Chemistry · Carboxylic Acids and Derivatives

NEET (UG) 2025 — Question 65

The correct order of decreasing acidity of the following aliphatic acids is :-

Option A:
$\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCOOH}>\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCOOH}>\mathrm{CH}_{3} \mathrm{COOH}$ $>\mathrm{HCOOH}$
Option B:
$\mathrm{CH}_{3} \mathrm{COOH}>\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCOOH}>\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCOOH}$ $>\mathrm{HCOOH}$
Option C:
$\mathrm{HCOOH}>\mathrm{CH}_{3} \mathrm{COOH}>\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCOOH}$ $>\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCOOH}$
Correct
Option D:
$\mathrm{HCOOH}>\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCOOH}>\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCOOH}$ $>\mathrm{CH}_{3} \mathrm{COOH}$

Answer: C

Step-by-step solution

Acidity of aliphatic carboxylic acids depends on the electron-releasing (+I) effect of alkyl groups attached to the $\ce{-COOH}$ carbon. More +I alkyl groups increase electron density on the carboxyl carbon, destabilise the conjugate base, and decrease acidity. $\ce{HCOOH}$ has no alkyl group, so it is the strongest acid. $\ce{CH3COOH}$ has one methyl group, so it is less acidic than formic acid. $\ce{(CH3)2CHCOOH}$ (isobutyric acid) has two methyl groups, so it is less acidic than acetic acid. $\ce{(CH3)3CCOOH}$ (pivalic acid) has three methyl groups, so it is the weakest acid. Therefore the decreasing acidity order is $\ce{HCOOH} > \ce{CH3COOH} > \ce{(CH3)2CHCOOH} > \ce{(CH3)3CCOOH}$ . Correct option: C.

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Exam: NEET (UG) 2025
Paper: 2025 paper
Subject: Chemistry
Chapter: Carboxylic Acids and Derivatives
Topic: Chemical Properties of Carboxylic Acids