What will be the correct order of the acidic strength?

Acetic acid < Benzoic acid < Formic acid  

The correct answer is option 4. Acetic acid < Benzoic acid < Formic acid.

Let us explore the acidic strengths of formic acid, acetic acid, and benzoic acid in detail by examining their molecular structures, the stability of their conjugate bases, and the effects of different substituents.

Acidity is a measure of how easily a compound donates a proton \((H^+)\). The strength of an acid is determined by the stability of its conjugate base (the species that remains after the proton is donated). The more stable the conjugate base, the stronger the acid.

Factors Affecting Acidity

Inductive Effect: Electron-withdrawing groups stabilize the conjugate base by dispersing negative charge, increasing acidity. Electron-donating groups have the opposite effect, decreasing acidity.

Resonance Effect: Conjugate bases that can delocalize the negative charge through resonance are more stable, increasing acidity.

Formic Acid \((HCOOH)\)

The conjugate base of formic acid is formate ion \((HCOO^-)\). Formic acid has no additional groups attached to the carboxyl group, meaning no electron-donating or withdrawing groups are influencing its conjugate base. The formate ion \((HCOO^-)\) is relatively stable because there are no alkyl groups to donate electron density. This makes formic acid quite acidic.

Acetic Acid \((CH_3COOH)\):

The conjugate base of acetic acid is acetate ion \((CH_3COO^-)\). The methyl group \((CH_3)\) attached to the carboxyl group is an electron-donating group. This slightly destabilizes the conjugate base \((CH_3COO^-)\) by increasing the electron density on the oxygen atoms. The acetate ion \((CH_3COO^-)\) is less stable compared to the formate ion due to the electron-donating effect of the methyl group. This makes acetic acid less acidic than formic acid.

Benzoic Acid \((C_6H_5COOH)\)

The conjugate base of benzoic acid Benzoate ion \((C_6H_5COO^-)\). The phenyl group \((C_6H_5)\) has a resonance effect where the conjugate base (benzoate ion) can delocalize the negative charge over the aromatic ring. This resonance stabilization makes the conjugate base more stable. The benzoate ion \((C_6H_5COO^-)\) is more stable than the acetate ion due to resonance stabilization but typically less stable than the formate ion. This makes benzoic acid more acidic than acetic acid but generally less acidic than formic acid.

Comparative Acidity

1. Formic Acid vs. Acetic Acid:

Formic acid is more acidic because it lacks electron-donating groups, making its conjugate base \((HCOO^-)\) more stable than the acetate ion \((CH_3COO^-)\).

2. Formic Acid vs. Benzoic Acid:

Formic acid is generally more acidic because the lack of electron-donating groups provides a highly stable conjugate base \((HCOO^-)\), while benzoic acid’s conjugate base \((C_6H_5COO^-)\) is stabilized by resonance but not as effectively as the formate ion.

3. Acetic Acid vs. Benzoic Acid:

Benzoic acid is more acidic than acetic acid due to the resonance stabilization of its conjugate base \((C_6H_5COO^-)\) compared to the inductive destabilization of the acetate ion  \((CH_3COO^-)\) by the methyl group.

Based on the effects of inductive and resonance stabilization, the correct order of acidic strength from weakest to strongest is:
Acetic acid < Benzoic acid < Formic acid.