Question

Which from following is a non benzenoid aromatic compound?

• A. Aniline
• B. Tropone
• C. Naphthalene
• D. Phenol

Hint:

Benzenoid aromatic compounds contain benzene rings. If aromaticity is present without a benzene ring, the compound is non-benzenoid.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Aromatic compounds are highly stable ring systems that fulfill specific criteria (Hückel's rule): they must be cyclic, planar, fully conjugated, and contain \((4n + 2) \pi\) electrons.
Aromatic compounds are broadly classified into two categories:
1. Benzenoid Aromatic Compounds: These contain one or more six-membered benzene rings in their structure.
2. Non-benzenoid Aromatic Compounds: These do not contain any benzene rings but still satisfy all the criteria for aromaticity (e.g., they might have 5-membered or 7-membered aromatic rings).
Step 2: Key Formula or Approach:
Evaluate each compound based on its structure to confirm if it contains a benzene ring (benzenoid) or if it achieves aromaticity through a different planar, conjugated system satisfying Hückel's rule (non-benzenoid).
Step 3: Detailed Explanation:
Let's look at the structure of each given option:
(A) Aniline: It consists of an amino group (\(-\text{NH}_2\)) directly attached to a benzene ring. Since it contains a benzene ring, it is a benzenoid compound.
(C) Naphthalene: Its structure consists of two fused benzene rings. It is clearly a benzenoid aromatic compound.
(D) Phenol: It consists of a hydroxyl group (\(-\text{OH}\)) directly attached to a benzene ring. Therefore, it is a benzenoid compound.
(B) Tropone: Its chemical name is 2,4,6-cycloheptatrien-1-one. It is a seven-membered carbon ring containing three alternating double bonds and one ketone functional group (\(\text{C}=\text{O}\)).
The high electronegativity of oxygen pulls electron density from the ring double bonds, creating a resonance structure where the oxygen is negatively charged and the ring carbon is positively charged (forming a tropylium cation-like core).
This core 7-membered ring then contains \(6 \pi\) electrons delocalized over 7 planar carbon atoms.
Since \(6 \pi\) electrons satisfy Hückel's rule (\(4n+2\) where \(n=1\)), the ring is aromatic.
However, because the ring has seven members and is not a 6-membered benzene ring, Tropone is classified as a non-benzenoid aromatic compound.
Step 4: Final Answer:
Tropone is the correct example of a non-benzenoid aromatic compound.