Aldehydes which do not contain α-hydrogen like HCHO, C6H5CHO,etc. undergo self oxidation and reduction on treatment with conc. alkali. In this reaction one molecule is oxidized to carboxylic acid and other molecule is reduced to alcohol. Thus, a mixture of an alcohol and a salt of carboxylic acid is formed by Cannizzaro’s reaction
Mechanism of Cannizzaro’s reaction
Taking benzaldehyde as an example, Cannizzaro’s reaction involves following steps:
Step I : In first step, there is nucleophilic addition of the hydroxide ion on the carbonyl group to give anion.
Step II : In this step, the anion transfers a hydride ion to the electron deficient carbonyl carbon of another aldehyde molecule to give carboxylic acid and alkoxide ion.
Step III : In this step, there is proton exchange between the carboxylic acid and the alkoxide ion to give salt of carboxylic acid and alcohol.
Crossed Cannizzaro’s reaction
When a mixture of formaldehyde and other aldehyde which has no α-hydrogen atom is treated with conc. alkali, formaldehyde is oxidized to carboxylic acid and another aldehyde is reduced to alcohol. This reaction is crossed cannizzaro’s reaction.
For example, benzyl alcohol and sodium formate is formed when a mixture of benzaldehyde and formaldehyde is treated with conc. NaOH.
Mechanism of crossed cannizzaro’s reaction
Taking a mixture of benzaldehyde and formaldehyde as an example, crossed cannizzaro’s reaction involves following steps:
Step-I : In first step, there is nucleophilic addition of the hydroxide ion on the carbonyl group of formaldehyde to give anion.
Step II : In this step, the anion transfers a hydride ion to the electron deficient carbonyl carbon of benzaldehyde molecule to give carboxylic acid and alkoxide ion.
Step III : In this step, there is proton exchange between the carboxylic acid and the alkoxide ion to give salt of carboxylic acid (sod. formate) and alcohol( benzyl alcohol).
Advantage of crossed- cannizzaro’s reaction:
In this reaction an aldehyde is treated with formaldehyde. Here, formaldehyde is oxidized to sodium formate and required alcohol is obtained from the reduction of the other aldehyde. Since both the aldehydes used are completely converted into the required products, there is no wastage of the valuable chemicals.
Why does methyl alcohol is not formed instead of sodium formate in crossed cannizzaro’s reaction?
The initial nucleophilic addition of hydroxide ion is faster on formaldehyde than on other aldehyde as there is no electron donating groups on formaldehyde. Hence, formic acid (sodium formate) is formed but not methyl alcohol.
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