Question

Which of the following molecules is not aromatic?

• A.
• B.
• C.
• D.

Hint:

To identify aromaticity: 1. Check for a cyclic, planar structure. 2. Count π-electrons in the conjugated system. 3. Verify if it follows \(4n + 2\) rule (\(n = 0, 1, 2,...\)).

Answer 1

The Correct Option is A

Aromaticity is determined by Hückel's rule (4n + 2 $\pi$-electrons in a cyclic, planar, conjugated system):

• (a) Cyclohexa-1,3-diene: 4 $\pi$-electrons (non-aromatic, violates Hückel’s rule).
• (b) Benzene: 6 $\pi$-electrons (aromatic).
• (c) Cyclopentadienyl anion: 6 $\pi$-electrons (aromatic).
• (d) Tropylium cation: 6 $\pi$-electrons (aromatic).

Only option (a) lacks aromaticity.
Thus, the correct answer is \(\boxed{1}\).

Answer 2

Step 1: Understand Hückel's rule for aromaticity.
A compound is aromatic if it is cyclic, planar, fully conjugated, and contains \((4n + 2)\) \(\pi\)-electrons, where \(n\) is a non-negative integer.

Step 2: Analyze each compound.

• (a) Cyclohexa-1,3-diene: It has 4 \(\pi\)-electrons (from two double bonds), which does not satisfy Hückel's rule (\(4n + 2\)). Therefore, it is non-aromatic.
• (b) Benzene: It has 6 \(\pi\)-electrons, satisfying Hückel's rule with \(n=1\). It is aromatic.
• (c) Cyclopentadienyl anion: It has 6 \(\pi\)-electrons (4 from double bonds plus 2 from the lone pair on the negatively charged carbon), satisfying Hückel's rule. It is aromatic.
• (d) Tropylium cation: It has 6 \(\pi\)-electrons in a planar cyclic conjugated system, satisfying Hückel's rule. It is aromatic.

Step 3: Conclusion.
Only option (a) — cyclohexa-1,3-diene — is non-aromatic.
Hence, the correct answer is \(\boxed{1}\).