohol.When crossed Cannizaro reaction is carried out between formaldehyde and benzaldehyde, formaldehyde always forms formate salt while benzaldehyde yields benzyl alcohol.

This is because carbonyl carbon of formaldehyde is more electrophilic than that of benzaldehyde. So, OH initially attacks at formaldehyde (due to electronic, statistical and steric factors) to form hydroxy alkoxide, which acts as hydride donor to finally form carboxylate while benzaldehyde accepts hydride to form alcohol finally.The presence of electron withdrawing substituent increases the rate of Cannizaro reaction while electron releasing substituent decreases the rate.

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