Question

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

• A. \(H_C > H_D > H_A > H_B\)
• B. H_B > H_A > H_D > H_C
• C. H_A > H_B > H_C > H_D
• D. \(H_C > H_A > H_D > H_B\)

Hint:

The key to determining acidity is analyzing the stability of the conjugate base. Factors like resonance, electronegativity, and hybridization play a crucial role. Carboxylic acids are generally much more acidic than carbon acids, which are more acidic than terminal alkynes. Alkynes with a negative charge adjacent to the triple bond are highly unstable.

Answer 1

The Correct Option is A

The acidity of a proton depends on the stability of the conjugate base formed after its removal. The more stable the conjugate base, the more acidic the proton.
Step 1: Acidity of \(H_C\)
\(H_C\) is the most acidic proton. Removal of \(H_C\) results in a carboxylate anion, which is highly stabilized by resonance.\\[6pt]
Step 2: Acidity of \(H_D\)
\(H_D\) is the second most acidic proton. Its removal forms a carbanion that is stabilized by resonance with the benzene ring.\\[6pt]
Step 3: Acidity of \(H_A\) vs \(H_B\)
\(H_A\) is more acidic than \(H_B\). The conjugate base formed after the removal of \(H_A\) is stabilized by resonance with the triple bond, resulting in a negative charge being distributed over two carbon atoms. However, the conjugate base formed after removal of \(H_B\) results in the negative charge adjacent to the triple bond, with no resonance stabilization. This is highly unstable due to the electron-withdrawing nature of the sp-hybridized carbon in the alkyne.
Step 4: Overall Acidity Order
Therefore, the correct order of acidity is:
\[H_C > H_D > H_A > H_B.\]
Conclusion: Option \((2)\) is correct.