11.
Arrange the following compounds in increasing order of acid strength:
I: Phenol, II: o-nitrophenol, III: m-nitrophenol, IV: 2,4-dinitrophenol, V: 2,4,6-
trinitrophenol
a. I < V < IV < II < III
b. V < IV < III < II < I
c. I < III < II < IV < V
d. I < II < III < IV < V
Phenol nitrophenol acid strength order follows increasing electron-withdrawing nitro group effects stabilizing the phenoxide ion. The correct arrangement is I < II < III < IV < V, matching option d. This MCQ tests substituent impacts on phenol acidity for CSIR NET exams.
Acidity Principles
Nitro groups (-NO₂) act as strong electron-withdrawers via inductive (-I) and resonance (-R) effects, enhancing phenol acidity by delocalizing phenoxide negative charge. Phenol (pKa ~10) serves as baseline; more nitro groups or ortho/para positions increase acidity most. Ortho/para nitro allow resonance stabilization, meta relies on weaker -I effect. Additional nitro groups amplify this progressively.
Compound Analysis
| Compound | Position | pKa (approx.) | Key Factor |
|---|---|---|---|
| I: Phenol | None | 9.95-10 | No substituents |
| II: o-Nitrophenol | Ortho | 7.17-7.23 | Strong -R/-I + H-bonding reduces slightly vs para |
| III: m-Nitrophenol | Meta | 8.28-8.4 | -I only, no resonance |
| IV: 2,4-Dinitrophenol | 2,4 (o/p) | ~4.0-4.5 | Two nitro, enhanced resonance |
| V: 2,4,6-Trinitrophenol (Picric acid) | 2,4,6 (o/p/o) | 0.3-0.71 | Three nitro, extreme stabilization |
Option Evaluation
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a. I < V < IV < II < III: Wrong; V (strongest) cannot precede IV/II.
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b. V < IV < III < II < I: Reverse order ignores nitro enhancement.
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c. I < III < II < IV < V: Swaps II/III; o- > m- due to resonance despite H-bonding.
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d. I < II < III < IV < V: Correct; matches pKa trend: phenol < o-NO₂ < m-NO₂ < 2,4-(NO₂)₂ < 2,4,6-(NO₂)₃.
