To determine which of the given compounds has aromatic character, we need to analyze each option based on the criteria for aromaticity. The criteria for a compound to be considered aromatic are:
1. The compound must be cyclic.
2. Each atom in the ring must be planar.
3. Each atom in the ring must be sp² hybridized.
4. The compound must follow Huckel's rule, which states that the number of π (pi) electrons must be of the form 4n + 2, where n is a non-negative integer (0, 1, 2, ...).
Now, let's evaluate each option:
### Option 1: Cyclopentadienyl Cation
- Structure: The cyclopentadienyl cation has a cyclic structure with 5 carbon atoms.
- Hybridization: Each carbon atom is sp² hybridized.
- π Electrons: The cation has 4 π electrons (since one of the double bonds is missing due to the positive charge).
- Huckel's Rule: For n = 1, 4n + 2 = 6. Since we have 4 π electrons, it does not satisfy Huckel's rule. Therefore, it is anti-aromatic.
### Option 2: Cyclopentadienyl Radical
- Structure: The cyclopentadienyl radical also has a cyclic structure with 5 carbon atoms.
- Hybridization: Each carbon atom is sp² hybridized.
- π Electrons: The radical has 5 π electrons (4 from the double bonds and 1 from the unpaired electron).
- Huckel's Rule: For n = 1, 4n + 2 = 6. Since we have 5 π electrons, it does not satisfy Huckel's rule. Therefore, it is not aromatic.
### Option 3: Cyclopentadienyl Anion
- Structure: The cyclopentadienyl anion has a cyclic structure with 5 carbon atoms.
- Hybridization: Each carbon atom is sp² hybridized.
- π Electrons: The anion has 6 π electrons (4 from the double bonds and 2 from the negative charge).
- Huckel's Rule: For n = 1, 4n + 2 = 6. Since we have 6 π electrons, it satisfies Huckel's rule. Therefore, it is aromatic.
### Option 4: Cyclopentadiene
- Structure: Cyclopentadiene has a cyclic structure with 5 carbon atoms.
- Hybridization: However, in cyclopentadiene, two of the carbon atoms are involved in a double bond, while the other three are sp³ hybridized.
- Planarity: The structure is not fully planar due to the sp³ hybridization.
- Huckel's Rule: It does not satisfy the conditions for aromaticity.
### Conclusion:
Among the four options, only the cyclopentadienyl anion satisfies all the criteria for aromaticity. Therefore, the correct answer is:
**Cyclopentadienyl Anion (Option 3)**
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