Carboxylic Acids and Esters

This guide covers subtopics 18.1, Carboxylic acids, and 18.2, Esters, from Topic 18 of Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Both are AS Level content, combined into one resource because esters (18.2) is a short, two-outcome subtopic that largely restates esterification and hydrolysis from the ester’s own perspective — the same reactions covered under 18.1 from the acid’s perspective — rather than introducing substantial new content of its own.

Before studying this

This resource assumes Alcohols: Reactions and Oxidation (esterification and further oxidation both start there) and Carbonyl Compounds: Aldehydes and Ketones (one production route to carboxylic acids goes via an aldehyde). A2 Topic 33 extends this topic with acyl chlorides and aromatic carboxylic acids — neither appears here, as both are A2-only.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — AS Level, Topic 18

18.1 Carboxylic acids — recalling the reactions by which carboxylic acids are produced (oxidation of a primary alcohol or aldehyde with acidified K₂Cr₂O₇ or KMnO₄ under reflux; hydrolysis of a nitrile with dilute acid or dilute alkali followed by acidification; hydrolysis of an ester with dilute acid or dilute alkali and heat followed by acidification); describing the reaction with reactive metals (salt + H₂), with alkalis (neutralisation), with carbonates (salt + H₂O + CO₂), esterification with an alcohol using concentrated H₂SO₄ as catalyst, and reduction by LiAlH₄ to a primary alcohol.

18.2 Esters — recalling the reaction by which esters are produced (an alcohol and a carboxylic acid, condensing with concentrated H₂SO₄ as catalyst); describing the hydrolysis of esters by dilute acid, and by dilute alkali with heat.

Producing carboxylic acids

Three routes, all reusing reactions from earlier organic resources:

• Oxidation of a primary alcohol or aldehyde, using acidified K₂Cr₂O₇ or KMnO₄ under reflux (contrast with distillation, which stops oxidation at the aldehyde stage — see Carbonyl Compounds).
• Hydrolysis of a nitrile with dilute acid or dilute alkali, followed by acidification.
• Hydrolysis of an ester with dilute acid or dilute alkali and heat, followed by acidification — the reverse of esterification, covered below.

Reactions of carboxylic acids

Carboxylic acids behave as weak Brønsted-Lowry acids (see Acids and Bases), so most of their reactions are the familiar acid reactions applied to this specific functional group:

• With reactive metals (a redox reaction, producing hydrogen gas): 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂
• With alkalis (neutralisation): CH₃COOH + NaOH → CH₃COONa + H₂O
• With carbonates (effervescence of CO₂ — a useful qualitative test for a carboxylic acid): 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
• Esterification, with an alcohol and concentrated H₂SO₄ as catalyst (usually under reflux): CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
• Reduction, using LiAlH₄ (a stronger reducing agent than NaBH₄, needed here because a carboxylic acid is harder to reduce than an aldehyde or ketone) to a primary alcohol: CH₃COOH + 4[H] → CH₃CH₂OH + H₂O

Esters: the same reaction, viewed from the other side

Esterification — condensation of a carboxylic acid and an alcohol, losing water, with concentrated H₂SO₄ as catalyst — is exactly the reaction already given above; 18.2 simply asks you to recognise it as the ester’s production route rather than a new reaction to learn.

Hydrolysis of an ester, though, is worth separating into two genuinely different cases:

• Acid hydrolysis (dilute acid, heat): the reverse of esterification, an equilibrium reaction that gives back the carboxylic acid and the alcohol. CH₃COOC₂H₅ + H₂O ⇌ CH₃COOH + C₂H₅OH
• Alkaline hydrolysis (dilute alkali, heat): goes to completion (not an equilibrium), and produces the carboxylate salt, not the free acid — because the alkali immediately deprotonates the carboxylic acid as it forms, removing it from the equilibrium and pulling the reaction fully towards the products. CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH

This is the same underlying chemistry used industrially to make soap from fats and oils (an ester of glycerol with long-chain carboxylic acids, hydrolysed with hot concentrated NaOH) — worth knowing as context, though fats and soap-making aren’t themselves part of this syllabus point.

Worked example. Ethyl ethanoate, CH₃COOC₂H₅, is heated under reflux with an excess of aqueous sodium hydroxide. Name the two organic products.

Alkaline hydrolysis breaks the ester into the alcohol and the carboxylate salt (not the free acid, since NaOH is in excess): the acyl (CH₃CO–) part becomes sodium ethanoate, CH₃COONa, and the alkoxy (–OC₂H₅) part becomes ethanol, C₂H₅OH.

Common mistakes

• Writing the product of alkaline ester hydrolysis as the free carboxylic acid. With alkali present, the acid is immediately deprotonated to its carboxylate salt — only acid hydrolysis gives the free acid back.
• Treating acid and alkaline hydrolysis as the same reaction with different reagents. One is a reversible equilibrium; the other goes to completion — this difference is often exactly what a question is testing.
• Using NaBH₄ to try to reduce a carboxylic acid. NaBH₄ isn’t a strong enough reducing agent for this functional group — only LiAlH₄ works.
• Forgetting the carbonate test’s diagnostic value. Effervescence with a carbonate is a simple, specific test that a carboxylic acid (not just any weak acid or alcohol) is present.

Quick revision checklist

• Three routes to a carboxylic acid: reflux-oxidation of an alcohol/ aldehyde, nitrile hydrolysis, ester hydrolysis
• Reactions with metals, alkalis, carbonates, esterification, and LiAlH₄ reduction
• Esterification and its reverse (hydrolysis) as the same equilibrium viewed from opposite directions
• Acid hydrolysis (equilibrium, gives the free acid) vs alkaline hydrolysis (complete, gives the carboxylate salt)

Related resources

• Alcohols: Reactions and Oxidation — oxidation to a carboxylic acid, and esterification from the alcohol’s side
• Carbonyl Compounds: Aldehydes and Ketones — the aldehyde-to-acid oxidation route
• Nitrogen Compounds: Amines and Nitriles — nitrile hydrolysis, the third route to a carboxylic acid
• Acids and Bases: The Brønsted-Lowry Theory — carboxylic acids as weak Brønsted-Lowry acids
• Carboxylic Acids and Acyl Chlorides — the A Level continuation, adding acyl chlorides and benzoic acid
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.