Question:
Which of the following has lowest $pK_a$?
Which of the following has lowest $pK_a$?
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Electron-withdrawing groups (EWG) stabilize the carboxylate anion, increasing acidity.
Updated On: Jun 10, 2026
- Structure 1
- Structure 2
- Structure 3
- Structure 4
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The Correct Option is C
Solution and Explanation
Step 1: Concept
$pK_a$ is inversely related to acid strength. Electron-withdrawing groups increase acidity (lower $pK_a$).
Step 2: Analysis
Nitro group ($NO_2$) is a strong electron-withdrawing group, significantly increasing acidity.
Step 3: Conclusion
Structure 3 (p-nitrobenzoic acid) is the strongest acid, hence lowest $pK_a$.
Final Answer: (C)
$pK_a$ is inversely related to acid strength. Electron-withdrawing groups increase acidity (lower $pK_a$).
Step 2: Analysis
Nitro group ($NO_2$) is a strong electron-withdrawing group, significantly increasing acidity.
Step 3: Conclusion
Structure 3 (p-nitrobenzoic acid) is the strongest acid, hence lowest $pK_a$.
Final Answer: (C)
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