To determine which of the given ring compounds obeys Huckel's rule, we need to analyze each compound based on the number of pi electrons they possess. Huckel's rule states that a compound is aromatic if it has a planar structure and contains \(4n + 2\) pi electrons, where \(n\) is a non-negative integer (0, 1, 2, ...).
Let's analyze each compound step by step:
### Step 1: Analyze Compound C4H4
1. **Structure**: The compound C4H4 can be represented as a cyclic structure with four carbons.
2. **Count Pi Electrons**: In this structure, there are 4 pi electrons (assuming all carbons are involved in pi bonding).
3. **Check Huckel's Rule**: For Huckel's rule, we need \(4n + 2\) pi electrons. If we set \(n = 0\), we get 2 pi electrons; if \(n = 1\), we get 6 pi electrons. Since 4 pi electrons do not satisfy \(4n + 2\), this compound does not obey Huckel's rule.
### Step 2: Analyze Compound C4H4+
1. **Structure**: The compound C4H4+ has a positive charge, indicating one carbon is sp3 hybridized.
2. **Hybridization**: The presence of an sp3 hybridized carbon implies that the compound is not planar (tetrahedral geometry).
3. **Conclusion**: Since the compound is not planar, it cannot obey Huckel's rule regardless of the number of pi electrons.
### Step 3: Analyze Compound C4H4−2
1. **Structure**: The compound C4H4−2 has two negative charges.
2. **Count Pi Electrons**: In this structure, we can count the pi electrons as follows: 1, 2, 3, 4, 5, and 6. Thus, there are 6 pi electrons.
3. **Check Huckel's Rule**: Setting \(n = 1\) gives us \(4(1) + 2 = 6\) pi electrons, which satisfies Huckel's rule. Therefore, this compound obeys Huckel's rule.
### Step 4: Analyze Compound C4H4 (again)
1. **Structure**: This compound is the same as the first one, C4H4.
2. **Count Pi Electrons**: As previously determined, it has 4 pi electrons.
3. **Check Huckel's Rule**: Again, since 4 pi electrons do not satisfy \(4n + 2\), this compound does not obey Huckel's rule.
### Conclusion
After analyzing all the compounds, we find that the only compound that obeys Huckel's rule is **C4H4−2**.
### Final Answer
The correct option is **C4H4−2**.
