Question

The correct increasing order of acidity of the following molecules is

Choose the correct answer from the options given below:

• A. (A), (B), (C), (D)
• B. (D), (C), (A), (B)
• C. (B), (A), (D), (C)
• D. (C), (B), (D), (A)

Hint:

Acidity of aromatic acids:
\(-M, -I\) groups (e.g., \(-\text{NO}_2\)) \(\rightarrow\) Increase acidity.
\(+M\) groups (e.g., \(-\text{OCH}_3, -\text{OH}\)) \(\rightarrow\) Decrease acidity.
Order: 4-methoxybenzoic acid \(<\) benzoic acid \(<\) 4-nitrobenzoic acid \(<\) 3,5-dinitrobenzoic acid.

Answer 1

The Correct Option is B

Concept:
The acidity of substituted benzoic acids depends on the stability of the carboxylate anion formed after deprotonation.
Electron-withdrawing groups (\(-\text{NO}_2\)) stabilize the negative charge on the carboxylate group through \(-I\) and \(-M\) effects, thereby increasing the acidity.
Electron-donating groups (\(-\text{OCH}_3\)) destabilize the conjugate base through \(+M\) electron donation into the ring, thereby decreasing the acidity.

Step 1: Effect of Electron-Donating Methoxy Group in Compound (D):
In 4-methoxybenzoic acid (D), the methoxy group (\(-\text{OCH}_3\)) at the para-position acts as a strong resonance electron-donating group (\(+M > -I\)).
It pumps electron density into the benzene ring and intensifies the negative charge on the carboxylate anion, destabilizing it.
Therefore, 4-methoxybenzoic acid is less acidic than unsubstituted benzoic acid (C).

Step 2: Reference Acidity of Benzoic Acid (C):
Benzoic acid (C) lacks any substituents on the phenyl ring.
It is more acidic than 4-methoxybenzoic acid (D), but less acidic than derivatives bearing electron-withdrawing nitro groups.

Step 3: Effect of Nitro Groups in Compounds (A) and (B):
In 4-nitrobenzoic acid (A), the nitro group at the para-position exerts both powerful \(-M\) and \(-I\) electron-withdrawing effects, significantly stabilizing the carboxylate anion and increasing acidity.
In 3,5-dinitrobenzoic acid (B), two nitro groups are present at both meta-positions.
Although the mesomeric effect does not operate at meta-positions, two strongly electronegative \(\text{NO}_2\) groups exert a combined inductive electron-withdrawing effect (\(-I\)), resulting in a stronger overall acidifying effect than a single para-nitro group (\(pK_a\) of 3,5-dinitrobenzoic acid \(\approx 2.82\) vs \(pK_a\) of 4-nitrobenzoic acid \(\approx 3.44\)).
Thus, compound (B) is the strongest acid in the series.

Step 4: Compiling the Increasing Acidity Order:
Arranging the compounds from weakest acid to strongest acid gives:
\[ \text{(D)} < \text{(C)} < \text{(A)} < \text{(B)} \] Final Answer:
The correct increasing order of acidity is (D), (C), (A), (B), corresponding to option (B).