See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: In resonance (canonical) structures, we evaluate their validity based on formal charges, electronegativity considerations, and octet rule compliance. A canonical structure is considered practically invalid if it places a positive charge on a more electronegative atom and a negative charge on a less electronegative atom, violating electronegativity principles. Step 1: Analyze Structure I (CH2=O). This is the neutral Lewis structure of formaldehyde with a C=O double bond. Carbon and oxygen both satisfy the octet rule (carbon has 4 bonds, oxygen has 2 bonds and 2 lone pairs). This is the most stable and valid canonical structure. Step 2: Analyze Structure II ((+)CH2-O(-)). Here carbon bears a positive charge and oxygen bears a negative charge, with a single C-O bond. Since oxygen is more electronegative than carbon, it is perfectly acceptable for oxygen to carry the negative charge and carbon to carry the positive charge. This structure is a valid (though less stable) canonical structure because the negative charge resides on the more electronegative atom (oxygen). Step 3: Analyze Structure III ((-)CH2-O(+)). Here carbon bears a negative charge and oxygen bears a positive charge, with a single C-O bond. This places a negative charge on carbon (less electronegative) and a positive charge on oxygen (more electronegative). This violates the basic electronegativity principle: a more electronegative atom should not carry a positive charge while a less electronegative atom carries a negative charge. Such charge separation is energetically very unfavorable and makes this structure practically not a valid canonical (resonance) contributor. Step 4: Why other options fail. Structure I is the primary valid structure. Structure II is a legitimate resonance contributor. Structure III is the one that violates electronegativity principles and is therefore practically not a valid canonical structure. Therefore, the correct answer is C.