Rank the following in order of decreasing importance as a contributing resonance structure to the molecular structure of acetone,CH₃COCH₃ (more important less important).

Question

Rank the following in order of decreasing importance as a contributing resonance structure to the molecular structure of acetone,CH₃COCH₃ (more important > less important).

Quizplus · Accepted Answer

Resonance structures 1 and 3 are equivalent, and both involve the movement of a double bond to an adjacent carbon atom to form a carbonyl group (C=O). 
Resonance structure 2 involves the movement of a double bond to an oxygen atom, but since oxygen is more electronegative than carbon, it is less stable than the other two structures. Therefore, the contributions of structures 1 and 3 are more important than that of structure 2.
Thus, the correct order of decreasing importance is 1 > 3 > 2, which corresponds to choice B.

Rank the Following in Order of Decreasing Importance as a Contributing