Four structures are given in options (a) to (d) . Examine them and select the aromatic structures.

Options

A.
B.
C.
D.

Explanation

To determine which of the given structures are aromatic, we need to follow 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. **Planarity**: The molecule must be planar, allowing for effective overlap of p-orbitals.
2. **Conjugated Double Bonds**: The molecule must have a continuous ring of p-orbitals, which typically means alternating single and double bonds.
3. **Hückel's Rule**: The molecule must have \(4n + 2\) π electrons, where \(n\) is a non-negative integer (0, 1, 2, ...).

### Step 2: Analyze Each Structure
Let’s examine each structure one by one based on the criteria above.

#### Structure 1: Cyclopropene with a Positive Charge
- **Planarity**: Yes, all carbons are sp² hybridized.
- **Conjugation**: There is one double bond, but it does not have conjugated double bonds.
- **π Electrons**: There are 2 π electrons (for \(n = 0\), \(4(0) + 2 = 2\)).
- **Conclusion**: This structure is aromatic.

#### Structure 2: A Compound with 8 π Electrons
- **Planarity**: Yes, all carbons are sp² hybridized.
- **Conjugation**: There are conjugated double bonds.
- **π Electrons**: There are 8 π electrons (for \(n = 2\), \(4(2) + 2 = 10\)).
- **Conclusion**: This structure is anti-aromatic (not aromatic).

#### Structure 3: Bifenyl
- **Planarity**: Yes, both benzene rings are planar and sp² hybridized.
- **Conjugation**: Each benzene ring has conjugated double bonds.
- **π Electrons**: Each benzene ring has 6 π electrons (for \(n = 1\), \(4(1) + 2 = 6\)).
- **Conclusion**: This structure is aromatic.

#### Structure 4: A Compound with 4 π Electrons
- **Planarity**: Yes, all carbons are sp² hybridized.
- **Conjugation**: There are conjugated double bonds.
- **π Electrons**: There are 4 π electrons (for \(n = 1\), \(4(1) + 2 = 6\)).
- **Conclusion**: This structure is anti-aromatic (not aromatic).

### Final Conclusion
The aromatic structures from the given options are:
- **Structure 1 (Cyclopropene with a Positive Charge)**
- **Structure 3 (Bifenyl)**