Question

Give the correct structure for A and B for the reaction below:

Answer 1

This reaction is a Cannizaro named reaction in which if the substrate does not have alpha hydrogen the substrate gets oxidized and reduced at the same time and gives a product. Here we have substrate as benzaldehyde and formaldehyde, both don’t have alpha hydrogens thus they will give Cannizaro reaction.

Complete step-by-step answer: In Cannizaro reaction, the substrate which will not have alpha hydrogen will react in the presence of concentrated sodium hydroxide and gives it’s oxidized and reduced form. We can use any other strong alkali in place of sodium hydroxide also. If we have aldehyde or ketone then its oxidized form is carboxylic acid while reduced form is alcohol.
In the above reaction benzaldehyde will react with concentrated sodium hydroxide and give benzyl alcohol which is its reduced form and on the other hand formaldehyde will get oxidized and give sodium salt of carboxylic acid.
The happening of reaction is a concerted step where there is transfer of a hydrogen atom from one group to another group. At last as we get sodium salt of carboxylic acid on further hydrolysis we get carboxylic acid. These types of named reactions are very important because the chances of mistakes are high in predicting the reaction.
Let’s see the reaction here,

Option $(1)$ is correct.

Note: When the substrate has alpha hydrogen it will react and give $\alpha ,\beta - \text{unsaturated hydroxyl compound}$. Thus the availability of hydrogen just next to the carbonyl group is important. The aldol condensation reaction goes through a carbanion formation which will further react with another molecule of carbonyl compound and attack as a nucleophile on the electrophilic center of the compound.