Correct order of basic strength of Pyrrole,

Pyridine,

and Piperidine

is:

Availability of the Nitrogen Lone Pair

The basic strength of an amine or a heterocyclic nitrogen compound depends directly on the availability of its lone pair of electrons on the nitrogen atom to accept a proton ($$\text{H}^+$$). If the lone pair is highly localized, the compound is more basic; if it is delocalized due to resonance or held tightly by a more electronegative orbital, the basicity drops significantly.

Detailed Analysis of Each Compound:

• 1. Piperidine

  Piperidine is a cyclic secondary aliphatic amine. The nitrogen atom is completely $$\text{sp}^3$$ hybridized, and its lone pair of electrons is highly localized in an $$\text{sp}^3$$ orbital. Since it does not participate in resonance and benefits from the electron-donating inductive effect ($$+I$$) of the adjacent alkyl ring chain, piperidine is the most basic compound among the three.




• 2. Pyridine

  In pyridine, the nitrogen atom is part of an aromatic ring and is $$\text{sp}^2$$ hybridized. The lone pair of electrons resides in an $$\text{sp}^2$$ hybrid orbital that lies within the plane of the ring, meaning it is perpendicular to the $$\pi$$ system and does not participate in aromaticity. It is available for protonation, but because an $$\text{sp}^2$$ hybridized nitrogen is more electronegative than an $$\text{sp}^3$$ nitrogen, it holds its electrons more tightly. Therefore, pyridine is significantly less basic than piperidine.




• 3. Pyrrole

  In pyrrole, the lone pair on the nitrogen atom is actively involved in the aromatic ring system to satisfy Hückel's rule ($$6\pi$$ electrons). Because these electrons are fully delocalized within the aromatic ring, donating them to a proton would destroy the molecule's aromatic stabilization. This makes pyrrole an exceptionally weak base, placing it as the least basic compound in the group.

Conclusion:

Combining the hybridization and delocalization factors, the correct decreasing order of basic strength is:

$$\mathbf{\text{Piperidine} > \text{Pyridine} > \text{Pyrrole}}$$

Answer: Option D — Piperidine > Pyridine > Pyrrole