Question

Resonance structures of propenal are given below. Which of these resonating structures is more stable? Give reason for your answer.

Answer 1

Stability of canonical structures:

1. A neutral canonical structure is more stable than charged canonical structures.

2. A canonical structure having more covalent bonds will be more stable.

3. A canonical structure in which all atoms having complete octet will be more stable

Canonical structure $I$ have more covalent bonds, all atoms have complete octet and this canonical form is neutral.

While canonical structure $II$ have less covalent bond, positively charged carbon have incomplete octet and the canonical form is charged species.

So, stability order should be $I>II$.