Question

Arrange the following compounds in increasing order of their acidic strengths: \[ CH_3CH_2COOH,\quad CH_3CH(CH_3)COOH,\quad CH_3CH_2CH_2COOH,\quad BrCH_2CH_2COOH \]

Hint:

−I group (like halogen) increases acidity. +I alkyl groups decrease acidity.

Answer 1

Concept: Acidity of carboxylic acids depends mainly on:

• Inductive effect of substituents
• Electron-withdrawing groups increase acidity
• Electron-donating alkyl groups decrease acidity

Step 1: Analyze substituent effects.
(1) CH\(_3\)CH(CH\(_3\))COOH: Branched alkyl group shows strong +I effect → least acidic.
(2) CH\(_3\)CH\(_2\)CH\(_2\)COOH: Longer alkyl chain → stronger +I effect than propionic acid → lower acidity.
(3) CH\(_3\)CH\(_2\)COOH: Moderate +I effect → higher acidity than above two.
(4) BrCH\(_2\)CH\(_2\)COOH: Bromine is electron-withdrawing (−I effect) → stabilizes carboxylate ion → most acidic.
Step 2: Arrange in increasing acidity. \[ CH_3CH(CH_3)COOH<CH_3CH_2CH_2COOH<CH_3CH_2COOH<BrCH_2CH_2COOH \]