Question

The increasing order of acidic strength of the following in aqueous solution is 
 

 

• A. IV < II < III < I
• B. I < III < II < IV
• C. I < II < III < IV
• D. III < I < II < IV

Hint:

Acidity Order: -M \textgreater -I \textgreater Standard \textgreater +I \textgreater +M groups. \( \text{NO}_2 \) is a stronger withdrawing group than \( \text{CN} \). \( \text{OCH}_3 \) is a strong donor due to resonance.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:

The acidity of benzoic acid derivatives depends on the substituent groups:
- Electron Withdrawing Groups (EWG): Increase acidity (-I, -M effects).
- Electron Donating Groups (EDG): Decrease acidity (+I, +M effects).

Step 2: Analyzing Substituents:

- I. -OCH₃ (Methoxy): Shows +M effect (strong resonance donation) and -I effect. The +M effect dominates at para position. It destabilizes the carboxylate ion. Weakest acid.
- II. H (Benzoic Acid): Standard reference.
- III. -CN (Cyano): Shows -I and -M effects (moderately strong EWG). Increases acidity.
- IV. -NO₂ (Nitro): Shows strong -I and strong -M effects (very strong EWG). Increases acidity significantly.

Step 3: Ordering:

Acidity Order: Strong EDG < Reference < Moderate EWG < Strong EWG.
-OCH₃ (I) < H (II) < -CN (III) < -NO₂ (IV)
Order: I < II < III < IV.

Step 4: Final Answer:
Option (C).