From the following compounds choose the one which is not aromatic ?

Options

A.
B.
C.
D.

Explanation

To determine which compound is not aromatic from the given options, we need to follow these steps:

### Step 1: Understand the Criteria for Aromaticity
A compound is considered aromatic if it meets the following three criteria:
1. **Cyclic Structure**: The compound must be in a ring form.
2. **Planarity**: The compound must be planar, meaning all atoms are in the same geometric plane.
3. **Hückel's Rule**: The compound must have a specific number of π (pi) electrons, which follows the formula \(4n + 2\), where \(n\) is a non-negative integer.

### Step 2: Analyze Each Compound
Now, let's analyze the compounds one by one based on the criteria mentioned above.

#### Compound A:
- **Structure**: A three-membered ring.
- **Pi Electrons**: 2 π electrons.
- **Cyclic**: Yes.
- **Planar**: Yes.
- **Hückel's Rule**:
- \(4n + 2 = 2\) implies \(4n = 0\) → \(n = 0\) (valid).
- **Conclusion**: This compound is aromatic.

#### Compound B:
- **Structure**: An eight-membered ring.
- **Pi Electrons**: 8 π electrons.
- **Cyclic**: Yes.
- **Planar**: Yes.
- **Hückel's Rule**:
- \(4n + 2 = 8\) implies \(4n = 6\) → \(n = 1.5\) (not valid).
- **Conclusion**: This compound is anti-aromatic, but due to its structure, it adopts a non-aromatic shape (tub shape), making it not aromatic.

#### Compound C:
- **Structure**: A five-membered ring.
- **Pi Electrons**: 6 π electrons.
- **Cyclic**: Yes.
- **Planar**: Yes.
- **Hückel's Rule**:
- \(4n + 2 = 6\) implies \(4n = 4\) → \(n = 1\) (valid).
- **Conclusion**: This compound is aromatic.

#### Compound D:
- **Structure**: A quasi-aromatic molecule (with a nitrogen atom).
- **Pi Electrons**: 6 π electrons (including the lone pair on nitrogen).
- **Cyclic**: Yes.
- **Planar**: Yes.
- **Hückel's Rule**:
- \(4n + 2 = 6\) implies \(4n = 4\) → \(n = 1\) (valid).
- **Conclusion**: This compound is aromatic.

### Final Conclusion
From the analysis, the compound that is **not aromatic** is **Compound B**.

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