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 acidic strength of substituted benzoic acids depends on the stability of their conjugate base, the benzoate anion.
Substituents on the benzene ring can stabilize or destabilize this anion through Inductive (I) and Mesomeric/Resonance (M) effects.
Step 2: Key Formula or Approach:
The approach is to evaluate the nature of each substituent:
- Electron Withdrawing Groups (EWGs): Disperse negative charge, stabilize the anion \(\rightarrow\) Increase acidity.
- Electron Donating Groups (EDGs): Intensify negative charge, destabilize the anion \(\rightarrow\) Decrease acidity.
Step 3: Detailed Explanation:
Let's analyze the given compounds and their substituents:
(I) Benzoic acid: The standard reference molecule with no substituents on the ring.
(II) 4-nitrobenzoic acid: The \(-\text{NO}_2\) group is strongly electron-withdrawing through both the -I and -M effects. It pulls electron density away from the ring, highly stabilizing the carboxylate anion. Thus, it is more acidic than (I).
(III) 3,4-dinitrobenzoic acid: Contains two strongly electron-withdrawing \(-\text{NO}_2\) groups. The combined -I and -M effects of two nitro groups stabilize the conjugate base to a much greater extent than a single nitro group. This will be the most acidic compound in the set.
(IV) 4-methoxybenzoic acid: The \(-\text{OCH}_3\) group is an electron-donating group. While it has a weak -I effect, its strong +M effect (due to the lone pair on oxygen resonating into the ring) dominates. It donates electron density into the ring, destabilizing the negative charge on the carboxylate anion. Therefore, it is less acidic than unsubstituted benzoic acid (I).
Arranging them in increasing order of acidity:
Least acidic (destabilized by EDG) < Standard < More acidic (stabilized by one EWG) < Most acidic (stabilized by two EWGs).
Therefore: 4-methoxybenzoic acid (IV) < benzoic acid (I) < 4-nitrobenzoic acid (II) < 3,4-dinitrobenzoic acid (III).
Step 4: Final Answer:
The correct increasing order is IV < I < II < III.