Question

Which among the following substituent groups increases the acidic strength of aromatic carboxylic acids?

• A. –OH
• B. –CH$_3$
• C. –NH$_2$
• D. –NO$_2$

Hint:

Electron-withdrawing groups such as –NO$_2$ increase the acidity of aromatic carboxylic acids by stabilizing the negative charge on the conjugate base.

Answer 1

The Correct Option is D

Step 1: Understanding acidic strength.
The acidic strength of aromatic carboxylic acids is influenced by the electron-withdrawing or electron-donating nature of substituents. Electron-withdrawing groups increase acidity by stabilizing the negative charge on the conjugate base.
Step 2: Analyzing the options.
(A) –OH: Hydroxyl groups are electron-donating, which decreases acidity.
(B) –CH$_3$: Methyl groups are electron-donating, which also reduces acidity.
(C) –NH$_2$: Amino groups are electron-donating, so they reduce the acidic strength.
(D) –NO$_2$: Nitro groups are electron-withdrawing and increase acidity by stabilizing the conjugate base.
Step 3: Conclusion.
The correct answer is (D) –NO$_2$ as it is an electron-withdrawing group that increases the acidic strength of carboxylic acids.