Question

Among the given organic compounds, the total number of aromatic compounds is

• A. 1
• B. 2
• C. 3
• D. 4

Answer 1

Answer: (A)

1

Answer 2

Here's the analysis of each compound for aromaticity:

Compound (A): This compound is a fused bicyclic system. It has two fused six-membered rings. There are four double bonds shown in the conjugated system.

1. Cyclic: Yes, it is a cyclic compound.
2. Planar: The fused ring system may not be planar due to the presence of the bridgehead carbons and the arrangement of double bonds. However, for aromaticity, planarity is a key requirement.
3. Conjugated: The double bonds are conjugated.
4. Hückel's Rule: The total number of pi electrons is $4 \times 2 = 8$. For aromaticity, the number of pi electrons must be of the form $(4n+2)$. Since $8 \neq 4n+2$ for any integer $n$, it does not satisfy Hückel's rule. If it were planar, it would be anti-aromatic. However, due to likely non-planarity, it is considered non-aromatic.

Compound (B): This compound consists of a benzene ring attached to a cyclohexene ring via a methylene group. The benzene ring is aromatic. However, the cyclohexene ring is not. The entire molecule is not aromatic as a whole. Generally, when asked about aromatic compounds, the entire molecule's aromaticity is considered. Thus, compound (B) is not an aromatic compound.

Compound (C): This compound is a fused bicyclic system consisting of a six-membered benzene ring fused to a five-membered ring. The five-membered ring contains a double bond and a lone pair of electrons (represented by two dots) on a carbon atom. This structure represents the Inden-1-ide anion.

1. Cyclic: Yes, it is a cyclic compound.
2. Planar: The benzene ring is planar, and the five-membered ring with delocalized pi electrons can achieve planarity.
3. Conjugated: The system is conjugated. The benzene ring contributes 6 pi electrons. The five-membered ring has a double bond (2 pi electrons) and a lone pair on a carbon atom (2 electrons) that participates in the pi system.
4. Hückel's Rule: The total number of pi electrons is $6$ (from benzene) $+ 2$ (from double bond) $+ 2$ (from lone pair) $= 10$. This fits Hückel's rule, as $10 = 4n+2$ with $n=2$. Therefore, compound (C) is aromatic.

Compound (D): This compound is a fused bicyclic system containing a benzene ring and a five-membered ring with a double bond and a carbocation.

1. Cyclic: Yes, it is a cyclic compound.
2. Planar: The benzene ring is planar. The presence of a carbocation and a double bond in the five-membered ring suggests it could be planar.
3. Conjugated: The system is conjugated. The benzene ring contributes 6 pi electrons. The double bond contributes 2 pi electrons. The carbocation contributes 0 pi electrons to the pi system.
4. Hückel's Rule: The total number of pi electrons is $6$ (from benzene) $+ 2$ (from double bond) $= 8$. This does not fit Hückel's rule $(4n+2)$. If planar, it would be anti-aromatic. Thus, compound (D) is not aromatic.

Based on the analysis, only compound (C) is aromatic.

The total number of aromatic compounds among the given options is 1.