Which of the following will show aromatic behaviour ?

Options

A.
B.
C.
D.

Answer

To determine which of the given compounds shows aromatic behavior, we need to apply Huckel's rule and 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. **Cyclic Structure**: The compound must be a ring.
2. **Planarity**: The compound must be planar, allowing for overlapping p-orbitals.
3. **Conjugation**: There must be a complete cyclic conjugation of pi electrons.
4. **Huckel's Rule**: The compound must have (4n + 2) pi electrons, where n is a non-negative integer (0, 1, 2, ...).

### Step 2: Analyze Each Compound
Let’s analyze the compounds one by one:

#### Compound 1:
- **Cyclic**: Yes, it is cyclic.
- **Planar**: Check if all atoms are sp2 hybridized.
- **Conjugation**: Look for alternating double bonds.
- **Pi Electrons**: Count the pi electrons and check if they fit Huckel's rule.

**Conclusion**: This compound does not have complete cyclic conjugation, so it is **not aromatic**.

#### Compound 2:
- **Cyclic**: Yes, it is a benzene ring.
- **Planar**: All carbon atoms in benzene are sp2 hybridized.
- **Conjugation**: Benzene has complete conjugation with alternating double bonds.
- **Pi Electrons**: Benzene has 6 pi electrons (C6H6), which fits Huckel's rule (4n + 2, where n=1).

**Conclusion**: This compound is **aromatic**.

#### Compound 3:
- **Cyclic**: Yes, it is cyclic.
- **Planar**: Check the hybridization of the atoms.
- **Conjugation**: Look for alternating double bonds.
- **Pi Electrons**: Count the pi electrons.

**Conclusion**: If all atoms are sp3 hybridized and there are no pi bonds, this compound is **not aromatic**.

#### Compound 4:
- **Cyclic**: Yes, it is cyclic.
- **Planar**: Check the hybridization.
- **Conjugation**: Look for pi bonds.
- **Pi Electrons**: Count the pi electrons.

**Conclusion**: If it has 4 pi electrons and does not fit Huckel's rule, it is **not aromatic**.

### Final Conclusion
Among the given compounds, only **Compound 2 (benzene)** shows aromatic behavior.