Which one of the following is an aromatic compound?

Options

A. Cyclopentadienyl cation
B. Cycloheptatrienyl cation
C. Cycloheptatrienyl anion
D. Cycloheptatriene

Explanation

To determine which of the given compounds is aromatic, we will follow Huckel's rule, which outlines the criteria for aromaticity. Let's break down the steps:

### Step 1: Understand Huckel's Rule
Huckel's rule states that for a compound to be aromatic, it must satisfy two main conditions:
1. The compound must be cyclic and planar.
2. The compound must have a cyclic cloud of π electrons that follows the formula \(4n + 2\), where \(n\) is a non-negative integer (0, 1, 2, ...).

### Step 2: Analyze Each Compound
We will analyze each option provided in the question to see if they meet the criteria for aromaticity.

#### Option A: Cyclopentadienyl cation
1. **Structure**: It has a 5-membered ring.
2. **Hybridization**: The carbon atoms are sp² hybridized, indicating they are in the same plane.
3. **π Electrons**: There are 2 π bonds, contributing 4 π electrons.
4. **Aromaticity Check**: Since it has 4 π electrons, it does not satisfy Huckel's rule (which requires \(4n + 2\)). Therefore, it is anti-aromatic.

#### Option B: Cycloheptatrienyl cation
1. **Structure**: It has a 7-membered ring.
2. **Hybridization**: All carbon atoms are sp² hybridized, so they are planar.
3. **π Electrons**: There are 3 π bonds, contributing 6 π electrons.
4. **Aromaticity Check**: Since it has 6 π electrons, it satisfies Huckel's rule (\(n=1\) gives \(4(1) + 2 = 6\)). Therefore, it is aromatic.

#### Option C: Anion (Cycloheptatrienyl anion)
1. **Structure**: Similar to option B but with an additional negative charge.
2. **Hybridization**: All carbon atoms remain sp² hybridized and planar.
3. **π Electrons**: There are still 3 π bonds, contributing 6 π electrons from the π bonds, plus 2 from the negative charge, totaling 8 π electrons.
4. **Aromaticity Check**: Since it has 8 π electrons, it follows the \(4n\) rule (where \(n=2\)). Therefore, it is anti-aromatic.

### Conclusion
Based on the analysis:
- **Option A** is anti-aromatic.
- **Option B** is aromatic.
- **Option C** is anti-aromatic.

Thus, the aromatic compound among the options is **Option B: Cycloheptatrienyl cation**.

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