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Q8.12(iii):
Arrange the following compounds in increasing order of their property as indicated: (iii) Benzoic acid, 4-Nitrobenzoic acid, 3,4-Dinitrobenzoic acid, 4-Methoxybenzoic acid (acid strength)

Solution :

Step 1: Theoretical Foundation of Acid Strength

The acid strength of substituted benzoic acids is determined by the stability of the conjugate base, the carboxylate anion ($C_6H_5COO^-$). Stability is enhanced by electron-withdrawing groups (EWG), which disperse the negative charge through inductive ($-I$) and resonance ($-R$) effects, and decreased by electron-donating groups (EDG), which intensify the negative charge through inductive ($+I$) or resonance ($+R$) effects [Per the Brønsted-Lowry Theory of Acids and Bases].

Step 2: Analysis of Substituent Effects

We evaluate the substituents attached to the benzene ring relative to the carboxylic acid group:

  • 4-Methoxybenzoic acid: The methoxy group ($-OCH_3$) exerts a strong $+R$ effect and a weak $-I$ effect. The net effect is electron-donating, which destabilizes the conjugate base, making it the least acidic.
  • Benzoic acid: Serves as the reference compound with no substituents.
  • 4-Nitrobenzoic acid: The nitro group ($-NO_2$) exerts both strong $-I$ and $-R$ effects. It is strongly electron-withdrawing, significantly stabilizing the conjugate base.
  • 3,4-Dinitrobenzoic acid: This compound features two nitro groups. One is in the para-position (strong $-I$ and $-R$) and one in the meta-position ($-I$ effect only). The combined inductive and resonance withdrawal makes this the most stable conjugate base and thus the strongest acid.

Step 3: Comparative Data Table

Compound Substituent Effect Electronic Nature
4-Methoxybenzoic acid $+R > -I$ Electron Donating
Benzoic acid None Neutral
4-Nitrobenzoic acid $-I, -R$ Electron Withdrawing
3,4-Dinitrobenzoic acid $2 \times (-I) + (-R)$ Strongly Electron Withdrawing

Step 4: Vectorial Representation of Electronic Effects

Increasing Acid Strength 4-Methoxybenzoic Benzoic Acid 4-Nitrobenzoic 3,4-Dinitrobenzoic

Step 5: Final Ordering

Based on the quantitative assessment of electron density at the reaction center, the acidity increases as the electron-withdrawing capability of the substituent increases. The arrangement in increasing order of acid strength is:

4-Methoxybenzoic acid < Benzoic acid < 4-Nitrobenzoic acid < 3,4-Dinitrobenzoic acid

Final Solution: The increasing order of acid strength is 4-Methoxybenzoic acid < Benzoic acid < 4-Nitrobenzoic acid < 3,4-Dinitrobenzoic acid.


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