Topic 23 of 34

Carboxylic acids are stabilised by their delocalised conjugate bases

Oxidation and nitrile hydrolysis make the group; substituents tune its acidity.

A-Level 9476 (2026-2027)

Carboxylic acids are stabilised by their delocalised conjugate bases

Oxidation and nitrile hydrolysis make the group; substituents tune its acidity.

Routes to a carboxylic acid
Starting group	Reagents and conditions	Carbon and nitrogen accounting
Primary alcohol or aldehyde	Acidified KMnO₄ or K₂Cr₂O₇, heat under reflux.	RCH₂OH → RCOOH, or RCHO → RCOOH; carbon count retained.
Nitrile, acidic hydrolysis	Dilute aqueous acid, heat.	RCN + 2H₂O + H⁺ → RCOOH + NH₄⁺. The nitrile carbon becomes the acid carbon.
Nitrile, alkaline hydrolysis	Dilute aqueous alkali and heat, followed by acidification.	RCN + H₂O + OH^- → RCOO^- + NH₃; acidification then gives RCOOH.

Carboxylic acids donate H⁺ from O-H. In the carboxylate, negative charge is delocalised over two oxygen atoms, making the C-O bonds equivalent in the resonance hybrid. This stabilises the conjugate base more effectively than in an alcohol, and generally more than in phenol, so carboxylic acids are appreciably stronger acids.

Chlorine substituents withdraw electron density through σ bonds and stabilise the carboxylate anion. Thus chloroethanoic acid is stronger than ethanoic acid; adding more chlorine atoms strengthens the electron-withdrawing effect. The inductive effect weakens with distance, so a chlorine closer to COOH generally increases acidity more than one farther away on an otherwise comparable chain.

Check your understandingWhy is CCl₃COOH more acidic than CH₃COOH?Think it through, then reveal the answer

The three electronegative chlorines withdraw electron density and stabilise the negative carboxylate conjugate base. The acid dissociation equilibrium is therefore more product-favoured. Do not describe the chlorine atoms as donating their lone pairs directly into this saturated carbon chain.