Consider the acidity of the carboxylic acids: (1) PhCOOH (2) $\mathrm{o}-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} \mathrm{COOH}$ (3) $\mathrm{p}-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} \mathrm{COOH}$ (4) $\mathrm{m}-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} \mathrm{COOH}$ Which of these is the most acidic?

(B) $\mathrm{o}-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} \mathrm{COOH}$

(C) $\mathrm{p}-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} \mathrm{COOH}$

(D) $\mathrm{m}-\mathrm{NO}_{2} \mathrm{C}_{6} \mathrm{H}_{4} \mathrm{COOH}$

JEE Main · 2004 · 24 Apr 2004 · Offline · Aldehydes Ketones and Carboxylic Acids

Consider the acidity of the carboxylic acids:
(1) PhCOOH
(2)

o - NO_{2} C_{6} H_{4} COOH

(3)

p - NO_{2} C_{6} H_{4} COOH

(4)

m - NO_{2} C_{6} H_{4} COOH

Which of these is the most acidic?

(A)PhCOOH
(B) $o - NO_{2} C_{6} H_{4} COOH$ ✓ Correct
(C) $p - NO_{2} C_{6} H_{4} COOH$
(D) $m - NO_{2} C_{6} H_{4} COOH$

Answer: (B)

Solution

The nitro group is strongly electron-withdrawing and raises acidity by drawing electron density off the carboxylate and stabilising the anion, so all three nitrobenzoic acids outdo plain benzoic acid. The ortho isomer is strongest of all: besides the inductive withdrawal felt most keenly at the neighbouring position, the ortho nitro group can hydrogen-bond to and steady the carboxylate, so

o - NO X_{2} C X_{6} H X_{4} COOH

is the most acidic.

JEE Main · 2004 · 24 Apr 2004 · Offline · Single correct option

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JEE Main · 2004 · 24 Apr 2004 · Offline · Aldehydes Ketones and Carboxylic Acids

Solution

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