Identify the odd species out Which of the species among the following is different from others ?

Options

A.
B.
C.
D.

Answer

To solve the problem of identifying the odd species out among the given options based on their aromatic or anti-aromatic nature, we will follow these steps:

### Step 1: Understand the Criteria for Aromaticity and Anti-aromaticity
- **Aromatic Compounds** must satisfy the following conditions:
1. The molecule must be cyclic.
2. The molecule must be planar.
3. The molecule must follow Huckel's rule, which states that it must have \(4n + 2\) π electrons, where \(n\) is a non-negative integer (0, 1, 2, ...).

- **Anti-aromatic Compounds** must satisfy:
1. The molecule must be cyclic.
2. The molecule must be planar.
3. The molecule must have \(4n\) π electrons.

### Step 2: Analyze Each Option
Let's analyze the given options one by one to determine whether they are aromatic or anti-aromatic.

#### Option 1: Cyclobutadiene
- Structure: Cyclic with 4 π electrons (2 double bonds).
- Analysis:
- Cyclic: Yes
- Planar: Yes
- π Electrons: 4 (follows \(4n\) rule, where \(n=1\)).
- Conclusion: **Anti-aromatic**

#### Option 2: Cyclobutene
- Structure: Cyclic with 4 π electrons (2 double bonds).
- Analysis:
- Cyclic: Yes
- Planar: Yes
- π Electrons: 4 (follows \(4n\) rule, where \(n=1\)).
- Conclusion: **Anti-aromatic**

#### Option 3: Cyclopentadienyl Anion
- Structure: Cyclic with 6 π electrons (2 double bonds + 1 lone pair).
- Analysis:
- Cyclic: Yes
- Planar: Yes
- π Electrons: 6 (follows \(4n + 2\) rule, where \(n=1\)).
- Conclusion: **Aromatic**

#### Option 4: Benzene
- Structure: Cyclic with 6 π electrons (3 double bonds).
- Analysis:
- Cyclic: Yes
- Planar: Yes
- π Electrons: 6 (follows \(4n + 2\) rule, where \(n=1\)).
- Conclusion: **Aromatic**

### Step 3: Identify the Odd Species
From our analysis:
- Option 1 (Cyclobutadiene) and Option 2 (Cyclobutene) are **anti-aromatic**.
- Option 3 (Cyclopentadienyl Anion) and Option 4 (Benzene) are **aromatic**.

Thus, the odd species out is **Option 1: Cyclobutadiene**, as it is the only one that is anti-aromatic.

### Final Answer
The odd species out is **Cyclobutadiene**.

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