To determine the stability order of the resonance structures of \( \text{CH}_3 - \text{CH} = \text{CH} - \text{CHO} \), we can evaluate each structure based on several key factors affecting resonance stability:
**Octet Rule Compliance**: The most stable resonance structure usually has atoms with completed octets.
**Charge Distribution**: Resonance structures with minimal formal charges are typically more stable.
**Electronegativity**: Negative charges are more stable on more electronegative atoms (e.g., oxygen).
Let's analyze each structure:
Structure I: The carbon atom in the chain has a negative charge. The electronegative oxygen has a positive charge, which is not favorable.
Structure II: The positive charge is on a carbon atom, while the negative charge is on oxygen. This is more favorable compared to Structure I.
Structure III: There are no charges, and all atoms comply with the octet rule. The double bond between carbon and oxygen is highly stable due to resonance.
Based on these observations, the stability order is:
Most Stable: Structure III – All atoms have full octets with no charges.
Intermediate Stability: Structure II – Favorable charge distribution with negative charge on oxygen.
Least Stable: Structure I – Unfavorable charge distribution. Negative charge on carbon and positive on oxygen.
Thus, the correct stability order is: III > II > I.
are isomeric compounds. 
are functional group isomers.