Question

The weakest among the following acids is

• A. peroxyacetic acid
• B. acetic acid
• C. chloroacetic acid
• D. trichloroacetic acid
• E. propanoic acid

Hint:

Carboxylic acids are strong because of resonance. Take away the resonance (as in peroxy acids), and the acidity drops drastically.

Answer 1

The Correct Option is A

Concept: Acid strength depends on the stability of the conjugate base formed after losing a proton. Carboxylic acids are stabilized by resonance between two equivalent oxygen atoms.

Step 1: Compare carboxylic acids (B, C, D, E).

• Chloro-substituents (C, D): Chlorine is electron-withdrawing (-I effect). It pulls electron density away, stabilizing the negative charge on the carboxylate ion. Trichloroacetic acid is the strongest.
• Alkyl groups (B, E): Alkyl groups are electron-donating (+I effect). Propanoic acid (with a larger ethyl group) is slightly weaker than acetic acid.

Step 2: Analyze Peroxyacetic acid (A).
Peroxyacetic acid (\(CH_3-CO-O-OH\)) contains a peroxy (\(-O-O-\)) linkage.
• When it loses a proton, the resulting negative charge is on an oxygen atom that is not in direct resonance with the carbonyl group.
• In a regular carboxylic acid, the charge is delocalized over two oxygens: \([O-C=O]^-\). In a peroxy acid, the charge is localized on one oxygen: \(R-CO-O-O^-\).
• This lack of resonance stabilization makes the conjugate base very unstable compared to carboxylate ions.

Step 3: Conclusion.
Because its conjugate base is much less stable, peroxyacetic acid is a significantly weaker acid than even simple propanoic acid.