Which one is not aromatic compound ?

Options

A.
B.
C.
D.

Answer

To determine which compound is not aromatic, we need to apply the criteria for aromaticity. An aromatic compound must satisfy the following conditions:

1. **Planarity**: The compound must be planar, meaning that all atoms in the ring must lie in the same plane.
2. **Cyclic Structure**: The compound must be cyclic, meaning it forms a ring.
3. **Hückel's Rule**: The compound must have a specific number of π (pi) electrons, which follows Hückel's rule: \(4n + 2\) π electrons, where \(n\) is a non-negative integer (0, 1, 2, ...).

Now, let's analyze the given compounds one by one:

### Step 1: Analyze the first compound
- **Structure**: Check if it is cyclic and planar.
- **Hybridization**: Each carbon atom should be sp² hybridized.
- **Pi Electrons**: Count the number of π electrons.

Assuming the first compound has 2 π electrons:
- It is cyclic and planar.
- It has 2 π electrons, which fits Hückel's rule when \(n = 0\) (since \(4(0) + 2 = 2\)).
- **Conclusion**: This compound is aromatic.

### Step 2: Analyze the second compound (Naphthalene)
- **Structure**: Check if it is cyclic and planar.
- **Hybridization**: Each carbon atom should be sp² hybridized.
- **Pi Electrons**: Count the number of π electrons.

Naphthalene has 10 π electrons:
- It is cyclic and planar.
- It has 10 π electrons, which fits Hückel's rule when \(n = 2\) (since \(4(2) + 2 = 10\)).
- **Conclusion**: This compound is aromatic.

### Step 3: Analyze the third compound (Biphenyl)
- **Structure**: Check if it is cyclic and planar.
- **Hybridization**: Each carbon atom should be sp² hybridized.
- **Pi Electrons**: Count the number of π electrons.

Biphenyl consists of two benzene rings:
- Each benzene ring has 6 π electrons, so biphenyl has a total of 12 π electrons.
- It is planar and cyclic.
- It has 12 π electrons, which fits Hückel's rule when \(n = 3\) (since \(4(3) + 2 = 14\) does not fit, but each ring is aromatic).
- **Conclusion**: This compound is aromatic.

### Step 4: Analyze the fourth compound
- **Structure**: Check if it is cyclic and planar.
- **Hybridization**: Each carbon atom should be sp² hybridized.
- **Pi Electrons**: Count the number of π electrons.

Assuming this compound has 4 π electrons:
- It is cyclic and planar.
- It has 4 π electrons, which does not satisfy Hückel's rule (since \(n\) cannot be a non-negative integer for 4 π electrons).
- **Conclusion**: This compound is not aromatic.

### Final Answer
The compound that is not aromatic is the fourth compound.

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