JEE Advanced · 2019 · 27 May 2019 · Paper 1 · Aldehydes Ketones and Carboxylic Acids

The correct order of acid strength of the following carboxylic acids is -

(A)I > III > II > IV
(B)III > II > I > IV
(C)II > I > IV > III
(D)I > II > III > IV✓ Correct

Answer: (D)

Solution

Acid strength increases with electron-withdrawing groups and decreases with electron-donating groups. Structure I (pKa 1.86) has strong electron-withdrawing groups making it most acidic. Structure II (pKa 4.3) is next. Structure III (pKa 4.5) has a methoxy group (electron-donating) making it weaker. Structure IV (pKa 4.88) is a simple aliphatic acid, the weakest. The order of decreasing acid strength (increasing pKa) is I > II > III > IV, which matches option (D).

JEE Advanced · 2019 · 27 May 2019 · Paper 1 · Single correct option

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