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JEE Main 2020 · Chemistry · General Organic Chemistry and Isomerism · medium

The correct order of acidic strength for o-nitrobenzoic acid (I), p-nitrobenzoic acid (II), benzoic acid (III) and p-methoxybenzoic acid (IV) is

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The correct order of acidic strength for oo-nitrobenzoic acid (I), pp-nitrobenzoic acid (II), benzoic acid (III) and pp-methoxybenzoic acid (IV) is
  1. A. I>II>III>IV\mathrm{I>II>III>IV}
  2. B. II>I>III>IV\mathrm{II>I>III>IV}
  3. C. I>III>II>IV\mathrm{I>III>II>IV}
  4. D. IV>III>II>I\mathrm{IV>III>II>I}
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Answer: (A) I>II>III>IV\mathrm{I>II>III>IV}

The −NO2-\mathrm{NO_2} group stabilises the carboxylate by −I-I/−M-M effects, so I and II are stronger than benzoic acid. The ortho isomer is strongest because the −I-I effect works at close range and steric inhibition of resonance twists the COOH\mathrm{COOH} out of plane (ortho effect). pp-OCH3\mathrm{OCH_3} donates by +M+M and destabilises the anion, so IV is weakest. pKa: 2.17, 3.44, 4.20, 4.47.
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