Q19 / 200 · Manipal (MET) 2010 · Chemistry

Consider the acidity of the carboxylic acids I. PhCOOH II. o- $\mathrm{NO}_2 \mathrm{C}_6 \mathrm{H}_4 \mathrm{COOH}$ III. $p-\mathrm{NO}_2 \mathrm{C}_6 \mathrm{H}_4 \mathrm{COOH}$ IV. $m-\mathrm{NO}_2 \mathrm{C}_6 \mathrm{H}_4 \mathrm{COOH}$ Which of the following order is correct?

  • A
    $\mathrm{I}\gt\mathrm{II}\gt\mathrm{III}\gt\mathrm{IV}$
  • B
    $\mathrm{II}\gt\mathrm{IV}\gt\mathrm{III}\gt\mathrm{I}$
  • C
    $\mathrm{II}\gt\mathrm{IV}\gt\mathrm{I}\gt\mathrm{III}$
  • Correct answer
    D
    $\mathrm{II}\lt\mathrm{III}\gt\mathrm{IV}\gt\mathrm{I}$

Solution

$\mathrm{-NO}_2$ group at any position shows electron withdrawing effect, thus acid strength is increased. But o-nitro benzoic acid believed to have ortho effect. As a result, resonance gets prevented. Hence, its acid strength is maximum, thus, the order of acid strength (II) $\lt$ (III) $\gt$ (IV) $\gt$ (I) (The effect is more at para position than meta.)

Hence, the correct answer is option: 'D'.

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