Q16. The CORRECT order of acidity of the following compounds is
(A) II > I > III
(B) II > III > I
(C) III > II > I
(D) III > I > II
Cyclopentadiene (II) exhibits the highest acidity among the three compounds due to the aromatic stabilization of its conjugate base, followed by cyclopentene (III) with a vinylic hydrogen, and cyclopentane (I) as the least acidic. The correct order is II > III > I (option B).
Compound Structures
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Cyclopentane (I): Fully saturated C5 ring with all sp³-hybridized carbons and equivalent C-H bonds (pKa >50).
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Cyclopentadiene (II): C5 ring with two conjugated double bonds and one sp³ CH₂ group (pKa ≈15-16); deprotonation occurs at the CH₂.
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Cyclopentene (III): C5 ring with one isolated C=C bond; relevant acidity from vinylic H on sp² carbon (pKa ≈44).
Acidity Analysis
Acidity depends on conjugate base stability: greater stability means lower pKa (stronger acid). Cyclopentadiene’s CH₂ loses H⁺ to form the cyclopentadienyl anion (6 π electrons, aromatic per Hückel’s rule), providing exceptional resonance delocalization. Cyclopentene’s vinylic H yields a less stable vinyl anion due to sp² hybridization (higher s-character holds electrons tighter), outperforming cyclopentane’s sp³ alkyl anion, which lacks any stabilization.
Option Breakdown
| Option | Order | Why Incorrect/Correct |
|---|---|---|
| (A) II > III | Wrong | Ignores III’s moderate acidity from sp² H (pKa 44 >50 for I). |
| (B) II > III > I | Correct | Matches pKa: 15 (II aromatic anion) < 44 (III vinyl) < >50 (I alkyl). |
| (C) III > II | Wrong | Vinyl anion far less stable than aromatic anion. |
| (D) III > I | Wrong | Fails to rank most acidic II. |
In organic chemistry for CSIR NET aspirants, understanding the acidity order cyclopentadiene cyclopentene cyclopentane hinges on conjugate base stability and hybridization. Cyclopentadiene (pKa ~15) tops the list as its deprotonation yields an aromatic cyclopentadienyl anion with 6 π electrons following Hückel’s rule (4n+2, n=1), offering unmatched delocalization.
Cyclopentene follows (pKa ~44) because its vinylic sp² hydrogen forms a vinyl anion; sp² carbons’ 33% s-character enhances acidity over sp³ but lacks aromaticity. Cyclopentane, with all sp³ C-H bonds (pKa >50), is least acidic—no resonance or hybridization advantage stabilizes its carbanion.
Why Aromaticity Dominates
Aromatic species gain ~36 kcal/mol extra stability, slashing cyclopentadiene’s pKa dramatically versus typical hydrocarbons. For CSIR NET, memorize: allylic/ benzylic positions boost acidity, but full aromaticity in anions (like here) is unbeatable.
Exam Tips
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Compare pKa trends: sp (25) > sp² (44) > sp³ (>50).
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Draw resonance for cyclopentadienyl anion to visualize equality of carbons.
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Practice similar: indene (pKa 20) less acidic than cyclopentadiene due to disrupted aromaticity.
Master acidity order cyclopentadiene cyclopentene cyclopentane for scoring in molecular orbital and acid-base sections.
