Question

Which of the following is not an aromatic species?

• A. Cycloheptatrienyl cation
• B. Cyclopentadienyl anion
• C. Cyclooctatetraene
• D. Naphthalene

Hint:

To be aromatic, a cyclic, conjugated molecule must be planar and have $(4n+2)$ $\pi$ electrons. If it is planar and has $4n$ $\pi$ electrons, it is anti-aromatic. If it is not planar, it is non-aromatic, regardless of the electron count. Cyclooctatetraene adopts a non-planar tub shape to avoid being anti-aromatic.

Answer 1

The Correct Option is C

To determine which of the given species is not aromatic, we need to apply Hückel's Rule. According to Hückel's Rule, a planar, cyclic molecule is aromatic if it has \(4n + 2\) π electrons, where \(n\) is a non-negative integer.

1. Cycloheptatrienyl cation: This species has 6 π electrons (3 double bonds), fitting the \(4n + 2\) rule (with \(n = 1\)). It is a planar and conjugated system. Therefore, it is aromatic.
2. Cyclopentadienyl anion: This species has 6 π electrons (due to the presence of 2 double bonds and an extra electron from the anion), again matching the \(4n + 2\) rule (with \(n = 1\)). It is aromatic.
3. Cyclooctatetraene: This species has 8 π electrons (4 double bonds), fitting the \(4n\) rule instead of \(4n + 2\). Cyclooctatetraene tends to adopt a tub-like conformation to avoid anti-aromaticity and is thus not aromatic.
4. Naphthalene: This compound has 10 π electrons (5 double bonds), which fits the \(4n + 2\) rule (with \(n = 2\)). It is aromatic.

From the above reasoning, Cyclooctatetraene is not aromatic as it does not satisfy the \(4n + 2\) π electron rule for aromaticity and instead adopts a non-planar structure to avoid being anti-aromatic.