See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: The 'most basic' nitrogen in a molecule is the one with the highest electron density available for protonation. Basicity of nitrogen depends on hybridization, resonance delocalization, and inductive effects. Step 1 – Option (a): The molecule contains a pyrrole N–H and a pyridine N. The pyrrole N lone pair is delocalized into the aromatic pi system (sp2, part of aromaticity), making it non-basic (pKa of conjugate acid ~−3.8). The pyridine N lone pair is in an sp2 orbital perpendicular to the pi system and is not delocalized, making it basic (pKa ~5.2). Therefore, the pyridine N is the most basic, and circling it is correct. Step 2 – Option (b): The molecule has a diethylamino group (sp3 N, tertiary amine, highly basic) attached to the benzene ring versus another N that is part of a strained or otherwise modified ring attached via the benzene. The N in the circled small ring (azetidine-type or similar N-ethyl ring) is an sp3 nitrogen not delocalized into the ring system; however, the key comparison is that the diethylamino N attached directly to the aromatic ring has its lone pair delocalized into the ring (aniline-type, less basic), whereas the circled N in the saturated ring is an sp3 nitrogen not conjugated with the aromatic ring, making it more basic. Circling the saturated ring N is correct. Step 3 – Option (c): The molecule contains multiple nitrogen groups: an amide N (H2N–C=O, lone pair delocalized into carbonyl, least basic), a CN nitrile (sp N, lone pair in sp orbital, very weakly basic), and two aliphatic NH2 groups. Among these, the aliphatic primary amine NH2 groups (sp3, not delocalized) are most basic. The circled NH2 is an aliphatic amine, which is indeed the most basic nitrogen in this molecule. Circling it is correct. Step 4 – Since all three representations (a), (b), and (c) correctly circle the most basic nitrogen atom in their respective molecules, option (d) 'All of these' is correct. Why other options fail individually: Each of (a), (b), (c) alone is only partially correct; the question asks which is the correct representation among the choices, and since all three individual representations are correct, the answer must be (d). Therefore, the correct answer is D.