Question

Assertion (A): The pK$_a$ of O$_2$N–CH$_2$–COOH is lower than that of CH$_3$–COOH. Reason (R): The –NO$_2$ group shows an electron-withdrawing effect, which increases the acidic character of O$_2$N–CH$_2$–COOH.

• A. Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
• B. Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
• C. Assertion (A) is true, but Reason (R) is false
• D. Assertion (A) is false, but Reason (R) is true.

Hint:

Electron-withdrawing groups increase acidity by stabilizing the conjugate base, while electron-donating groups decrease acidity by destabilizing it.

Answer 1

The Correct Option is A

The nitro (-NO$_2$) group is a strong electron-withdrawing group (-I effect) that increases the stability of the carboxylate anion, making O$_2$N–CH$_2$–COOH more acidic than CH$_3$–COOH. Since higher acidity corresponds to a lower pK$_a$, Assertion (A) is true, and the Reason (R) correctly explains it.