Which of the following heterocyclic compounds would have aromatic character ?

Options

A.
B.
C.
D.

Answer

To determine which of the given heterocyclic compounds has aromatic character, we will apply Huckel's rule. This rule states that for a compound to be considered aromatic, it must meet the following criteria:

1. The compound must be cyclic.
2. The compound must be planar (all atoms in the same plane).
3. The compound must have a continuous ring of p-orbitals (conjugated system).
4. The compound must follow Huckel's rule for π electrons, which states that the number of π electrons must be equal to 4n + 2, where n is a non-negative integer (0, 1, 2, ...).

Now, let's analyze each option step by step:

### Step 1: Analyze Option A
- **Hybridization**: All atoms are sp² hybridized.
- **Cyclic Structure**: Yes, it is cyclic.
- **Count π Electrons**: There are 4 π bonds, contributing 8 π electrons (4 bonds × 2 electrons per bond).
- **Huckel's Rule**: 8 π electrons do not satisfy Huckel's rule (4n + 2). This means it is anti-aromatic.

### Step 2: Analyze Option B
- **Hybridization**: Nitrogens may be sp² hybridized.
- **Cyclic Structure**: Yes, it is cyclic.
- **Count π Electrons**: Each nitrogen contributes 2 π electrons from their lone pairs, totaling 8 π electrons.
- **Huckel's Rule**: 8 π electrons do not satisfy Huckel's rule (4n + 2). This means it is anti-aromatic.

### Step 3: Analyze Option C
- **Hybridization**: Similar to Option B, with sp² hybridization.
- **Cyclic Structure**: Yes, it is cyclic.
- **Count π Electrons**: Again, 8 π electrons are counted.
- **Huckel's Rule**: 8 π electrons do not satisfy Huckel's rule (4n + 2). This means it is anti-aromatic.

### Step 4: Analyze Option D
- **Hybridization**: The nitrogen lone pair participates in the π system, making it sp².
- **Cyclic Structure**: Yes, it is cyclic.
- **Count π Electrons**: There are 6 π electrons (2 from each of the 3 bonds).
- **Huckel's Rule**: 6 π electrons satisfy Huckel's rule (4n + 2 with n = 1). This means it is aromatic.

### Conclusion
Based on the analysis, the compound that exhibits aromatic character is **Option D**.

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