Question

Assertion: Aromatic aldehydes and formaldehyde undergo Cannizzaro reaction. Reason: Aromatic aldehydes are almost as reactive as formaldehyde.

Answer 1

Analysing the assertion

Aromatic aldehydes and formaldehyde do not contain $\alpha -$hydrogen and thus undergo Cannizzaro reaction. For example, benzaldehyde reacts with conc.$KOH$solution and undergoes Cannizzaro’s reaction.


Cannizzaro’s reaction of formaldehyde is shown below:


So, the assertion is correct.

Analysing the reason

Formaldehyde is more reactive than aromatic aldehydes in nucleophilic addition reactions. This is due to the electron-donating resonance effect of the aromatic rings which makes carbonyl carbon less electron deficient. The attack of a nucleophile on less electron deficient carbon is slower than the attack on formaldehyde. Another reason for slow reaction of aromatic aldehydes as compared to formaldehyde is the steric crowding.

A nucleophile faces more difficulty while attacking on more crowded aromatic aldehydes and hence the reaction is slower.


So, the reason is incorrect.
Mechanism of Reaction
Step 1: Nucleophilic attack on the carbonyl group to give a anion.


Step 2: Conversion of the anion to a carboxylate ion and another aldehyde molecule to an alkoxide ion.


Step 3: Protonation of methoxide to methanol.


Conclusion: Assertion is a correct statement but reason is wrong statement. So, option (C) is the correct answer.