Which of the following is not an aromatic compound ?

Options

A.
B.
C.
D.

Answer

To determine which of the given compounds is not an aromatic compound, we need to apply the four criteria for aromaticity:

1. **Cyclic Structure**: The compound must be cyclic.
2. **Planarity**: The compound must be planar.
3. **Delocalization of π-electrons**: The π-electrons must be delocalized over the structure.
4. **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 each option provided in the question.

### Step 1: Analyze Option A - Thiophene
- **Structure**: Thiophene is a 5-membered ring containing sulfur.
- **Count π-electrons**: It has 4 π-electrons from the double bonds and 2 from the lone pair on sulfur, totaling 6 π-electrons.
- **Check Hückel's Rule**: For \(n = 1\), \(4n + 2 = 6\). This satisfies Hückel's rule.
- **Cyclic and Planar**: Thiophene is cyclic and planar.
- **Conclusion**: Thiophene is aromatic.

### Step 2: Analyze Option B - Cyclohexadiene
- **Structure**: Cyclohexadiene is a 6-membered ring with two double bonds.
- **Count π-electrons**: It has 4 π-electrons (2 from each double bond).
- **Check Hückel's Rule**: For \(n = 0\), \(4n + 2 = 2\); for \(n = 1\), \(4n + 2 = 6\). It does not satisfy Hückel's rule with 4 π-electrons.
- **Cyclic and Planar**: Cyclohexadiene is cyclic and planar.
- **Conclusion**: Cyclohexadiene is not aromatic.

### Step 3: Analyze Option C - Cyclohexatrienyl Cation
- **Structure**: This is a 7-membered ring with 3 double bonds and a positive charge.
- **Count π-electrons**: It has 6 π-electrons (2 from each double bond).
- **Check Hückel's Rule**: For \(n = 1\), \(4n + 2 = 6\). This satisfies Hückel's rule.
- **Cyclic and Planar**: The structure is cyclic and planar.
- **Conclusion**: Cyclohexatrienyl cation is aromatic.

### Step 4: Analyze Option D - Furan
- **Structure**: Furan is a 5-membered ring with one oxygen atom.
- **Count π-electrons**: It has 4 π-electrons (2 from the double bonds and 2 from the lone pair on oxygen).
- **Check Hückel's Rule**: For \(n = 1\), \(4n + 2 = 6\). This satisfies Hückel's rule.
- **Cyclic and Planar**: Furan is cyclic and planar.
- **Conclusion**: Furan is aromatic.

### Final Conclusion
After analyzing all options, we find that the compound that is **not aromatic** is **Option B - Cyclohexadiene**.