Question:
Among the following, compound with the lowest \(pK_a\) value is:
Among the following, compound with the lowest \(pK_a\) value is:
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Ortho effect (intramolecular H-bonding) increases acidity significantly → lowest \(pK_a\) for o-hydroxy benzoic acid.
Updated On: Apr 16, 2026
- o-hydroxy benzoic acid
- m-hydroxy benzoic acid
- p-hydroxy benzoic acid
- benzoic acid
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The Correct Option is A
Solution and Explanation
Concept:
Lower \(pK_a\) means stronger acid. The ortho effect enhances acidity.
Step 1: Ortho effect. In o-hydroxy benzoic acid (salicylic acid), the -OH group is ortho to -COOH. It forms an intramolecular hydrogen bond with the carboxylate ion after deprotonation, stabilizing the conjugate base.
Step 2: Comparison.
• o-hydroxy benzoic acid: Intramolecular H-bond stabilizes conjugate base → strongest acid → lowest \(pK_a\).
• m- and p-hydroxy benzoic acids: No intramolecular H-bond; electronic effects (resonance/inductive) but less effective than ortho effect.
• Benzoic acid: No additional -OH group → weakest acid among these.
Step 1: Ortho effect. In o-hydroxy benzoic acid (salicylic acid), the -OH group is ortho to -COOH. It forms an intramolecular hydrogen bond with the carboxylate ion after deprotonation, stabilizing the conjugate base.
Step 2: Comparison.
• o-hydroxy benzoic acid: Intramolecular H-bond stabilizes conjugate base → strongest acid → lowest \(pK_a\).
• m- and p-hydroxy benzoic acids: No intramolecular H-bond; electronic effects (resonance/inductive) but less effective than ortho effect.
• Benzoic acid: No additional -OH group → weakest acid among these.
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