Question

Which one of the following is aromatic?

• A. Cycloheptatrienyl cation
• B. Cycloheptatriene
• C. Cyclooctatetraene
• D. Cyclopentadienyl cation

Hint:

The tropylium ion (C\(_7\)H\(_7^+\)) is a classic example of an aromatic carbocation with 6 \(\pi\)-electrons.

Answer 1

The Correct Option is A

To determine which of the given compounds is aromatic, we should apply Hückel's rule for aromaticity. According to Hückel's rule, a compound is considered aromatic if it satisfies the following criteria:

• It is cyclic.
• It is planar.
• It is fully conjugated (i.e., every atom in the ring must have a p-orbital for continuous overlap).
• It must contain \(4n + 2\) π electrons (where \(n\) is a non-negative integer).

Now, let's analyze each option:

1. Cycloheptatrienyl cation: This ion is formed by the removal of a hydride ion (\(H^-\)) from cycloheptatriene, creating a positive charge. It has a conjugated system of 6 π electrons (follows \(4n + 2 = 6\) with \(n = 1\)), and it is planar and cyclic. Therefore, it satisfies Hückel's rule and is aromatic.
2. Cycloheptatriene: Although it is cyclic and planar, it does not have a fully conjugated π electron system due to the presence of a non-aromatic CH\(_2\) group, and it does not fulfill the \(4n + 2\) rule. Thus, it is not aromatic.
3. Cyclooctatetraene: This compound is cyclic and has 8 π electrons. However, it is not planar but rather adopts a tub-shaped conformation, which prevents it from having a completely conjugated π system, and it does not satisfy the \(4n + 2\) rule since \(8 = 4n\). Therefore, it is non-aromatic.
4. Cyclopentadienyl cation: Although this is planar and cyclic, it has only 4 π electrons (\(4n = 4\)), thus not fulfilling the \(4n + 2\) π electron count required for aromaticity. Hence, this is non-aromatic.

In conclusion, the Cycloheptatrienyl cation satisfies all the criteria for aromaticity, making it aromatic.