See image — Amines Chemistry Question

💡 Solution & Explanation

Concept: Basicity of amines depends on the availability of the lone pair on nitrogen. Electron-withdrawing groups decrease basicity, while electron-donating groups increase it. Additionally, when the lone pair is directly involved in resonance with an electron-withdrawing group, the nitrogen becomes much less basic. Step 1: Identify the structures. - o-, m-, p-Aminoacetophenone: These are acetophenones (Ph-CO-CH3) with an -NH2 group at ortho, meta, or para position respectively. The -NH2 group is the basic site. - Acetanilide: This is Ph-NH-CO-CH3, where the nitrogen is directly bonded to the carbonyl group (an amide linkage). Step 2: Analyze basicity of aminoacetophenones. - In o-Aminoacetophenone: The -NH2 lone pair is reduced in availability by the electron-withdrawing -COCH3 group at ortho position. Additionally, intramolecular hydrogen bonding between -NH2 and the carbonyl oxygen can further tie up the lone pair, reducing basicity. - In p-Aminoacetophenone: The -NH2 lone pair is delocalized into the ring and toward the para -COCH3 group through resonance, significantly reducing basicity compared to m-isomer. - In m-Aminoacetophenone: The -COCH3 group withdraws electrons inductively but cannot delocalize the -NH2 lone pair through resonance at meta position, so basicity is less reduced. Step 3: Analyze basicity of Acetanilide. - In acetanilide (Ph-NH-CO-CH3), the nitrogen lone pair is delocalized directly into the carbonyl group via amide resonance (N lone pair -> C=O). This conjugation is much stronger than any indirect resonance in aminoacetophenones. - Amide nitrogen lone pairs are notoriously non-basic (pKaH of acetanilide ~0.5, compared to ~2-4 for aminoacetophenones) because of direct resonance donation into the carbonyl. - This makes acetanilide by far the weakest base among all four options. Step 4: Why other options fail. - o-, m-, p-Aminoacetophenone all have a free -NH2 group whose lone pair, even if partially delocalized, is not as strongly tied up as in an amide bond. Their conjugate acids are more stable than that of acetanilide, making them stronger bases than acetanilide. Therefore, the correct answer is D.