Chemistry · Carboxylic Acids and Derivatives

JEE Main 2024 — 8 April, Shift 2 — Question 70

The correct sequence of acidic strength of the following aliphatic acids in their decreasing order is :

$\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}, \mathrm{CH}_{3} \mathrm{COOH}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}$ , $\mathrm{HCOOH}$

Option A:
$\mathrm{HCOOH}>\mathrm{CH}_{3} \mathrm{COOH}>\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}>$ $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}$
Correct
Option B:
$\mathrm{HCOOH}>\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}>$ $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}>\mathrm{CH}_{3} \mathrm{COOH}$
Option C:
$\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}>\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}>$ $\mathrm{CH}_{3} \mathrm{COOH}>\mathrm{HCOOH}$
Option D:
$\mathrm{CH}_{3} \mathrm{COOH}>\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}>$ $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}>\mathrm{HCOOH}$

Answer: A

Step-by-step solution

Acid strength in aliphatic carboxylic acids decreases with increasing $+I$ (electron–donating) effect of alkyl groups. Formic acid $(\mathrm{HCOOH})$ has no alkyl group and is therefore the strongest. Among the remaining acids: $\mathrm{CH_3COOH > CH_3CH_2COOH > CH_3CH_2CH_2COOH}$ due to increasing $+I$ effect with longer alkyl chains.

Hence the decreasing order of acidic strength is $\mathrm{HCOOH > CH_3COOH > CH_3CH_2COOH > CH_3CH_2CH_2COOH}$ .

Thus, the correct answer is Option A.

Answer key and solution verified before publishing.

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Exam: JEE Main 2024
Paper: 8 April, Shift 2
Subject: Chemistry
Chapter: Carboxylic Acids and Derivatives
Topic: Chemical Properties of Carboxylic Acids