See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: Enol content (keto-enol tautomerism equilibrium) is favored by factors that stabilize the enol form relative to the keto form. Key factors include: (1) conjugation of the enol double bond with existing unsaturation, (2) intramolecular hydrogen bonding in 1,3-dicarbonyl systems, (3) aromaticity in the enol form, (4) ring strain relief, and (5) electronic effects (charge). Analysis of option (c): - First compound: 2-cyclopenten-1-one anion (the anion of 2-cyclopentenone). In this anion, the alpha carbon is deprotonated. The resulting carbanion/enolate is stabilized by the adjacent carbonyl AND the existing double bond (extended conjugation/aromatic-like 6pi system in the ring). The enolate form is highly stabilized — effectively, the anion itself IS the enol(ate) form, meaning enol content is very high (essentially 100% enolate). - Second compound: cyclopentanone (saturated, neutral). Cyclopentanone has modest enol content (~a few percent), much less than the first compound in this pair when we consider the anion context, but the question compares enol tautomer populations. More precisely: The first compound in (c) is the anion of 2-cyclopentenone. This anion is a fully delocalized, aromatic 6pi cyclopentadienyl-like enolate — it exists essentially entirely in the enol(ate) form (enol content ~100%). The second compound is cyclopentanone, which has very low enol content (<<1% in normal conditions, ~0.01%). Thus the second compound (cyclopentanone) has dramatically LESS enol content than the first compound (the cyclopentenone anion which is essentially all enolate). Why other options fail: (a) Protonation of cyclopropanone gives the cyclopropanone cation. The cation actually has increased enol content relative to neutral cyclopropanone because the enol form is more stable for the cation (cyclopropenyl cation aromatic stabilization of enol form). So second compound does NOT have less enol content — it has more. (b) First compound appears to be cyclopentane-1,2-dione or similar with positive charge; second is 2-cyclopenten-1-one. The conjugated enone (second) actually has less enol content than a 1,3-diketone due to loss of enolization driving force, but the charge complicates this and the ordering may not clearly show second < first in the intended direction. (d) 'None of these' is incorrect since (c) satisfies the condition. Therefore, the correct answer is C.