HNCO (isocyanic acid) has following resonating structures :
`underset("I")(H-N=C=O)harrunderset("II")(H-overset(Theta)N-C-=O^(o+))harrunderset("III")(H-overset(o+)N-=C-O^(Theta))`

Options

A. `IgtIIIgtII`
B. `IgtIIgtIII`
C. `IIgtIIIgtI`
D. `IIgtIgtIII`

Answer

To determine the stability of the resonating structures of isocyanic acid (HNCO), we will analyze the given structures based on several stability criteria.

### Step-by-Step Solution:

1. **Identify the Resonating Structures:**
- Structure I: \( H-N=C=O \)
- Structure II: \( H-\overset{\delta^-}{N}-C^{\delta+}=\overset{\delta^+}{O} \)
- Structure III: \( H-\overset{\delta^+}{N}=\overset{\delta^-}{C}-O \)

2. **Evaluate the Charge Distribution:**
- Structure I has no formal charges, making it a neutral structure.
- Structure II has a negative charge on nitrogen and a positive charge on carbon.
- Structure III has a negative charge on oxygen and a positive charge on nitrogen.

3. **Apply Stability Criteria:**
- **Uncharged Species:** Structure I is the most stable because it is uncharged.
- **Negative Charge on Electronegative Atom:** In Structure III, the negative charge is on oxygen (more electronegative than nitrogen), making it more stable than Structure II, where the negative charge is on nitrogen.
- **Positive Charge on Electropositive Atom:** In Structure II, the positive charge is on carbon, which is less stable compared to Structure III where the positive charge is on nitrogen.

4. **Rank the Structures:**
- Based on the analysis:
- Structure I is the most stable (no charges).
- Structure III is more stable than Structure II because it has the negative charge on the more electronegative atom (oxygen).
- Therefore, the order of stability is: I > III > II.

5. **Final Conclusion:**
- The correct order of stability of the resonating structures is: **I > III > II**.