Hückel's rule: (4n+2) π electrons for planar cyclic conjugation (aromaticity) (Planar, cyclic, fully conjugated species with (4n+2) π electrons is aromatic (special)
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Planar, cyclic, fully conjugated species with (4n+2) π electrons is aromatic (special stability). 4n π → antiaromatic if planar; non-planar can relieve antiaromaticity.
What each symbol means
| Symbol | What it stands for |
|---|---|
| n | Non-negative integer in Hückel formula |
| π electrons | Count all electrons in conjugated p orbitals (lone pairs on sp² hybrid atoms may count if in plane of π system) |
When to use this
Must be cyclic, planar (or effectively planar), and every atom in ring conjugated p system. Benzene: 6 π e⁻ → n=1.