Which of the following is not aromatic?

Options

A. Benzene
B. Cyclopropenly cation
C. Tropylium cation
D. Cyclopentadieny cation

Explanation

To determine which of the following compounds is not aromatic, we need to apply Huckel's rule and check the characteristics of each compound. For a compound to be classified as aromatic, it must fulfill the following criteria:

1. **Cyclic Structure**: The compound must be cyclic.
2. **Planarity**: The compound must be planar.
3. **Fully Conjugated**: The compound must have a continuous overlap of p-orbitals.
4. **Huckel's Rule**: The compound must have \(4n + 2\) π electrons, where \(n\) is a non-negative integer.

Let's analyze the given options step by step:

### Step 1: Analyze Benzene
- **Structure**: Benzene is a six-membered ring with alternating double bonds.
- **Cyclic**: Yes, it is cyclic.
- **Planar**: Yes, it is planar.
- **Fully Conjugated**: Yes, all carbon atoms are sp² hybridized allowing for π overlap.
- **π Electrons**: Benzene has 6 π electrons.
- Calculation: \(n = 1\) → \(4(1) + 2 = 6\)
- **Conclusion**: Benzene is aromatic.

### Step 2: Analyze Cyclopropanil Cation
- **Structure**: Cyclopropanil cation is a three-membered ring with a positive charge.
- **Cyclic**: Yes, it is cyclic.
- **Planar**: Yes, it is planar.
- **Fully Conjugated**: Yes, the positive charge allows for π overlap.
- **π Electrons**: It has 2 π electrons.
- Calculation: \(n = 0\) → \(4(0) + 2 = 2\)
- **Conclusion**: Cyclopropanil cation is aromatic.

### Step 3: Analyze Tropolium Cation
- **Structure**: Tropolium cation is a seven-membered ring with a positive charge.
- **Cyclic**: Yes, it is cyclic.
- **Planar**: Yes, it is planar.
- **Fully Conjugated**: Yes, it has continuous overlap of p-orbitals.
- **π Electrons**: It has 6 π electrons.
- Calculation: \(n = 1\) → \(4(1) + 2 = 6\)
- **Conclusion**: Tropolium cation is aromatic.

### Step 4: Analyze Cyclopentadiene Cation
- **Structure**: Cyclopentadiene cation is a five-membered ring with a positive charge.
- **Cyclic**: Yes, it is cyclic.
- **Planar**: Yes, it is planar.
- **Fully Conjugated**: Yes, it has continuous overlap of p-orbitals.
- **π Electrons**: It has 4 π electrons.
- Calculation: \(n = 1\) → \(4(1) + 2 = 6\) (not satisfied, as it has 4)
- **Conclusion**: Cyclopentadiene cation is anti-aromatic (it satisfies the \(4n\) rule).

### Final Conclusion
Among the options analyzed, the compound that is **not aromatic** is the **cyclopentadiene cation**.