Lesson 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)

I can prepare carboxylic acids by oxidation and hydrolysis.Syllabus
Syllabus 9476, 11.8(a). Prepare carboxylic acids by oxidation and hydrolysis.
I can explain carboxylic-acid and chloroethanoic-acid acidity.Syllabus
Syllabus 9476, 11.8(c). Explain carboxylic-acid and chloroethanoic-acid acidity.

Carboxylic acids are stabilised by their delocalised conjugate bases

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

Make a guess. You are not marked.

Which is the stronger acid, ethanoic acid or chloroethanoic acid?

Ethanoic acid, as it has no heavy Cl atom
Ethanoic acid, as Cl donates electrons to COO^-
Chloroethanoic acid, as Cl stabilises the anion

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Chloroethanoic acid, as Cl stabilises the anion

The electronegative chlorine draws electron density away from the carboxylate group, spreading its negative charge and stabilising the anion.

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 release H⁺ from their O-H group. In the carboxylate ion, the negative charge is spread over two oxygen atoms, so the two C-O bonds become the same. This makes the anion more stable than the anion of an alcohol, and usually more stable than phenoxide. So carboxylic acids are clearly stronger acids.

Chlorine atoms pull electron density away through the σ bonds and spread out the negative charge of the carboxylate ion. This makes the anion more stable, so chloroethanoic acid is stronger than ethanoic acid. More chlorine atoms pull more strongly and make the acid stronger still. The effect weakens with distance, so a chlorine closer to the COOH group raises the acid strength more.

Check your understanding

Why is CCl₃COOH a stronger acid than CH₃COOH?

The three Cl atoms donate lone pairs into the chain and push out the H⁺.
CCl₃COOH has a larger Mr, so it releases more H⁺ in solution.
The three Cl atoms withdraw electrons and stabilise the CCl₃COO^- anion.

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The three Cl atoms withdraw electrons and stabilise the CCl₃COO^- anion.

Electronegative chlorines pull electron density away from the carboxylate, spreading its negative charge. A more stable conjugate base makes dissociation more favourable.