See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: Resonating structures (resonance structures) are different representations of the same molecule that differ only in the distribution of electrons, NOT in the connectivity (bonding framework) of atoms. The positions of atoms must remain the same in all resonance structures. Step 1: Analyze option (a). CH3 — N = C = S (methyl isothiocyanate) and CH3 — S — C ≡ N (methyl thiocyanate). In the first structure, the connectivity is CH3 attached to N, which is attached to C, which is attached to S. In the second structure, the connectivity is CH3 attached to S, which is attached to C, which is attached to N. The atom connectivity has changed: in one structure N is directly bonded to CH3, while in the other S is directly bonded to CH3. This means the positions of atoms have changed, so these are NOT resonance structures of each other — they are structural isomers (constitutional isomers), specifically functional group isomers. Step 2: Analyze option (b). CH3 — C(+) = O and CH3 — C ≡ O(+) differ only in electron distribution (double bond vs triple bond between C and O, with charge relocation). The atomic connectivity (CH3-C-O) is the same. These ARE valid resonance structures. Step 3: Analyze option (c). CH3 — C(=O) — OH (acetic acid) and CH3 — C(O−) = O(+) — H differ only in electron distribution. The connectivity CH3-C-O-H remains the same with charge delocalization. These ARE valid resonance structures. Step 4: Analyze option (d). CH2 = CH — C ≡ N and CH2(+) — CH = C = N(−) differ only in electron distribution through conjugation. The connectivity CH2-CH-C-N is the same throughout. These ARE valid resonance structures. Step 5: Conclusion. Only option (a) has structures where the atomic connectivity differs (N bonded to CH3 vs S bonded to CH3), making them structural isomers, not resonance structures. Why other options fail: Options (b), (c), and (d) all maintain the same atomic connectivity and differ only in electron pair distribution, satisfying the requirement for resonance structures. Therefore, the correct answer is A.