Question

Which of the following carboxylic acid has the highest $pK_a$ value?

• A. $O_2N-CH_2-COOH$
• B. $F-CH_2-COOH$
• C. $HCOOH$
• D. $CN-CH_2COOH$
• E. $Cl-CH_2COOH$

Hint:

Remember: Strong EWG $\to$ High $K_a$ $\to$ Low $pK_a$. For "highest $pK_a$", just look for the molecule with the weakest electron-withdrawing groups.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
pKa is the negative logarithm of the acid dissociation constant (Ka).
- Stronger acid \(\rightarrow\) higher Ka \(\rightarrow\) lower pKa.
- Weaker acid \(\rightarrow\) lower Ka \(\rightarrow\) higher pKa.
Step 2: Key Formula or Approach:
Electron-withdrawing groups (EWG) like -NO\textsubscript{2}, -CN, -F, and -Cl increase acidity by stabilizing the conjugate base, thereby lowering pKa.
Step 3: Detailed Explanation:
1. Options A, B, D, and E are all substituted acetic acids containing strong EWGs. These groups exert a -I effect, making these acids much stronger than plain acetic acid.
2. Formic acid (HCOOH) does not have any EWG on the carbon, but it is generally stronger than acetic acid.
3. However, compared to the nitro, cyano, or halo-substituted acetic acids, Formic acid is the weakest acid among the list. Thus, it has the lowest Ka and the highest pKa.
Step 4: Final Answer:
HCOOH has the highest pKa value.