What is the correct order of increasing acid strength?

The correct answer is Option (3) → Ethanol < Phenol < Acetic acid < Chloroacetic acid

An acid is stronger when its conjugate base (the anion left after losing H⁺) is more stable. The smaller the pK_a, the stronger the acid.

• Ethanol, \( \ce{C2H5OH} \) — first (weakest). In the ethoxide ion the negative charge is localised on one oxygen; pK_a is about 16.

• Phenol, \( \ce{C6H5OH} \) — second. In the phenoxide ion the negative charge is delocalised over the ring, which makes the ion more stable; pK_a is 10.

• Acetic acid, \( \ce{CH3COOH} \) — third. The acetate ion has two equivalent resonance structures, and its negative charge is delocalised over two electronegative oxygen atoms, so it is more stable than the phenoxide ion; pK_a is 4.76.

• Chloroacetic acid, \( \ce{ClCH2COOH} \) — fourth (strongest). Chlorine is an electron withdrawing group; it pulls electron density away from the carboxylate ion and stabilises it further, so it is a stronger acid than acetic acid.

NCERT (2026-27 edition):
7.4.4 Chemical Reactions – “… In alkoxide ion, the negative charge is localised on oxygen while in phenoxide ion, the charge is delocalised. The delocalisation of negative charge (structures I-V) makes phenoxide ion more stable and favours the ionisation of phenol.”

8.9.1 Reactions Involving Cleavage of O–H Bond – “… The pK_a of hydrochloric acid is –7.0, where as pK_a of trifluoroacetic acid (the strongest carboxylic acid), benzoic acid and acetic acid are 0.23, 4.19 and 4.76, respectively. Smaller the pK_a, the stronger the acid (the better it is as a proton donor). … 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 conjugate base of carboxylic acid, a carboxylate ion, is stabilised by two equivalent resonance structures in which the negative charge is at the more electronegative oxygen atom. … Thus, the carboxylate ion is more stabilised than phenoxide ion, so carboxylic acids are more acidic than phenols.”

Effect of substituents on the acidity of carboxylic acids – “… Electron withdrawing groups increase the acidity of carboxylic acids by stabilising the conjugate base through delocalisation of the negative charge by inductive and/or resonance effects.”