Answer the question on basis of passage given below:

Carboxylic acids are weaker than mineral acids, but they are stronger acids than alcohols and many simple phenols (\(pK_a\) is ∼16 for ethanol and 10 for phenol) The higher acidity of carboxylic acids as compared to phenols can be understood similarly. The conjugate base of carboxylic acid, a carboxylate ion, is stabalized by two equivalent resonance structures in which the negative charge is at the more electronegative oxygen atom. The conjugate base of phenol,a phenoxide ion, has non-equivalent resonance structures in which the negative charge is at the less electronegative carbon atom. Further, the negative charge is delocalized over two electronegative oxygen atoms in carboxylate ion whereas it is less effectively delocalized over one oxygen atom and less electronegative carbon atoms in phenoxide ion.Thus, the carboxylate ion is more established than phenoxide ion, so carboxylic acids are more acidic than phenols.

Effect of substituents on the acidity of carboxylic acids: substituents may affect the stability of the conjugate base and thus, also affect the acidity of the carboxylic acids. Electron withdrawing group increase the acidity of carboxylic acids by stabilizing the conjugate base through delocalization of the negative charge by by inductive and /or resonance effects. Conversely, electron donating groups decrease the acidity by destabilising the conjugate base.

Arrange the following acids in order of their increasing acidity:

A. 4-Methoxybenzoic acid

B. 4-Nitrobenzoic acid

C. 4-Chloro benzoic acid

D. 4-Methylbenzoic acid

E. Benzoic acid

Choose the correct answer from the options given below:

A, D, E, C, B

The correct answer is option 3. A, D, E, C, B.

The acidity of benzoic acid derivatives is influenced by substituents attached to the benzene ring. These substituents can be either electron-withdrawing or electron-donating.

Electron-Withdrawing Groups (EWGs):

Effects on Acidity: EWGs increase the acidity of the carboxyl group by stabilizing the negative charge on the carboxylate ion formed after deprotonation. This stabilization makes it easier for the acid to lose a proton (H⁺).

Examples: Nitro group \((NO_2)\), chloro group \((Cl)\).

Electron-Donating Groups (EDGs):

Effects on Acidity: EDGs decrease the acidity of the carboxyl group by increasing the electron density around the carboxylate ion, making it less stable. This makes it harder for the acid to lose a proton.

Examples: Methoxy group \((OCH_3)\), methyl group \((CH_3)\).

Analysis of Each Acid:

A. 4-Methoxybenzoic Acid :

Substituent: Methoxy group \((OCH_3)\).

Effect: The methoxy group is an electron-donating group through resonance. It increases electron density on the benzene ring and the carboxyl group, making the carboxylate ion less stable. Thus, the acidity decreases.

Relative Acidity: Least acidic among the given acids.

D. 4-Methylbenzoic Acid:

Substituent: Methyl group \((CH_3)\).

Effect: The methyl group is an electron-donating group via inductive effects. It also increases electron density on the carboxyl group, but its effect is less than that of the methoxy group.

Relative Acidity: Slightly more acidic than 4-methoxybenzoic acid but still less acidic than benzoic acid.

E. Benzoic Acid:

Substituent: No additional substituents.

Effect: Serves as the baseline for comparison. Its acidity is influenced by the inherent properties of the carboxyl group without additional electronic effects from substituents.

Relative Acidity: Intermediate acidity, used as a reference.

C. 4-Chlorobenzoic Acid:

Substituent: Chloro group \((Cl)\).

Effect: The chloro group is an electron-withdrawing group via inductive effects. It pulls electron density away from the carboxyl group, making the carboxylate ion more stable and thus increasing acidity.

Relative Acidity: More acidic than benzoic acid.

B. 4-Nitrobenzoic Acid:

Substituent: Nitro group \((NO_2)\).

Effect: The nitro group is a strong electron-withdrawing group through both inductive and resonance effects. It significantly stabilizes the carboxylate ion, making the acid much more acidic.

Relative Acidity: Most acidic among the given acids.

Summary of Acidity Order:

Least Acidic: 4-Methoxybenzoic Acid (A) due to strong electron-donating effects.

Next Least Acidic: 4-Methylbenzoic Acid (D) due to moderate electron-donating effects.

Intermediate: Benzoic Acid (E) as the baseline.

More Acidic: 4-Chlorobenzoic Acid (C) due to electron-withdrawing effects.

Most Acidic: 4-Nitrobenzoic Acid (B) due to strong electron-withdrawing effects.

Final Order of Increasing Acidity: A (4-Methoxybenzoic Acid) < D (4-Methylbenzoic Acid) < E (Benzoic Acid) < C (4-Chlorobenzoic Acid) < B (4-Nitrobenzoic Acid)

This matches option 3: A, D, E, C, B