Aromaticity: Relative Stability of the π-System explains how delocalized π-electrons in cyclic conjugated systems provide extra stability compared to non-aromatic compounds.
Aromaticity: Relative Stability of the π-System
- All three compounds—pyrrole, furan, and thiophene—are aromatic because they each have:
- A 5-membered ring
- 4 carbon atoms contributing 4 π-electrons
- One heteroatom (N, O, or S) that donates a lone pair into the π-system, contributing 2 more π-electrons
- Total = 6 π-electrons, satisfying Hückel’s Rule (4n + 2, where n = 1)
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Comparison of Aromaticity
| Compound | Heteroatom | Lone Pair Delocalization | Aromatic Stability (Relative) |
| Thiophene | Sulfur (S) | Larger size, less electronegative, better orbital overlap | Most aromatic |
| Pyrrole | Nitrogen (N) | Lone pair contributes to π-system; moderate overlap | Moderate |
| Furan | Oxygen (O) | Highly electronegative, prefers to hold lone pair; poorer overlap | Least aromatic |
Why?
- Oxygen (furan) is the most electronegative → reluctant to share its lone pair → weaker delocalization
- Nitrogen (pyrrole) is less electronegative than oxygen → better delocalization
- Sulfur (thiophene) is larger and less electronegative → best overlap with π-system despite its size
So, aromatic stability decreases in the order:
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Thiophene > Pyrrole > Furan
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