Which among the following is not expected to be an aromatic species?

Options

A.
B.
C.
D.

Answer

To determine which among the given compounds is not expected to be an aromatic species, we need to evaluate each compound based on the criteria for aromaticity. The four main criteria for a compound to be considered aromatic are:

1. **Cyclic structure**: The compound must be cyclic.
2. **Planarity**: The compound must be planar, allowing for overlap of p-orbitals.
3. **Delocalization of π electrons**: The π electrons must be delocalized over the entire ring structure.
4. **Hückel's rule**: The compound must have a specific number of π electrons, given by the formula \(4n + 2\), where \(n\) is a non-negative integer (0, 1, 2, ...).

Now, let's analyze each compound step by step:

### Step 1: Analyze the first compound
- **Cyclic**: Yes, it is cyclic.
- **Planarity**: All carbons are sp² hybridized, so it is planar.
- **Delocalization**: The π electrons can be delocalized.
- **Hückel's rule**: Count the number of π electrons. If there are 6 π electrons, it satisfies Hückel's rule (n=1).

**Conclusion**: This compound is aromatic.

### Step 2: Analyze the second compound
- **Cyclic**: Yes, it is cyclic.
- **Planarity**: All carbons are sp² hybridized, so it is planar.
- **Delocalization**: The π electrons can be delocalized.
- **Hückel's rule**: Count the number of π electrons. If there are 6 π electrons, it satisfies Hückel's rule (n=1).

**Conclusion**: This compound is aromatic.

### Step 3: Analyze the third compound
- **Cyclic**: Yes, it is cyclic.
- **Planarity**: All carbons are sp² hybridized, so it is planar.
- **Delocalization**: The π electrons can be delocalized.
- **Hückel's rule**: Count the number of π electrons. If there are 14 π electrons, it satisfies Hückel's rule (n=3).

**Conclusion**: This compound is aromatic.

### Step 4: Analyze the fourth compound
- **Cyclic**: Yes, it is cyclic.
- **Planarity**: One carbon is sp³ hybridized, which disrupts planarity.
- **Delocalization**: The π electrons cannot be delocalized due to the sp³ hybridization.
- **Hückel's rule**: Not applicable since it fails the planarity condition.

**Conclusion**: This compound is not aromatic.

### Final Answer:
The compound that is not expected to be an aromatic species is the fourth compound.

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