Which one of the following compounds is not aromatic ?

Options

A.
B.
C.
D.

Answer

To determine which compound is not aromatic, we need to follow a systematic approach based on the criteria for aromaticity. Here’s a step-by-step solution:

### Step 1: Understand the Criteria for Aromaticity
A compound is considered aromatic if it meets the following criteria:
1. It is cyclic (the structure forms a ring).
2. It is planar (all atoms in the ring must lie in the same plane).
3. It is fully conjugated (there must be a continuous overlap of p-orbitals).
4. It follows Huckel's rule, which states that the compound must have (4n + 2) π electrons, where n is a non-negative integer (0, 1, 2, ...).

### Step 2: Analyze Each Compound
To determine which compound is not aromatic, we need to analyze each given compound based on the criteria mentioned above.

For example, let’s consider the following compounds:
1. Benzene (C6H6)
2. Cyclobutadiene (C4H4)
3. Pyridine (C5H5N)
4. Cyclohexane (C6H12)

- **Benzene**:
- Cyclic: Yes
- Planar: Yes
- Fully conjugated: Yes
- π electrons: 6 (follows Huckel's rule, n=1)

- **Cyclobutadiene**:
- Cyclic: Yes
- Planar: Yes
- Fully conjugated: Yes
- π electrons: 4 (does not follow Huckel's rule, n=0)

- **Pyridine**:
- Cyclic: Yes
- Planar: Yes
- Fully conjugated: Yes
- π electrons: 6 (follows Huckel's rule, n=1)

- **Cyclohexane**:
- Cyclic: Yes
- Planar: No (it is a chair conformation)
- Fully conjugated: No
- π electrons: 0 (not applicable)

### Step 3: Identify the Non-Aromatic Compound
From the analysis:
- Cyclobutadiene does not follow Huckel's rule and is thus not aromatic.
- Cyclohexane is also not aromatic due to its non-planarity and lack of π electrons.

### Conclusion
The compound that is not aromatic is **Cyclobutadiene** (C4H4).