See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: Resonance structures vs. Tautomers. Resonance structures: These are different representations of the SAME compound where only electrons are delocalized (moved), not atoms or protons. The connectivity of atoms does not change; resonance structures are not real separate compounds but rather contribute to the resonance hybrid. Tautomers: These are distinct structural isomers that interconvert by migration of a proton (H atom) and a shift of bonding electrons. They are real, separate (though interconverting) compounds. Analysis of each option: (a) The pair shows aniline and its resonance structure where the lone pair on nitrogen is delocalized into the ring, giving positive charge on N and negative charge at the para carbon. Only electron pairs move, no atoms migrate. This IS a resonance relationship. Correctly labeled as Resonance. (b) The pair shows cyclohex-2-en-1-one (a non-aromatic ketone) and phenol (an aromatic compound with OH). These two structures differ by migration of a proton (H moves from enol OH in phenol form to give the keto form) along with shift in double bond positions. This is keto-enol tautomerism. Correctly labeled as Tautomers. (c) The pair shows a thioaldehyde/thiol compound and its charged form where electrons have shifted. The structures differ only in electron distribution (S becomes S- and charges shift), with no migration of atoms. This IS a resonance relationship. Correctly labeled as Resonance. (d) The pair shows two forms of a protonated ester. In a protonated ester, the two structures shown (one with positive charge on the carbonyl carbon and one with positive charge on the oxygen) differ only in the position of electrons (pi electrons and lone pairs), NOT in the position of any atom or proton. Both structures have the same atomic connectivity with only electron redistribution. Therefore, these are RESONANCE structures of the protonated ester, NOT tautomers. Labeling them as Tautomers is INCORRECT. The incorrect relation is in option (d) because the two protonated ester structures shown are resonance structures (only electrons move, no proton migration), but they are incorrectly labeled as tautomers. Therefore, the correct answer is D.