Which of the following species is aromatic :

Options

A.
B.
C.
D.

Explanation

To determine which of the given species is aromatic, we need to apply 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 must be cyclic.
2. It must be planar.
3. It must have complete conjugation (p-orbitals must overlap).
4. It must follow Huckel's rule, which states that it should have \(4n + 2\) π electrons, where \(n\) is a non-negative integer.

### Step 2: Analyze Each Compound
We will analyze each of the four compounds given in the question to see if they meet the aromaticity criteria.

#### Compound 1:
- **Structure:** 5-membered cyclic ring with a positive charge.
- **Electrons:** Contains 4 π electrons.
- **Analysis:**
- It does not satisfy Huckel's rule (\(4n\) instead of \(4n + 2\)).
- Therefore, it is **anti-aromatic**.

#### Compound 2:
- **Structure:** A ring with a positive charge and a double-bonded oxygen.
- **Electrons:** The π electrons do not allow for complete cyclic conjugation.
- **Analysis:**
- There is no complete cyclic conjugation.
- Therefore, it is also **anti-aromatic**.

#### Compound 3:
- **Structure:** Another cyclic compound.
- **Electrons:** Contains 4 or 5 π electrons.
- **Analysis:**
- Similar to the first two, it does not meet Huckel's rule.
- Therefore, it is **anti-aromatic**.

#### Compound 4:
- **Structure:** A fused 5-membered and 3-membered ring.
- **Electrons:** Can be shown to have 6 π electrons through resonance.
- **Analysis:**
- This compound can exhibit complete cyclic conjugation.
- It satisfies Huckel's rule with \(n = 1\) (6 π electrons).
- Therefore, it is **aromatic**.

### Step 3: Conclusion
Based on the analysis, the only compound that is aromatic is **Compound 4**.

### Final Answer
The correct option is **Compound 4**.

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