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.

The correct order of decreasing acid strength of acetic acid (A), formic acid (B), Benzoic acid (C) is:

B, C, A

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

To determine the correct order of decreasing acid strength among acetic acid (A), formic acid (B), and benzoic acid (C), we need to carefully analyze the structure and substituents of each acid, as well as how these factors influence their acidity.

Detailed Analysis of Acidity

1. Formic Acid (B) (\(HCOOH\)):

Formic acid is the simplest carboxylic acid with a carboxyl group (\(-COOH\)) directly attached to a hydrogen atom.

Acidity: The \( pK_a \) of formic acid is approximately 3.75, making it a relatively strong acid among the carboxylic acids listed here.

Reason: Formic acid is more acidic than acetic acid primarily because there are no electron-donating groups attached to the carboxyl group. The absence of additional alkyl groups means there are no inductive effects to destabilize the carboxylate ion.

2. Acetic Acid (A) (\(CH_3COOH\)):

Acetic acid has a carboxyl group (\(-COOH\)) attached to a methyl group (\(-CH_3\)).

Acidity: The \( pK_a \) of acetic acid is about 4.76, making it less acidic than formic acid.

Reason: The methyl group in acetic acid is an electron-donating group via inductive effects. This electron-donating effect increases the electron density around the carboxyl group, reducing the stability of the carboxylate ion formed after deprotonation. This makes acetic acid less acidic compared to formic acid.

3. Benzoic Acid (C) (\(C_6H_5COOH\)):

Benzoic acid has a carboxyl group (\(-COOH\)) attached to a benzene ring (\(C_6H_5\)).

Acidity: The \( pK_a \) of benzoic acid is about 4.2, which is less than formic acid but can be more acidic than acetic acid in some contexts.

Reason: The phenyl group of benzoic acid provides resonance stabilization to the carboxylate ion formed after deprotonation. The delocalization of the negative charge on the carboxylate ion over the benzene ring stabilizes the ion more effectively than in acetic acid. However, benzoic acid is generally less acidic than formic acid because the resonance stabilization effect is not as strong as the intrinsic acid strength of formic acid.

Comparative Summary:

B. Formic Acid : Strongest acid among the three, with the lowest \( pK_a \). This is due to the lack of electron-donating groups and the strong intrinsic acidity of the simplest carboxylic acid.

C. Benzoic Acid : More acidic than acetic acid due to the resonance stabilization provided by the phenyl group, but less acidic than formic acid.

A. Acetic Acid : Weakest acid among the three, with the highest \( pK_a \), because the methyl group donates electron density to the carboxyl group, reducing the stability of the carboxylate ion.

Thus, the correct option is:3. B, C, A