Nitrogen containing compounds are basic in nature and their basic strength depends on ease of availability of lone pair of electrons on nitrogen atoms. For the following nitrogen containing compounds what will be correct order of basicity?
| A. |  |
| B. |
|
| C. |  |
| D. |  |
| E. |  |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A > D > B > E > C
The basic strength of a nitrogen compound depends on how freely the lone pair on nitrogen is available to take up a proton. Electron-releasing alkyl groups make it more available; delocalisation into a ring and greater s-character of the orbital holding it make it less available.
A. Piperidine — a cyclic secondary aliphatic amine; the sp3 nitrogen carries two alkyl (ring) carbons, so the lone pair is the most available (pKb about 2.9).
D. Cyclohexylamine — a primary aliphatic amine with one alkyl group on nitrogen; slightly weaker than piperidine (pKb about 3.3).
B. Pyridine — the lone pair is not part of the aromatic ring, but it sits in an sp2 orbital (more s-character, held closer to the nucleus), so pyridine is a much weaker base (pKb about 8.8).
E. Aniline — the lone pair is delocalised into the benzene ring by resonance, so it is even less available (pKb 9.38).
C. Pyrrole — the lone pair is part of the aromatic sextet of the ring; protonating nitrogen would destroy aromaticity, so pyrrole is practically non-basic.
Shortcut: only Option (4) begins with A (piperidine) and ends with C (pyrrole).
Note: The question is from beyond the NCERT textbook.