Alcohols Phenols And Ethers

Detailed Explanation of the Acidity Order: IV > III > I > II

Acidity of aromatic carboxylic acids (like benzoic acid derivatives) is influenced mainly by two effects:

• −I effect (inductive electron withdrawal): Electron-withdrawing groups (EWGs) stabilize the conjugate base by dispersing negative charge, increasing acidity.
• +M effect (mesomeric electron donation): Electron-donating groups (EDGs) through resonance donate electron density to the ring, destabilizing the conjugate base and decreasing acidity.

Let’s evaluate each compound (I to IV):

1. Compound I: Ortho-substituted benzoic acid
   The substituent is not specified, but if it's an electron-donating group (EDG), ortho effect still makes this more acidic than meta or para substituted EDG groups.
   Ortho effect: Even EDGs at the ortho position can increase acidity due to steric inhibition of resonance, preventing lone pair donation into the ring.
2. Compound II: Para-substituted with an electron-donating group (e.g., –OH, –OCH₃)
   The +M effect from the para position increases electron density on the ring, destabilizing the conjugate base. Least acidic.
3. Compound III: Meta-substituted with an electron-withdrawing group (e.g., –NO₂)
   The −I effect is significant in the meta position (though no resonance with carboxylate anion), which helps stabilize the conjugate base. More acidic than I and II.
4. Compound IV: Ortho-substituted with a strong electron-withdrawing group (like –NO₂ or –Cl)
   This combines:
   • −I effect from the EWG
   • Ortho effect (steric inhibition of resonance of the carboxyl group)

Final Acidity Order: IV > III > I > II

Correct Answer: (D): IV > III > I > II