Question

What is the correct order of acidity of the protons marked A-D in the given compounds ?

• A. $H_C>H_D>H_B>H_A$
• B. $H_C>H_D>H_A>H_B$
• C. $H_D>H_C>H_B>H_A$
• D. $H_C>H_A>H_D>H_B$

Answer 1

Answer: (B)

$H_C>H_D>H_A>H_B$

Answer 2

The acidity of a proton is determined by the stability of the conjugate base formed upon its removal.

1. $H_C$ is a carboxylic acid proton, forming a resonance-stabilized carboxylate anion.
2. $H_D$ is alpha to a carbonyl and benzylic, stabilized by resonance with the phenyl ring and inductive effect of the carbonyl.
3. $H_A$ is a terminal alkyne proton, forming a resonance-stabilized acetylide anion due to the sp hybridization of the carbon.
4. $H_B$ is benzylic and alpha to an alkyne, stabilized by resonance with the phenyl ring.

Comparing the factors: Carboxylate resonance > alpha-carbonyl/benzylic stabilization > sp hybridization > benzylic/alkyne stabilization. Thus, the order of acidity is $H_C > H_D > H_A > H_B$.