See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: The most acidic C-H or N-H bond is the one whose conjugate base (carbanion or amide anion) is most stabilized by resonance or inductive effects. Step 1 - Identify each hydrogen's environment: - Ha: C-H on the carbon situated between TWO carbonyl groups (flanked by C=O on both sides, i.e., a 1,3-dicarbonyl or malonyl-type active methine). - Hb: C-H on a carbon alpha to ONE carbonyl group. - Hc: C-H on a carbon alpha to a nitrogen (not adjacent to carbonyl). - Hd: C-H on a carbon between two nitrogen atoms (hemiaminal-type carbon). - He (H on N): N-H adjacent to a carbonyl (amide N-H, typically pKa ~17-25). Step 2 - Assess stabilization of conjugate base: - Removing Ha gives a carbanion stabilized by TWO flanking carbonyl groups simultaneously (analogous to malonate or dimedone active methine). This is double resonance stabilization: the negative charge can be delocalized into both C=O groups, forming a stable enolate on both sides. - Removing Hb gives a mono-enolate stabilized by one carbonyl (pKa ~20 for simple ketone alpha-H). - Removing Hc gives a carbanion with no adjacent carbonyl stabilization; only weak inductive effect from distant nitrogen. - Removing Hd gives a carbanion between two N atoms; nitrogen lone pairs can destabilize by electron donation, and there is no carbonyl to stabilize. - He is an amide N-H; amide N-H is less acidic than active methine between two carbonyls (amide pKa ~25 vs. active methine between two carbonyls pKa ~13). Step 3 - Compare acidities: Active methine (Ha) between two carbonyls is the most acidic because the resulting carbanion is stabilized by conjugation with TWO carbonyl groups simultaneously (analogous to malonic ester with pKa ~13). All other positions lack this double resonance stabilization. Step 4 - Why other options fail: - Hb: only single carbonyl stabilization, less acidic than Ha. - Hc: no carbonyl adjacent, least acidic C-H. - Hd: between two N atoms, no carbonyl, not more acidic than Ha. - He (N-H, amide): pKa ~25, less acidic than doubly-activated methine. Therefore, the correct answer is A.