Aromaticity
Hardchemistry
Which is aromatic?
Select the correct option:
Solution
Incorrect! Answer:
Tropylium cation (C7H7+)
- Hückel's Rule: For a compound to be aromatic, it must be Cyclic, Planar, Completely Conjugated, and contain (4n+2)π electrons.
- Evaluating Tropylium (C7H7+):
- 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 4n+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 (4n).
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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 (4n+2)π electrons.
- Evaluating Tropylium (C7H7+):
- 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 4n+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 (4n).
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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