Which of the following species is aromatic?

Options

A.
B.
C.
D.

Answer

To determine which of the given species is aromatic, we need to follow a systematic approach based on the criteria for aromaticity. Here’s a step-by-step solution:

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

### Step 2: Analyze Each Compound
Now, we will analyze each compound provided in the question based on the above criteria.

#### Compound A:
- **Cyclic**: Yes
- **Planar**: Yes (all carbons are sp² hybridized)
- **Conjugation**: Yes (there are alternating double bonds)
- **π Electrons**: Count the π electrons. If there are 4 π electrons, it follows \(4n\) rule, making it anti-aromatic (not aromatic).

**Conclusion**: Anti-aromatic.

#### Compound B:
- **Cyclic**: Yes
- **Planar**: No (contains sp³ hybridized carbon)
- **Conjugation**: No (due to non-planarity)

**Conclusion**: Non-aromatic.

#### Compound C:
- **Cyclic**: Yes
- **Planar**: No (due to the presence of a non-planar structure)
- **Conjugation**: Yes, but non-planarity leads to non-aromatic.

**Conclusion**: Non-aromatic.

#### Compound D:
- **Cyclic**: Yes
- **Planar**: Yes (all atoms are in the same plane)
- **Conjugation**: Yes (there are alternating double bonds)
- **π Electrons**: Count the π electrons. If there are 6 π electrons, it follows Huckel's rule (\(4n + 2\) with \(n = 1\)).

**Conclusion**: Aromatic.

### Final Answer
Based on the analysis, the aromatic compound is **Compound D**.

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