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JEE Main 2025 · Chemistry · General Organic Chemistry and Isomerism · easy

The correct order of acid strength of p-nitrobenzoic acid (I), p-chlorobenzoic acid (II), benzoic acid (III) and p-methoxybenzoic acid (IV) in water…

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The correct order of acid strength of pp-nitrobenzoic acid (I), pp-chlorobenzoic acid (II), benzoic acid (III) and pp-methoxybenzoic acid (IV) in water is
  1. A. I > II > III > IV
  2. B. IV > III > II > I
  3. C. II > I > III > IV
  4. D. I > III > II > IV
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Answer: (A) I > II > III > IV

Electron-withdrawing groups stabilise the carboxylate anion. −NO2-\mathrm{NO_2} withdraws strongly by −M-M and −I-I (pKa about 3.4), −Cl-\mathrm{Cl} withdraws by −I-I which outweighs its +M+M (pKa about 4.0), −H-\mathrm{H} is the reference (4.2), while −OCH3-\mathrm{OCH_3} donates by +M+M and destabilises the anion (4.5). Order: I > II > III > IV.
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