Which of the following species is not aromatic ?

Options

A.
B.
C.
D.

Answer

To determine which of the given species is not aromatic, we need to evaluate each option based on the four criteria for aromaticity:

1. The compound must be cyclic.
2. The compound must be planar.
3. There must be a delocalization of π electrons.
4. The compound must satisfy Huckel's rule, which states that there must be \(4n + 2\) π electrons, where \(n\) is a non-negative integer.

Let's analyze each option step by step:

### Step 1: Analyze Cyclopentadienyl Anion
- **Cyclic**: Yes, it is a five-membered ring.
- **Planar**: Yes, all carbon atoms are sp² hybridized, allowing for planarity.
- **Delocalization of π electrons**: There are 6 π electrons (2 from each of the two double bonds and 2 from the negative charge).
- **Huckel's Rule**: For \(n = 1\), \(4(1) + 2 = 6\) π electrons. This matches the 6 π electrons present.

**Conclusion**: Cyclopentadienyl anion is aromatic.

### Step 2: Analyze Cyclopentadiene Cation
- **Cyclic**: Yes, it is a five-membered ring.
- **Planar**: Yes, it is planar due to sp² hybridization.
- **Delocalization of π electrons**: There are 4 π electrons (2 from the two double bonds; the positive charge does not contribute).
- **Huckel's Rule**: For \(n = 1\), \(4(1) + 2 = 6\) π electrons. However, we only have 4 π electrons.

**Conclusion**: Cyclopentadiene cation is not aromatic (non-aromatic).

### Step 3: Analyze Cyclopropene Cation
- **Cyclic**: Yes, it is a three-membered ring.
- **Planar**: Yes, it is planar due to sp² hybridization.
- **Delocalization of π electrons**: There are 2 π electrons (from the double bond).
- **Huckel's Rule**: For \(n = 0\), \(4(0) + 2 = 2\) π electrons, which matches the 2 π electrons present.

**Conclusion**: Cyclopropene cation is aromatic.

### Step 4: Analyze Cycloheptatrienyl Cation
- **Cyclic**: Yes, it is a seven-membered ring.
- **Planar**: Yes, it is planar due to sp² hybridization.
- **Delocalization of π electrons**: There are 6 π electrons (from the three double bonds).
- **Huckel's Rule**: For \(n = 1\), \(4(1) + 2 = 6\) π electrons, which matches the 6 π electrons present.

**Conclusion**: Cycloheptatrienyl cation is aromatic.

### Final Conclusion
The only compound that does not satisfy the criteria for aromaticity is the **Cyclopentadiene Cation**.

### Summary of Results
- Cyclopentadienyl anion: Aromatic
- Cyclopentadiene cation: **Not Aromatic**
- Cyclopropene cation: Aromatic
- Cycloheptatrienyl cation: Aromatic

Thus, the answer to the question is **Cyclopentadiene Cation**.

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