Question

Which among the following substituent groups decreases the acidic strength of aromatic carboxylic acid?

• A. $-\mathrm{NH_2}$
• B. $-\mathrm{CN}$
• C. $-\mathrm{Cl}$
• D. $-\mathrm{NO_2}$

Hint:

Electron-donating groups decrease acidity by destabilizing the conjugate base, while electron-withdrawing groups increase acidity by stabilizing it.

Answer 1

The Correct Option is A

Step 1: Understanding acidic strength of aromatic carboxylic acids.
The acidic strength of aromatic carboxylic acids depends on the stability of the carboxylate ion formed after dissociation. Electron-withdrawing groups stabilize the negative charge on the carboxylate ion and hence increase acidity, while electron-donating groups destabilize it and decrease acidity.

Step 2: Effect of substituent groups.
Electron-donating groups increase electron density on the aromatic ring and the carboxylate ion, reducing its stability. This leads to a decrease in acidic strength. Electron-withdrawing groups have the opposite effect and increase acidity.

Step 3: Analysis of given options.
(A) $-\mathrm{NH_2$:} This is a strong electron-donating group due to its +M (resonance donating) effect. It destabilizes the carboxylate ion and thus decreases acidic strength.
(B) $-\mathrm{CN$:} This is a strong electron-withdrawing group due to its $-I$ and $-M$ effects, which increases acidity.
(C) $-\mathrm{Cl$:} Chlorine shows a $-I$ effect, which increases acidity despite its weak +M effect.
(D) $-\mathrm{NO_2$:} Nitro group is a very strong electron-withdrawing group, significantly increasing acidic strength.

Step 4: Conclusion.
Among the given substituents, only the amino group donates electrons strongly and decreases the acidic strength of aromatic carboxylic acids.