14. What is the order of acidity of the labeled protons?
a. A>B>C>D>E
b. C>E>D>A>B
c. E>B>C>A>D
d. D>E>B>A>C

The most acidic proton is E, followed by B, then C, then A, and finally D, so the correct option is (c) E > B > C > A > D.​​

Introduction

Understanding the order of acidity of the labeled protons in multifunctional organic molecules is essential for predicting reaction mechanisms, enolate formation, and selectivity in synthesis. In this β‑keto ester–carboxylic acid system, different protons experience resonance stabilization and inductive effects to very different extents, which dramatically alters their acidity.​

Identify each labeled proton

• Proton A: Vinylic proton on the C=C bond conjugated to a carbonyl; deprotonation gives an anion where negative charge is only weakly stabilized by π‑conjugation.​

• Proton B: α‑Proton of a β‑keto ester (between two carbonyl groups), giving a highly resonance‑stabilized 1,3‑dicarbonyl enolate on deprotonation.​

• Proton C: α‑Proton next to a single ketone carbonyl; its conjugate base is a simple enolate, less stabilized than the β‑keto ester enolate.​

• Proton D: Ring methylene proton remote from strong electron‑withdrawing groups; its conjugate base is a typical alkyl carbanion with almost no stabilization.​

• Proton E: O–H proton of a carboxylic acid group, whose conjugate base (carboxylate) is strongly stabilized by resonance over two oxygens.​

Reasoning for the correct acidity order

1. Most acidic: E (carboxylic acid O–H).
   Carboxylic acids have pKa around 4–5 because the conjugate base delocalizes negative charge over two oxygens; this makes proton E the easiest to remove.​

2. Next: B (β‑keto ester α‑H).
   The α‑proton between two carbonyls has pKa around 10–11 due to extensive resonance stabilization and inductive withdrawal from both carbonyl groups, so it is much more acidic than a normal ketone α‑H.​

3. Then: C (simple ketone α‑H).
   A single carbonyl stabilizes the conjugate base via one enolate resonance pattern, giving typical pKa near 19–20; this is less acidic than B but still more acidic than vinylic or alkyl protons lacking such stabilization.​​

4. Then: A (vinylic H on conjugated C=C).
   Vinylic protons conjugated with a carbonyl may benefit from limited conjugation but lack the strong resonance and inductive stabilization seen in carbonyl α‑protons, so their acidity is weaker than that of C.​

5. Least acidic: D (remote sp³ C–H).
   Simple alkane‑like C–H bonds have pKa around 50, as the resulting carbanion has almost no stabilization, making D the least acidic proton in the molecule.​​

Thus the decreasing order of acidity is E > B > C > A > D, matching option (c).

Analysis of each option

• Option (a) A > B > C > D > E – Incorrect.
  This would require a vinylic C–H to be more acidic than a β‑keto ester α‑H and more acidic than a carboxylic acid O–H, which contradicts typical pKa data where carboxylic acids and β‑dicarbonyl compounds are far stronger acids.​

• Option (b) C > E > D > A > B – Incorrect.
  This suggests a simple ketone α‑H is more acidic than a carboxylic acid and places the extremely weakly acidic alkyl proton D ahead of B, which is inconsistent with known acidity trends.​

• Option (c) E > B > C > A > D – Correct.
  This order follows standard pKa ranges: carboxylic acid O–H (strongest), β‑keto ester α‑H, ketone α‑H, vinylic H, and finally remote alkyl H (weakest).​

• Option (d) D > E > B > A > C – Incorrect.
  Ranking the unactivated sp³ C–H (D) as more acidic than a carboxylic acid (E) contradicts the fundamental principle that carboxylic acids are among the strongest common organic acids, while alkyl C–H bonds are among the weakest.​