Understanding Basicity of Pyridine and Pyrrole

This question asks us to analyze two statements about the basicity of pyridine and pyrrole and determine their truthfulness and if one explains the other.

Analyzing Statement (a): Basicity Comparison

Statement (a) says that pyridine and pyrrole are both weak bases, but pyridine is much more basic than pyrrole.

• Pyridine is an aromatic heterocyclic amine. Its nitrogen atom has a lone pair of electrons.
• Pyrrole is also an aromatic heterocyclic amine. Its nitrogen atom also has a lone pair of electrons.

Basicity is related to the ability of an atom, specifically the nitrogen atom in this case, to donate its lone pair of electrons to accept a proton (H$^+$). A stronger base is better at accepting a proton.

Experimentally, pyridine (pKb $\approx$ 8.75, pKa of conjugate acid $\approx$ 5.25) is indeed a much stronger base than pyrrole (pKb $\approx$ 13.6, pKa of conjugate acid $\approx$ -3.6). Pyrrole is a very weak base, almost neutral or even slightly acidic in some contexts due to the acidity of its N-H proton.

Therefore, statement (a) is true: both are weak bases (compared to aliphatic amines), but pyridine is significantly more basic than pyrrole.

Analyzing Statement (b): Lone Pair Localization and Delocalization

Statement (b) describes the location of the nitrogen lone pair in pyridine and pyrrole and its role in aromaticity.

• In Pyridine: Pyridine is a six-membered ring containing nitrogen. It has three pi $(\pi)$ bonds within the ring. The nitrogen atom is sp$^{2}$ hybridized. The lone pair of electrons on the nitrogen atom is located in an sp$^{2}$ hybrid orbital that is perpendicular to the pi system of the ring. This lone pair is not part of the 6 pi electrons required for the ring's aromaticity. The aromaticity comes from the three double bonds' pi electrons (6 $\pi$ electrons). The lone pair is available for donation to a proton.
• In Pyrrole: Pyrrole is a five-membered ring containing nitrogen. It has two pi $(\pi)$ bonds. The nitrogen atom is sp$^{2}$ hybridized. The lone pair of electrons on the nitrogen atom is located in a p orbital that is parallel to the pi system of the two double bonds. For pyrrole to be aromatic (which it is, obeying Hückel's rule), the lone pair on the nitrogen atom must be delocalized into the ring's pi system. The aromatic system consists of the 4 pi electrons from the two double bonds plus the 2 electrons from the nitrogen's lone pair, totaling 6 pi electrons. This makes the lone pair part of the aromatic ring and less available for donation.

Therefore, statement (b) is true: the lone pair in pyridine is localized and not part of the aromatic system, while the lone pair in pyrrole is delocalized and essential for its aromaticity.

Connecting Statement (b) to Statement (a)

Basicity is determined by the availability of the lone pair to accept a proton.

• In pyridine, the nitrogen lone pair is localized and readily available to bond with a proton.
• In pyrrole, the nitrogen lone pair is delocalized into the aromatic ring. Protonating the nitrogen would remove these electrons from the pi system, disrupting the aromaticity and making the resulting species less stable. This makes the nitrogen lone pair in pyrrole far less available for protonation.

Because the lone pair in pyridine is readily available while the lone pair in pyrrole is involved in aromaticity and thus less available, pyridine is a much stronger base than pyrrole. Statement (b) provides the reason (the difference in lone pair availability due to involvement in the aromatic system) for the difference in basicity described in statement (a).

Thus, statement (b) is the correct explanation for statement (a).

Based on this analysis, both statements are true, and statement (b) correctly explains statement (a).