Q.17 The CORRECT order of basicity for the following compounds is
(A) I > II > III
(B) II > III > I
(C) II > I > III
(D) III > I > II
Imidazole (II) exhibits the highest basicity among the three compounds, followed by pyrazole (I), with 1,2,4-triazole (III) being the least basic. The correct order is II > I > III, corresponding to option (C).
Structures Identified
Compound I shows pyrazole: a five-membered ring with nitrogens at positions 1 (NH, pyrrole-like) and 2 (N, pyridine-like).
Compound II represents imidazole: nitrogens at positions 1 (NH) and 3.
Compound III is 1H-1,2,4-triazole: nitrogens at 1 (NH), 2, and 4.
Basicity Analysis
Basicity depends on the availability of the pyridine-like nitrogen lone pair for protonation and resonance stabilization in the conjugate acid. Higher pKa of the conjugate acid indicates stronger basicity: imidazole (~7.0), pyrazole (~2.5), 1,2,4-triazole (~2.4).
In imidazole, symmetric resonance in the imidazolium ion delocalizes the positive charge effectively across two equivalent nitrogens.
Pyrazole’s conjugate acid has less effective delocalization due to adjacent nitrogens, reducing charge spread.
Option Evaluation
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(A) I > II > III: Incorrect; pyrazole is weaker than imidazole due to poorer resonance.
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(B) II > III > I: Incorrect; 1,2,4-triazole’s extra nitrogen destabilizes the cation more than in pyrazole.
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(C) II > I > III: Correct; matches pKa order and resonance stability.
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(D) III > II > I: Incorrect; triazole has lowest basicity from three nitrogens crowding the ring.
The order of basicity pyrazole imidazole 1,2,4-triazole is a key concept in heterocyclic chemistry for CSIR NET Life Sciences aspirants. Imidazole tops the list due to optimal resonance, followed by pyrazole and then 1,2,4-triazole.
pKa Values Comparison
| Compound | Conjugate Acid pKa | Key Factor |
|---|---|---|
| Imidazole (II) | ~7.0 | Symmetric resonance delocalization |
| Pyrazole (I) | ~2.5 | Adjacent N reduces charge spread |
| 1,2,4-Triazole (III) | ~2.4 | Extra N destabilizes cation |
This azole basicity order stems from protonation on the sp² nitrogen lone pair.
Resonance in Conjugate Acids
Imidazole’s imidazolium ion shares charge equally: both nitrogens contribute to aromatic sextet stability. Pyrazole’s pyrazolium shows imbalanced resonance due to N-N adjacency, lowering basicity. 1,2,4-Triazole’s third nitrogen further electron-withdraws, yielding the weakest base.
CSIR NET Exam Relevance
For competitive exams, remember: two nitrogens with separated lone pairs (imidazole) > adjacent (pyrazole) > three nitrogens (triazole). Practice with pKa trends for quick scoring.
