Which among the following is not an aromatic species?

Options

A.
B.
C.
D.

Answer

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

1. The compound must be cyclic.
2. The compound must be planar.
3. Each atom in the ring must be sp² hybridized.
4. The compound must follow Huckel's rule, which states that the number of π electrons should be equal to \(4n + 2\), where \(n\) is a non-negative integer (0, 1, 2, ...).

Let's analyze each compound one by one:

### Step 1: Analyze the First Compound
- **Structure**: Assume it is a simple cyclic compound like cyclobutadiene.
- **Hybridization**: Each carbon is sp² hybridized.
- **Cyclic and Planar**: Yes, it is cyclic and planar.
- **Number of π Electrons**: There are 2 π electrons.
- **Huckel's Rule**: For \(n = 0\), \(4n + 2 = 2\). This satisfies Huckel's rule.

**Conclusion**: This compound is aromatic.

### Step 2: Analyze the Second Compound (Furan)
- **Structure**: Furan is a five-membered ring with one oxygen atom.
- **Hybridization**: Each carbon is sp² hybridized, and the oxygen can contribute a lone pair to the π system.
- **Cyclic and Planar**: Yes, it is cyclic and planar.
- **Number of π Electrons**: There are 6 π electrons (4 from carbon and 2 from the oxygen).
- **Huckel's Rule**: For \(n = 1\), \(4n + 2 = 6\). This satisfies Huckel's rule.

**Conclusion**: This compound is aromatic.

### Step 3: Analyze the Third Compound (Thiophene)
- **Structure**: Thiophene is a five-membered ring with one sulfur atom.
- **Hybridization**: Each carbon is sp² hybridized, and the sulfur can also contribute a lone pair to the π system.
- **Cyclic and Planar**: Yes, it is cyclic and planar.
- **Number of π Electrons**: There are 6 π electrons (4 from carbon and 2 from sulfur).
- **Huckel's Rule**: For \(n = 1\), \(4n + 2 = 6\). This satisfies Huckel's rule.

**Conclusion**: This compound is aromatic.

### Step 4: Analyze the Fourth Compound (NH)
- **Structure**: This compound is cyclic but may have sp³ hybridized atoms.
- **Hybridization**: The nitrogen atom is sp³ hybridized.
- **Cyclic and Planar**: It is cyclic but not planar due to the sp³ hybridization.
- **Number of π Electrons**: There are no π electrons available.
- **Huckel's Rule**: It does not satisfy Huckel's rule since it has no π electrons.

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

### Final Answer
Among the given compounds, the fourth compound (NH) is not an aromatic species.

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