AIIMS 2009 Chemistry Aromatic Compounds Assertion Reason Question

To understand the question, let's break down the assertion and reason provided, and then analyze why the correct answer is option B.

Assertion:

p-N,N⁻dimethylaminobenzaldehyde undergoes benzoin condensation.

Explanation: Benzoin condensation is a reaction where two molecules of an aldehyde condense in the presence of a base to form a β-hydroxy ketone (benzoin). In this case, p-N,N⁻dimethylaminobenzaldehyde, which has an aldehyde group (-CHO) attached to a benzene ring substituted with a dimethylamino group, can participate in this reaction. The dimethylamino group is an electron-donating group and activates the aromatic ring, making the aldehyde group more reactive towards nucleophilic attack.

Thus, the assertion is true.

Reason:

The aldehydic (-CHO) group is meta directing.

Explanation: In electrophilic aromatic substitution, the directing effects of substituents are crucial. Electron-donating groups, like the dimethylamino group, are typically ortho/para-directing due to their ability to stabilize the carbocation intermediate formed during the reaction. The aldehydic (-CHO) group, however, is a deactivating group and does not direct substitution effectively. This is often misunderstood; while the -CHO group can influence the reactivity, it does not direct substitutions to the meta position effectively.

Therefore, the reason provided is false because the -CHO group is actually a deactivating group and does not direct substitution to the meta position. Instead, it typically directs to the ortho or para positions relative to the electron-donating group.

Conclusion:

Since the assertion is true and the reason is false, the correct answer is option B: "Both assertion and reason are true but reason is not the correct explanation of assertion."

Why Other Options are Incorrect:

• Option A: Both assertion and reason are true, and reason is the correct explanation of assertion. This is incorrect because the reason is false.
• Option C: Assertion is true but reason is false. This option does not match the wording of option B, which accurately describes the situation.
• Option D: Both assertion and reason are false. This is incorrect because the assertion is true.

Key Concepts:

• Benzoin Condensation: A reaction involving the dimerization of aldehydes in the presence of a base.
• Directing Effects: The influence of substituents on the position of further substitutions on the aromatic ring. Electron-donating groups direct substitutions to ortho/para positions, while electron-withdrawing groups generally deactivate the ring and do not lead to meta substitution.

In summary, the assertion about p-N,N⁻dimethylaminobenzaldehyde undergoing benzoin condensation is valid, while the reasoning about the meta-directing nature of the -CHO group is flawed. Thus, the correct answer remains option B.