Question

The correct increasing order of the acid strength of benzoic acid (I), 4-nitrobenzoic acid (II), 3,4-dinitrobenzoic acid (III) and 4-methoxybenzoic acid (IV) is

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

Hint:

Acid strength for benzoic acids follows: \(EWG\) at para/meta > Benzoic Acid > \(EDG\) at para/meta. Remember that ortho-substituted benzoic acids are usually stronger than benzoic acid due to the "Ortho Effect."

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The acidity of substituted benzoic acids is influenced by the electronic effects of the substituents. Electron-withdrawing groups (EWG) stabilize the carboxylate anion and increase acidity, while electron-donating groups (EDG) destabilize it and decrease acidity.

Step 2: Detailed Explanation:
1. Methoxy group \((-\text{OCH}_3)\) at the para position acts as an EDG due to resonance \((+R\) effect), making IV the weakest acid.
2. Benzoic acid (I) has no substituent and serves as the reference.
3. Nitro group \((-\text{NO}_2)\) is a strong EWG \((-I\) and \(-R\) effects). One nitro group in II increases acidity.
4. Two nitro groups in III exert a massive withdrawing effect, making it the strongest acid in the series.
5. Therefore, the order is: \(\text{4-methoxybenzoic acid} < \text{benzoic acid} < \text{4-nitrobenzoic acid} < \text{3,4-dinitrobenzoic acid}\).

Step 3: Final Answer
The correct increasing order is IV < I < II < III.