Question:
Which of the following carboxylic acid has the highest $pK_a$ value?
Which of the following carboxylic acid has the highest $pK_a$ value?
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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.
Updated On: Jun 26, 2026
- $O_2N-CH_2-COOH$
- $F-CH_2-COOH$
- $HCOOH$
- $CN-CH_2COOH$
- $Cl-CH_2COOH$
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Verified By Collegedunia
The Correct Option is C
Solution and Explanation
Step 1: Understanding the Concept:
Acidity is inversely related to $pK_a$ ($pK_a = -\log K_a$). A higher $pK_a$ value indicates a weaker acid.
Step 2: Detailed Explanation:
1. Acidity and Inductive Effect: Electron-withdrawing groups (EWGs) increase acidity by stabilizing the conjugate base (carboxylate ion) via the $-I$ effect. Stronger $-I$ groups result in lower $pK_a$ values.
2. Analysis of Substituents:
- $-NO_2$ (Nitro) is a very strong EWG.
- $-CN$ (Cyano) is a strong EWG.
- $-F$ and $-Cl$ (Halogens) are significant EWGs.
3. All the substituted acids (A, B, D, E) have an EWG attached to the $\alpha$-carbon, which pulls electron density away and makes them stronger than formic acid.
4. Formic Acid ($HCOOH$): It has only a hydrogen atom attached to the carboxyl group. Hydrogen has effectively zero inductive effect compared to these groups. Therefore, $HCOOH$ is the weakest acid among the list, meaning it has the highest $pK_a$.
Step 3: Final Answer:
$HCOOH$ has the highest $pK_a$ value.
Acidity is inversely related to $pK_a$ ($pK_a = -\log K_a$). A higher $pK_a$ value indicates a weaker acid.
Step 2: Detailed Explanation:
1. Acidity and Inductive Effect: Electron-withdrawing groups (EWGs) increase acidity by stabilizing the conjugate base (carboxylate ion) via the $-I$ effect. Stronger $-I$ groups result in lower $pK_a$ values.
2. Analysis of Substituents:
- $-NO_2$ (Nitro) is a very strong EWG.
- $-CN$ (Cyano) is a strong EWG.
- $-F$ and $-Cl$ (Halogens) are significant EWGs.
3. All the substituted acids (A, B, D, E) have an EWG attached to the $\alpha$-carbon, which pulls electron density away and makes them stronger than formic acid.
4. Formic Acid ($HCOOH$): It has only a hydrogen atom attached to the carboxyl group. Hydrogen has effectively zero inductive effect compared to these groups. Therefore, $HCOOH$ is the weakest acid among the list, meaning it has the highest $pK_a$.
Step 3: Final Answer:
$HCOOH$ has the highest $pK_a$ value.
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