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 apply the criteria for aromaticity. A compound is considered aromatic if it meets the following three criteria:

1. **Cyclic Structure**: The compound must be cyclic.
2. **Planarity**: The compound must be planar to allow for overlap of p-orbitals.
3. **Hückel's Rule**: The compound must have (4n + 2) π electrons, where n is a non-negative integer (0, 1, 2, ...).

Now, let's analyze the options provided in the question step by step:

### Step 1: Check for Cyclic Structure
- Ensure that all the given species are cyclic. If any of the species is not cyclic, it cannot be aromatic.

### Step 2: Count the π Electrons
- For each cyclic species, count the total number of π electrons. This includes π electrons from double bonds and any lone pairs that participate in resonance.

### Step 3: Apply Hückel's Rule
- For each species, check if the total number of π electrons fits the formula (4n + 2). If it does, the species is aromatic; if it does not, it is not aromatic.

### Step 4: Check Hybridization
- Confirm that all atoms in the ring are sp² hybridized. This is important for the overlap of p-orbitals necessary for aromaticity.

### Example Analysis
Assuming we have four species to analyze:

1. **Species A**: Cyclic, 6 π electrons (n=1) → Aromatic
2. **Species B**: Cyclic, 10 π electrons (n=2) → Aromatic
3. **Species C**: Cyclic, 8 π electrons (does not fit 4n + 2) → Not Aromatic
4. **Species D**: Cyclic, 10 π electrons (n=2) → Aromatic

### Conclusion
From the analysis, **Species C** is the one that does not meet the criteria for aromaticity because it does not satisfy Hückel's rule.

### Final Answer
The species that is not aromatic is **Species C**.

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