Question

What will be the order of acidity of the following acids, starting with the highest?

(I) p-methoxy benzoic acid
(II) benzoic acid
(III) p-nitro benzoic acid

• A. I $>$ II $>$ III
• B. II $>$ I $>$ III
• C. I $=$ III $>$ II
• D. III $>$ II $>$ I

Hint:

- $-NO_2$ (strong EWG) $\uparrow$ acidity - $-OCH_3$ (EDG) $\downarrow$ acidity - More stabilization of conjugate base $\Rightarrow$ stronger acid

Answer 1

The Correct Option is D

Concept: The acidity of benzoic acid derivatives depends on the nature of substituents:
• Electron-withdrawing groups increase acidity (stabilize conjugate base)
• Electron-donating groups decrease acidity (destabilize conjugate base)

Step 1: Identify substituents.

• I: $-OCH_3$ (electron-donating group, +M effect)
• II: No substituent (reference: benzoic acid)
• III: $-NO_2$ (strong electron-withdrawing group, -I and -M effect)

Step 2: Effect on acidity.

• $-NO_2$ stabilizes conjugate base $\Rightarrow$ highest acidity
• No substituent $\Rightarrow$ moderate acidity
• $-OCH_3$ donates electrons $\Rightarrow$ lowest acidity

Step 3: Write correct order.
\[ \text{III} > \text{II} > \text{I} \]

Step 4: Final conclusion.
Thus, the correct order is: \[ III > II > I \]