Aromaticity

Question

Which is aromatic?

RankGuru · Accepted Answer

1. **Hückel's Rule**: For a compound to be aromatic, it must be **Cyclic, Planar, Completely Conjugated**, and contain **$(4n+2) \pi$ electrons**.
2. **Evaluating Tropylium ($C_7H_7^+$)**:
   - It is a 7-membered cyclic ring.
   - It is planar and the positive charge is delocalized over all carbons.
   - It has $3$ double bonds $= 6$ $\pi$ electrons ($n=1$), satisfying $4n+2$.
   - Result: **Aromatic**.
3. **Evaluating others**:
   - **Cyclobutadiene**: $4$ $\pi$ electrons (Antiaromatic).
   - **Cyclopentadienyl cation**: $4$ $\pi$ electrons (Antiaromatic).
   - **Cyclooctatetraene**: Actually non-planar (tub-shaped) and has $8$ $\pi$ electrons. Even if planar, it would be antiaromatic ($4n$).

Aromaticity

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