Which of the following is not aromatic ?

Options

A. Benzene
B. Cyclooctatetraenyl dianion
C. Tropylium cation
D. Cyclopentadienyl cation

Answer

To determine which of the given compounds is not aromatic, we need to analyze each option based on the criteria for aromaticity. Aromatic compounds must meet the following conditions:

1. They must be cyclic.
2. They must be planar.
3. They must have a continuous overlap of p-orbitals, allowing for delocalized π-electrons.
4. They must follow Huckel's rule, which states that they should have \(4n + 2\) π-electrons, where \(n\) is a non-negative integer (0, 1, 2, ...).

Let's analyze the options:

### Step 1: Identify the compounds
1. **Benzene** - C₆H₆
2. **Cyclopentadienyl cation**
3. **Potassium cation (K⁺)**
4. **Cyclopentadiene**

### Step 2: Check each compound for aromaticity

1. **Benzene**:
- Benzene is a six-membered ring with alternating double bonds (C₆H₆).
- It is cyclic, planar, and has 6 π-electrons (n=1 in Huckel's rule).
- **Conclusion**: Benzene is aromatic.

2. **Cyclopentadienyl cation**:
- Cyclopentadienyl cation is derived from cyclopentadiene (C₅H₆) by removing a hydrogen ion (H⁺).
- It has 5 carbon atoms and is cyclic and planar.
- The cation has 6 π-electrons (4 from the double bonds and 2 from the positive charge).
- **Conclusion**: Cyclopentadienyl cation is aromatic.

3. **Potassium cation (K⁺)**:
- K⁺ is not a hydrocarbon and does not have a cyclic structure or π-electrons.
- **Conclusion**: K⁺ is not aromatic.

4. **Cyclopentadiene**:
- Cyclopentadiene is a five-membered ring with two double bonds.
- It has 4 π-electrons (which makes it anti-aromatic).
- **Conclusion**: Cyclopentadiene is not aromatic.

### Final Conclusion:
From the analysis, the compound that is **not aromatic** is the **Potassium cation (K⁺)**.

### Summary of Steps:
1. Identify the compounds.
2. Analyze each compound for cyclic structure, planarity, and π-electron count.
3. Apply Huckel's rule to determine aromaticity.
4. Conclude which compound is not aromatic.