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

Hardchemistry

Which is aromatic?

Select the correct option:

Cyclobutadiene

Tropylium cation (C7H7+)

Cyclooctatetraene (planar)

Cyclopentadienyl cation

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About This Question

Subject: chemistry
Chapter: hydrocarbons
Topic: aromaticity
Difficulty: Hard
Year: 2025

Solution

Correct Answer:

Tropylium cation (C7H7+)

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

This hard difficulty chemistry question is from the chapter hydrocarbons, covering the topic of aromaticity. It appeared in the 2025 exam.

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Aromaticity

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Aromaticity

Select the correct option:

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