See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Step 1 – Identify the acidic protons in each compound. All four compounds can lose a proton alpha to a carbonyl (C–H acidity). The key factor is how well the resulting carbanion/enolate is stabilized. Compound I – 1,3-cyclohexanedione: The alpha carbon between the two ketone carbonyls is flanked by TWO carbonyl groups. This gives very strong stabilization of the enolate by double resonance delocalization. This is a 1,3-diketone, pKa ~ 5–7 (relatively very acidic). Compound II – delta-valerolactone (tetrahydropyran-2-one): This is a simple lactone with only ONE carbonyl. The alpha proton next to a single ester-type carbonyl is weakly acidic. Esters stabilize enolates less effectively than ketones because the oxygen lone pairs donate back into the carbonyl, raising the pKa. pKa ~ 30+. Compound III – tetrahydropyran-4-one: This is a cyclic ketone with an endocyclic oxygen in the ring at position 1 and ketone at position 4. The ring oxygen is two carbons away from the carbonyl. The alpha protons are alpha to a single ketone carbonyl, but the inductive effect of the ring oxygen (–I effect) slightly increases acidity compared to a plain ketone. However, the oxygen is not directly stabilizing the enolate through resonance. pKa slightly lower than a simple ketone (~20) but much higher than 1,3-diketone. Compound IV – methyl 4-oxocyclohexane-1-carboxylate: This compound has a ketone (C4) and a methyl ester (C1) on the same cyclohexane ring. The C2/C6 protons (alpha to the ketone) and C1 proton (alpha to both the ketone and the ester if in a 1,3-relationship or at least under inductive influence) can be considered. Critically, the proton at C1 is alpha to the ester group AND feels the inductive withdrawal of the ketone at C4 (beta-keto ester type relationship through the ring). A beta-keto ester is significantly more acidic than a simple ketone (pKa ~ 11–13) because the enolate is stabilized by both the ester and ketone. This compound is the MOST acidic among the four. Step 2 – Rank by acidity (increasing = least acidic to most acidic). - Compound II (simple lactone, single carbonyl, ester character): LEAST acidic (highest pKa, ~30+) - Compound III (cyclic ketone with ring O, single carbonyl, modest inductive effect): less acidic than I or IV but more acidic than II - Compound I (1,3-diketone, two carbonyls stabilizing enolate): more acidic than III - Compound IV (beta-keto ester embedded in ring, both ketone and ester stabilizing): MOST acidic Increasing order of acidic strength: II < III < I < IV This matches option (d). Why other options fail: - (a) III < I < IV < II: Places II as most acidic, which is wrong (lactone is least acidic). - (b) II < I < IV < III: Places III as most acidic, which is wrong. - (c) I < III < IV < II: Places II as most acidic and I as least, both incorrect. Therefore, the correct answer is D.