Question

In the following, strongest Acid is :

• A. CH$_3$CH$_2$COOH
• B. CH$_3$COOH
• C. C$_6$H$_5$COOH
• D. C$_6$H$_5$CH$_2$COOH

Hint:

General order of acidity: Formic Acid >Benzoic Acid >Acetic Acid. Aromatic carboxylic acids are generally stronger than aliphatic ones (except Formic).

Answer 1

The Correct Option is C

We compare the acid strengths based on the stability of their conjugate bases and pK$_a$ values.
Acetic acid (CH$_3$COOH) has a pK$_a$ of 4.76. Propanoic acid (CH$_3$CH$_2$COOH) has a pK$_a$ of 4.88.
Benzoic acid (C$_6$H$_5$COOH) has a pK$_a$ of 4.20. The phenyl group exerts an electron-withdrawing effect (due to sp$^2$ carbons) which stabilizes the carboxylate ion relative to alkyl groups.
Phenylacetic acid (C$_6$H$_5$CH$_2$COOH) has a pK$_a$ of 4.31. The -I effect of the phenyl ring is diminished by the intervening CH$_2$ group compared to direct attachment in Benzoic acid.
Since a lower pK$_a$ indicates a stronger acid, Benzoic acid (pK$_a$ 4.20) is the strongest among the given options.