JEE Chemistry 2024 Question with Solution

Given: Three contributing structures $I$, $II$ and $III$ are to be compared for relative stability.

Find: The correct order of relative stability.

The stability of resonance structures mainly depends on these factors:

1. Octet Rule: Structures in which all atoms, especially second-period elements like C, N and O, have a complete octet are generally more stable.
2. Charge Separation: Structures with minimal charge separation are more stable. A neutral structure is more stable than a charged one.
3. Negative Charge on Electronegative Atoms: Structures that place negative charge on more electronegative atoms like O are more stable.
4. Positive Charge on Electropositive Atoms: Structures with positive charge on less electronegative atoms are more stable.

Analyzing the given resonance structures:

1. Structure I: All atoms have complete octet shells and there is no charge separation, so it is the most stable.
2. Structure II: There is a positive charge on carbon and a negative charge on oxygen. Because charge separation is present, it is less stable than $I$, but the charges are on comparatively appropriate atoms.
3. Structure III: Carbon has a negative charge and oxygen has a positive charge. This is the least favorable charge distribution, so it is the least stable.

Therefore, the order of relative stability is $I > II > III$. Hence, the correct option is A.

Given: Compare the stability of contributing structures $I$, $II$ and $III$.

Find: The most stable to least stable order.

Use the quick resonance check:

• Neutral structure is preferred over charge-separated structures.
• If charges are present, negative charge on oxygen is better than negative charge on carbon.

So, Structure I is most stable because it is neutral. Between the charged structures, Structure II is more stable than Structure III because the negative charge is on oxygen in $II$, whereas in $III$ it is on carbon.

Thus, the correct order is $I > II > III$ and the correct option is A.