Nitrogen atoms are known to have basic (proton acceptor) properties.Consider the following nitrogen heterocycle (imidazole) and the nitrogen atoms labelled 1 - 3.Which (a - f) ranks the nitrogen atoms in order of most basic to least.

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Begin by identifying the three nitrogens in the given imidazole-containing system and recalling their electronic contexts.
Option a: 1, 2, 3. If we read this as the left-ring N (1) being most basic, followed by the right-ring N-H (2), and then the side-chain amino (3) last, this is inaccurate because the N-H in the ring (pyrrolic nitrogen) is not basic at all due to its involvement in the aromatic sextet; its lone pair is part of the aromatic system, so it cannot readily accept a proton. The ranking here misplaces a non-basic site above a basic one.
Option b: 1, 3, 2. This suggests the left-ring N (1) is the most basic, then the side-chain amine (3), and finally the ring N-H (2). While it correctly elevates the side-chain amine above the ring N-H, it incorrectly places the ring N (1) ahead of the highly basic aliphatic amine (3). In truth, the aliphatic amine is typically more basic than the ring-like pyridine nitrogen due to less delocalization and a higher electron density on the lone pair.
Option c: 2, 1, 3. Here the ring N-H (2) is stated as most basic, which is inconsistent with the chemistry: the N-H nitrogen is not a strong base because its lone pair is not available for protonation; it is part of the aromatic system. This option severely overestimates the basicity of the N-H site and underestimates the basicity of the other nitrogens.
Option d: 2, 3, 1. This ranking places the N-H (2) as most basic, which is incorrect for the same reason as above. Then it assigns the side-chain amine (3) to be more basic than the left-ring N (1), which is plausible, but since the first item is wrong, the overall order is flawed.
Option e: 3, 1, 2. This sequence starts with the side-chain primary amine (3) as the most basic, reflecting its aliphatic nature and high availability of the lone pair. The next is the left-ring nitrogen (1), which is pyridine-like and contributes a lone pair that is available for protonation, giving moderate basicity. Finally, the N-H in the ring (2) is least basic because its lone pair is incorporated into the aromatic sextet, making protonation unfavorable and drastically reducing basicity. This option aligns with typical acid–base trends in imidazole derivatives.
Option f: 3, 2, 1. This ordering places the side-chain amine (3) first, then the N-H (2), and then the left-ring N (1). While the first position is reasonable, placing the non-basic N-H (2) ahead of the ring N (1) contradicts the established understanding that the pyridine-like ring nitrogen (1) is more basic than the N-H in imidazole. The sequence overall misrepresents the relative basicities.
In summary, the key idea is that the side-chain amine (3) is the most basic due to its aliphatic environment, the left-ring pyridine-like nitrogen (1) is moderately basic, and the ring N-H (2) is the least basic because its lone pair is delocalized into the aromatic ring. The option that mirrors this most closely is the one labeled with a sequence 3 > 1 > 2, which corresponds to option e.

My LMS Subjects Quiz 3

Nitrogen atoms are known to have basic (proton acceptor) properties.Consider the following nitrogen heterocycle (imidazole) and the nitrogen atoms labelled 1 - 3.Which (a - f) ranks the nitrogen atoms in order of most basic to least.

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