Question

Arrange the following in decreasing order of acidic strength.
I. Benzoic acid
II. $4 -$nitrobenzoic acid
III. $3,4 -$dinitrobenzoic acid
IV. $4 -$methoxy benzoic acid

A.III>II>I>IV
B.III>II>IV>I
C.II>III>IV>I
D.I>II>III>IV

Answer 1

In general, the strength of an acid in a compound is directly proportional to the soundness of the acid’s conjugate base. In other words, an acid that incorporates a more stable conjugate base is going to be more acidic than an acid that encompasses a less stable conjugate base. Electron withdrawing groups and electron-donating groups have a significant role to play in determining the acidity of substituted benzoic acids.

Complete step by step solution:
Let us first understand what are electron-withdrawing and electron-donating groups
-Electron-withdrawing groups-The conjugate base of benzoic acid is stabilized by the electron-withdrawing groups (EWG) which makes the acid more acidic by delocalizing the charge of the carboxylate ion. Electron-withdrawing groups deactivate the benzene ring to electrophilic attack thereby making benzoic acids more acidic.
-Electron-donating groups-The conjugate base of benzoic acid is destabilized by the electron-donating groups (EDG). This makes the benzoic acid less acidic by pushing more electron density toward the negative charge in the carboxylate. Electron-donating groups further activate the benzene ring to electrophilic attack and make benzoic acids less acidic.
-Since electron-donating groups decrease the acidic strength, $4 -$ methoxy benzoic acid is a weaker acid than benzoic acid.
-Also, since electron-withdrawing groups increase the acid strength, therefore, both $4 -$nitrobenzoic acid and $3,4 -$ dinitrobenzoic acid are stronger acids than benzoic acid.
-Moving forward, due to the presence of an additional $N{O_2}$​ at meta position concerning $COOH$ group, $3,4 -$ dinitrobenzoic acid
thus the overall acid strength increase in the order:
$4 -$ methoxy benzoic acid < benzoic acid < $4 -$ nitrobenzoic acid < $3,4 -$ dinitrobenzoic acid.
So, the correct option is A).

Note:
When an electron-releasing or an electron-withdrawing species is introduced to a series of atoms (generally a carbon chain), the corresponding negative or electric charge is relayed through the carbon chain by the atoms belonging thereto. This causes a permanent dipole to arise within the molecule and is spoken because of the inductive effect. Using the inductive effect, we will predict the acidity and basicity of organic compounds.If ${K_a}$ acid is high, it is a strong acid but if $p{K_a}$ acid is high, it is said to be a weak acid $\,\,\left[ {pka{\text{ }} = {\text{ }} - log\left( {ka} \right)} \right]\,\,$ Same logic applies to bases.