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 \(\propto\) Stability of conjugate base \(\propto\) Electron withdrawing power of substituent. Just remember: Withdrawers make it stronger, Donors make it weaker!

Answer 1

The Correct Option is C

Concept: The acid strength of substituted benzoic acids is governed by the electronic effects (inductive and resonance) of the substituents on the benzene ring.
• Electron Withdrawing Groups (EWG): Groups like $-NO_2$ withdraw electron density from the ring, stabilizing the carboxylate anion and increasing acidity. The more EWGs present, the stronger the acid.
• Electron Donating Groups (EDG): Groups like $-OCH_3$ donate electron density into the ring, destabilizing the carboxylate anion and decreasing acidity.
• Order of Effects: Nitro group has strong $-I$ and $-M$ effects, while Methoxy group has a strong $+M$ effect that outweighs its $-I$ effect at the para position.

Step 1: Evaluate the effect of each substituent. - (III) 3,4-dinitrobenzoic acid: Contains two strong electron-withdrawing nitro groups. This will be the strongest acid.
- (II) 4-nitrobenzoic acid: Contains one strong electron-withdrawing nitro group. It is stronger than benzoic acid but weaker than the dinitro derivative.
- (I) Benzoic acid: The reference standard with no substituents.
- (IV) 4-methoxybenzoic acid: The methoxy group ($+M$) increases electron density on the ring, making it the weakest acid.

Step 2: Determine the increasing order. Combining these observations, the order of increasing acid strength is: 4-methoxybenzoic acid (IV) $<$ benzoic acid (I) $<$ 4-nitrobenzoic acid (II) $<$ 3,4-dinitrobenzoic acid (III). This corresponds to Option (C).