Q.13 The order of resonance energy for the following molecules is
(A) (1) > (3) > (2) > (4) (B) (1) > (3) > (4) > (2)
(C) (1) > (4) > (2) > (3) (D) (1) > (4) > (3) > (2)
Question Overview
The order of resonance energy for the following molecules is asked:
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Benzene
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Pyrrole
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Thiophene
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Furan
Options given:
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(A) (1) > (3) > (2) > (4)
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(B) (1) > (3) > (4) > (2)
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(C) (1) > (4) > (2) > (3)
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(D) (1) > (4) > (3) > (2)
Correct Answer
✅ Option (A): (1) > (3) > (2) > (4)
Concept: Resonance Energy & Aromatic Stabilization
Resonance energy is a measure of extra stability gained by a molecule due to delocalization of π-electrons.
Higher resonance energy = greater aromatic stabilization.
Key factors affecting resonance energy:
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Planarity
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Continuous conjugation
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Ability of heteroatom lone pairs to participate in π-delocalization
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Electronegativity of heteroatoms
Step-by-Step Explanation of Each Molecule
1. Benzene (Highest Resonance Energy)
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Fully conjugated six-π-electron system
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Perfect orbital overlap
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No heteroatoms disturbing electron density
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Maximum aromatic stabilization
✅ Highest resonance energy
3. Thiophene
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Sulfur contributes one lone pair to the aromatic sextet
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Larger size of sulfur allows better delocalization
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Lone pair donation is moderate, maintaining aromaticity
✅ Second highest resonance energy
2. Pyrrole
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Nitrogen donates its lone pair to the aromatic sextet
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Nitrogen is more electronegative than sulfur
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Lone pair donation is stronger, slightly reducing stability
⚠️ Lower resonance energy than thiophene
4. Furan (Lowest Resonance Energy)
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Oxygen is highly electronegative
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Poor overlap with π-system
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Lone pair donation is least effective
❌ Lowest resonance energy
Final Resonance Energy Order
Benzene>Thiophene>Pyrrole>Furan\textbf{Benzene} > \textbf{Thiophene} > \textbf{Pyrrole} > \textbf{Furan} (1)>(3)>(2)>(4)\boxed{(1) > (3) > (2) > (4)}
Why Other Options Are Incorrect
❌ Option (B): (1) > (3) > (4) > (2)
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Incorrectly places furan above pyrrole
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Oxygen’s high electronegativity destabilizes aromaticity
❌ Option (C): (1) > (4) > (2) > (3)
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Places thiophene last, which contradicts known aromatic stabilization trends
❌ Option (D): (1) > (4) > (3) > (2)
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Again overestimates furan’s stability
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Underestimates thiophene’s effective delocalization
Exam Tip (CSIR NET / GATE / JAM)
📌 Mnemonic to remember heteroaromatic stability:
S > N > O
(Less electronegative → better π-donation → higher resonance energy)
Conclusion
The correct order of resonance energy is governed by how effectively each ring maintains aromatic electron delocalization. Benzene remains the benchmark, while heteroatoms introduce varying degrees of stabilization depending on size and electronegativity.
✅ Correct Answer: Option (A)
