Question:
Which one of the following carboxylic acids has the highest $\text{p}K_a$ value?}
Which one of the following carboxylic acids has the highest $\text{p}K_a$ value?}
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Stronger electron-donating alkyl groups increase $\text{p}K_a$ by destabilizing the carboxylate ion.
Updated On: Jun 12, 2026
- Benzoic acid ($\text{C}_6\text{H}_5\text{COOH}$)
- Phenylacetic acid ($\text{C}_6\text{H}_5\text{CH}_2\text{COOH}$)
- Ethanoic acid ($\text{CH}_3\text{COOH}$)
- Propanoic acid ($\text{CH}_3\text{CH}_2\text{COOH}$)
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Verified By Collegedunia
The Correct Option is D
Solution and Explanation
Concept:
$\text{p}K_a = -\log K_a$. A higher $\text{p}K_a$ means a weaker acid. Acid strength depends on stability of the conjugate base ($\text{RCOO}^-$). Electron-withdrawing groups stabilize the conjugate base and increase acidity, while electron-donating groups destabilize it and decrease acidity.
Step 1: Benzoic acid.
The phenyl ring shows a mild $-I$ effect, stabilizing the carboxylate ion. Hence it is relatively more acidic.
Step 2: Phenylacetic acid.
The phenyl group is separated by $-CH_2-$, reducing resonance but still showing $-I$ effect, making it moderately acidic.
Step 3: Ethanoic acid.
The methyl group shows $+I$ effect, slightly decreasing acidity.
Step 4: Propanoic acid.
The ethyl group shows stronger $+I$ effect than methyl group, further destabilizing the carboxylate ion, making it the weakest acid. Conclusion:
Propanoic acid is the weakest acid among the given options, hence it has the highest $\text{p}K_a$ value.
Step 1: Benzoic acid.
The phenyl ring shows a mild $-I$ effect, stabilizing the carboxylate ion. Hence it is relatively more acidic.
Step 2: Phenylacetic acid.
The phenyl group is separated by $-CH_2-$, reducing resonance but still showing $-I$ effect, making it moderately acidic.
Step 3: Ethanoic acid.
The methyl group shows $+I$ effect, slightly decreasing acidity.
Step 4: Propanoic acid.
The ethyl group shows stronger $+I$ effect than methyl group, further destabilizing the carboxylate ion, making it the weakest acid. Conclusion:
Propanoic acid is the weakest acid among the given options, hence it has the highest $\text{p}K_a$ value.
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