JEE Challenger
More from Aldehydes, Ketones and Carboxylic Acids
ChemMedAdv 2016 • Paper 2

Acidity Order of Substituted Benzoic Acid Derivatives

Select the correct sequence of decreasing acidity among the provided chemical species:

Question Diagram 1

Options

A

I>II>III>IV\text{I} > \text{II} > \text{III} > \text{IV}

Correct
B

III>I>II>IV\text{III} > \text{I} > \text{II} > \text{IV}

C

III>IV>II>I\text{III} > \text{IV} > \text{II} > \text{I}

D

I>III>IV>II\text{I} > \text{III} > \text{IV} > \text{II}

Topics & Concepts

Aldehydes, Ketones and Carboxylic AcidsCarboxylic Acids and DerivativesInductive EffectResonance

Step-by-Step Solution

To determine the correct order of acidity for the given substituted benzoic acids, we analyze the stability of their respective conjugate bases (carboxylate anions) by considering inductive (−I-\text{I}), mesomeric/resonance (+M+\text{M}), and ortho/hydrogen-bonding effects:

  1. Compound I (2,6-dihydroxybenzoic acid) and Compound II (2-hydroxybenzoic acid):

    • Ortho Effect and Hydrogen Bonding: Hydroxyl (−OH-\text{OH}) groups at the ortho position exert a strong ortho effect. Upon deprotonation, the carboxylate anion (−COO−-\text{COO}^-) is stabilized through intramolecular hydrogen bonding with the adjacent hydroxyl group(s).
    • Comparison between I and II: Compound I possesses two ortho-−OH-\text{OH} groups, forming a highly stable, doubly intramolecularly hydrogen-bonded conjugate base. Compound II has only one ortho-−OH-\text{OH} group. Thus, Compound I is significantly more acidic than Compound II: I>II\text{I} > \text{II}

  2. Compound III (3-hydroxybenzoic acid) and Compound IV (4-hydroxybenzoic acid):

    • Meta Position (Compound III): At the meta position, the electron-donating mesomeric effect (+M+\text{M}) of the −OH-\text{OH} group cannot operate on the carboxylate group. Only the electron-withdrawing inductive effect (−I-\text{I}) acts, which stabilizes the conjugate base and increases acidity relative to phenol-substituted systems.
    • Para Position (Compound IV): At the para position, the electron-donating resonance effect (+M+\text{M}) of the −OH-\text{OH} group operates strongly and outweighs its electron-withdrawing inductive effect (−I-\text{I}). This increases the electron density on the carboxylic group, destabilizing the carboxylate anion and decreasing its acidity.
    • Thus, Compound III is more acidic than Compound IV: III>IV\text{III} > \text{IV}

  3. Overall Acidic Strength Comparison:

    • Due to the dominant ortho effect and strong hydrogen bonding stabilization in conjugate bases, the ortho-substituted compounds (I and II) are far more acidic than the meta (III) and para (IV) substituted compounds.

Combining these findings gives the overall decreasing order of acidity: I>II>III>IV\text{I} > \text{II} > \text{III} > \text{IV}

This corresponds to Option (A).

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