Why is II a weaker base than I?

• A. It's not. I is the weaker base.
• B. Using II as a base breaks aromaticity.
• C. II is a smaller ring size.
• D. II has an acidic hydrogen.

Answer: (B)

Verified step by step guidance

1. Identify the structures: Structure I is pyridine, a six-membered aromatic ring with a nitrogen atom. Structure II is pyrrole, a five-membered aromatic ring with a nitrogen atom and an attached hydrogen.
2. Understand aromaticity: Aromatic compounds are stable due to delocalized π-electrons. Pyridine (I) maintains aromaticity when acting as a base, as the lone pair on nitrogen is not part of the aromatic system. Pyrrole (II), however, loses aromaticity if the nitrogen's lone pair is used as a base, as it is part of the aromatic system.
3. Consider the role of the nitrogen atom: In pyridine (I), the nitrogen atom has a lone pair that is not involved in the aromatic π-system, making it available for protonation, thus acting as a base. In pyrrole (II), the nitrogen's lone pair is part of the aromatic π-system, making it less available for protonation.
4. Evaluate the impact of aromaticity on basicity: Using pyrrole (II) as a base disrupts its aromaticity, which is energetically unfavorable. This makes pyrrole a weaker base compared to pyridine, which retains its aromaticity upon protonation.
5. Conclude the reasoning: The correct answer is that using II (pyrrole) as a base breaks aromaticity, making it a weaker base than I (pyridine).