Which of the following is not aromatic?

Options

A. Benzene
B. Cyclopentadienyl cation
C. Cyclopropenyl cation
D. Tropylium cation

Explanation

To determine which of the given compounds is not aromatic, we will apply Huckel's rule and analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Aromaticity**:
- A compound is considered aromatic if it meets the following criteria:
1. It must be cyclic.
2. It must be planar (all atoms in the same plane).
3. It must have a continuous ring of p orbitals (delocalized π electrons).
4. It must have a total of \(4n + 2\) π electrons, where \(n\) is a non-negative integer.

2. **Analyzing Option A: Benzene**:
- Benzene has a six-membered carbon ring.
- Each carbon is sp² hybridized, making the molecule planar.
- Benzene has 6 π electrons (from 3 double bonds).
- According to Huckel's rule: \(n = 1\) gives \(4(1) + 2 = 6\).
- Benzene is aromatic.

3. **Analyzing Option B: Cyclopentadienyl Cation**:
- The cyclopentadienyl cation has a five-membered ring with a positive charge.
- Each carbon is sp² hybridized, making the molecule planar.
- The cation has 4 π electrons (from 2 double bonds).
- According to Huckel's rule: \(n = 1\) gives \(4(1) = 4\).
- Since it has 4 π electrons, it is anti-aromatic.

4. **Analyzing Option C**:
- The structure of the compound in option C is not specified, but let's assume it has a positive charge and 2 π electrons.
- If it has 2 π electrons, according to Huckel's rule: \(n = 0\) gives \(4(0) + 2 = 2\).
- Thus, this compound is aromatic.

5. **Analyzing Option D**:
- The structure of option D is also not specified, but let's assume it has a 7-membered ring with 3 π bonds.
- If it has 6 π electrons (from 3 double bonds), according to Huckel's rule: \(n = 1\) gives \(4(1) + 2 = 6\).
- Thus, this compound is aromatic.

### Conclusion:
Based on the analysis:
- Option A (Benzene) is aromatic.
- Option B (Cyclopentadienyl Cation) is anti-aromatic.
- Option C is aromatic.
- Option D is aromatic.

**Final Answer**: The compound that is not aromatic is **Option B: Cyclopentadienyl Cation**.