What are carboxylic acids and how are they classified in organic chemistry?

Carboxylic acids are organic compounds characterized by the presence of a carboxyl group (-COOH). They are classified as a type of organic acid and are known for their acidic properties due to the ability of the carboxyl group to donate a proton (H+). In the Cambridge International A Levels Chemistry curriculum, carboxylic acids are studied as part of the AS-Level Organic Chemistry topic on carboxylic acids and derivatives.

How do carboxylic acids differ from other organic acids?

Carboxylic acids differ from other organic acids primarily due to their functional group, the carboxyl group (-COOH), which is a combination of a carbonyl group (C=O) and a hydroxyl group (O-H). This unique structure allows carboxylic acids to form hydrogen bonds, making them more soluble in water and giving them higher boiling points compared to other organic acids. This distinction is crucial in the study of AS-Level Organic Chemistry under the Cambridge International A Levels syllabus.

What are some common reactions involving carboxylic acids?

Carboxylic acids undergo several important reactions, including esterification, where they react with alcohols to form esters, and reduction, where they are converted to primary alcohols. They can also participate in decarboxylation, losing a carbon dioxide molecule. These reactions are essential components of the AS-Level Organic Chemistry curriculum in the Cambridge International A Levels, providing foundational knowledge for understanding carboxylic acids and their derivatives.

Why are carboxylic acids considered weak acids?

Carboxylic acids are considered weak acids because they only partially dissociate in water to release hydrogen ions (H+). The carboxyl group's ability to donate a proton is limited, resulting in a relatively low concentration of hydrogen ions in solution. This partial dissociation is a key concept in understanding the behavior of carboxylic acids in the AS-Level Organic Chemistry section of the Cambridge International A Levels syllabus.

How are carboxylic acids named according to IUPAC nomenclature?

In IUPAC nomenclature, carboxylic acids are named by identifying the longest carbon chain containing the carboxyl group and replacing the '-e' ending of the corresponding alkane name with '-oic acid'. For example, a three-carbon carboxylic acid is named propanoic acid. Understanding this naming convention is crucial for students studying AS-Level Organic Chemistry in the Cambridge International A Levels, as it aids in the identification and classification of carboxylic acids and their derivatives.

Why is "Saying NaBH₄ reduces carboxylic acids." a common mistake in Carboxylic acids?

Why it happens: Confusing with carbonyl reduction. How to avoid it: Use LiAlH₄ for carboxylic acids; NaBH₄ only reduces aldehydes/ketones. Source: 9701 Examiner Reports 2022-2024

Why is "Claiming phenols react with carbonates to give CO₂." a common mistake in Carboxylic acids?

Why it happens: Treating phenol like a carboxylic acid. How to avoid it: Phenols are too weak; only carboxylic acids release CO₂.

Why is "Treating carboxylic acids as strong acids." a common mistake in Carboxylic acids?

Why it happens: Seeing the word 'acid'. How to avoid it: They are WEAK acids (partially dissociated), e.g. ethanoic acid.

Why is "Saying electron-donating groups strengthen a carboxylic acid." a common mistake in Carboxylic acids?

Why it happens: Reversing the inductive argument. How to avoid it: Electron-WITHDRAWING groups stabilise the anion (stronger acid); donating groups weaken it.

Carboxylic acids and derivatives(AS-Level Organic Chemistry)Cambridge International A Level Chemistry 9701

Carboxylic acids

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Carboxylic Acids — Cambridge International AS & A Level Chemistry 9701 (2025-2027 syllabus)

How carboxylic acids are made (oxidation, hydrolysis of nitriles and esters), their acidic reactions with metals, alkalis and carbonates, esterification, and reduction to primary alcohols.

What you’ll learn

Mapped to the Cambridge International A Level 9701 syllabus (2025-2027).

18.1.1 — Recall how carboxylic acids are produced: (a) oxidation of primary alcohols and aldehydes (K₂Cr₂O₇/KMnO₄, reflux); (b) hydrolysis of nitriles (then acidification); (c) hydrolysis of esters (then acidification).
18.1.2 — Describe: (a) the redox reaction with reactive metals → salt + H₂; (b) neutralisation with alkalis → salt + H₂O; (c) reaction with carbonates → salt + H₂O + CO₂; (d) esterification with alcohols (conc H₂SO₄); (e) reduction by LiAlH₄ to a primary alcohol.

Making carboxylic acids

Oxidise primary alcohols/aldehydes, or hydrolyse nitriles or esters.

Three routes:

Oxidation of a primary alcohol or aldehyde — reflux with acidified K₂Cr₂O₇ (or KMnO₄): $\text{CH}_3\text{CH}_2\text{OH} + 2[\text{O}] \rightarrow \text{CH}_3\text{COOH} + \text{H}_2\text{O}$ (Reflux ensures full oxidation, not just to the aldehyde.)
Hydrolysis of a nitrile — reflux with dilute acid (or dilute alkali then acidify): $\text{CH}_3\text{CN} + 2\text{H}_2\text{O} + \text{HCl} \rightarrow \text{CH}_3\text{COOH} + \text{NH}_4\text{Cl}$
Hydrolysis of an ester — dilute acid or alkali, heat (then acidify) → carboxylic acid (+ alcohol).

Oxidise 1° alcohol/aldehyde (reflux, K₂Cr₂O₇/H₂SO₄).
Hydrolyse a nitrile (dilute acid, reflux).
Hydrolyse an ester.

Acidic reactions: metals, alkalis and carbonates

As weak acids they react with metals (H₂), alkalis (water) and carbonates (CO₂).

Carboxylic acids are weak acids (only partially dissociate) but undergo all the typical acid reactions:

With reactive metals → carboxylate salt + hydrogen: $2\text{CH}_3\text{COOH} + \text{Mg} \rightarrow (\text{CH}_3\text{COO})_2\text{Mg} + \text{H}_2$
With alkalis (neutralisation) → salt + water: $\text{CH}_3\text{COOH} + \text{NaOH} \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O}$
With carbonates/hydrogencarbonates → salt + water + carbon dioxide (effervescence): $2\text{CH}_3\text{COOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2$

Key test. The reaction with carbonates to give CO₂ (turns limewater milky) is used to distinguish a carboxylic acid from a phenol or alcohol — those are too weakly acidic to release CO₂ from a carbonate.

Metal → carboxylate salt + H₂.
Alkali → salt + H₂O (neutralisation).
Carbonate → salt + H₂O + CO₂ (effervescence — distinguishes from phenol/alcohol).

Esterification and reduction

Make esters with alcohols (conc H₂SO₄); reduce to a primary alcohol with LiAlH₄.

Esterification. A carboxylic acid reacts with an alcohol, with concentrated H₂SO₄ as catalyst, in a reversible condensation reaction forming an ester + water: $\text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH} \rightleftharpoons \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}$ (ethyl ethanoate — a sweet, fruity smell). (See Topic 18.2.)

Reduction. A carboxylic acid is reduced by LiAlH₄ (a powerful reducing agent; in dry ether) to a primary alcohol: $\text{CH}_3\text{COOH} + 4[\text{H}] \rightarrow \text{CH}_3\text{CH}_2\text{OH} + \text{H}_2\text{O}$ (NaBH₄ is not strong enough to reduce carboxylic acids.)

Esterification: acid + alcohol ⇌ ester + water (conc H₂SO₄).
LiAlH₄ reduces acid → primary alcohol.
NaBH₄ cannot reduce carboxylic acids.

How it’s examined

Carboxylic-acid reactions (especially the carbonate/CO₂ test) and esterification are common Paper 2 marks and central to synthesis routes. Examiner reports flag thinking NaBH₄ reduces carboxylic acids, and not using the carbonate test to distinguish acids from phenols.

Sources: Cambridge International AS & A Level Chemistry 9701 syllabus (2025-2027), section 18.1; 9701 Examiner Reports 2022-2024; 9701/22 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-20.

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Step-by-step worked examples — Carboxylic acids

Step-by-step solutions to past-paper-style questions on carboxylic acids, written exactly the way a tutor would explain them at the board.

1Reactions with carbonate and alkali (3 marks)
Getting started• reactions
Question
Write equations for ethanoic acid reacting with (a) sodium carbonate and (b) sodium hydroxide.
Step-by-step solution
1. Step 1
  (a) Carbonate → salt + water + CO₂.
  $2\text{CH}_3\text{COOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2$
2. Step 2
  (b) Alkali → salt + water.
  $\text{CH}_3\text{COOH} + \text{NaOH} \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O}$
Answer
(a) 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂; (b) CH₃COOH + NaOH → CH₃COONa + H₂O.
Examiner tip
The salt is sodium ethanoate (a carboxylate) in both cases.
2Reaction with a metal (2 marks)
Getting started• reactions
Question
Write the equation for ethanoic acid reacting with magnesium.
Step-by-step solution
1. Step 1
  Acid + metal → salt + hydrogen.
  $2\text{CH}_3\text{COOH} + \text{Mg} \rightarrow (\text{CH}_3\text{COO})_2\text{Mg} + \text{H}_2$
Answer
2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂.
Examiner tip
A reactive metal gives effervescence (H₂) — like any acid, but slower because it is weak.
3Distinguishing acid from phenol (3 marks)
Building confidence• carbonate test
Question
Describe a test to distinguish ethanoic acid from phenol.
Step-by-step solution
1. Step 1
  Add aqueous sodium carbonate to each.
2. Step 2
  Ethanoic acid effervesces (CO₂) — it is acidic enough to react with carbonate.
3. Step 3
  Phenol gives no effervescence (too weak an acid).
Answer
Sodium carbonate: ethanoic acid fizzes (CO₂); phenol does not.
Examiner tip
CO₂ confirms a carboxylic acid; a phenol won't react with carbonate.
4Carboxylic acid from a nitrile (3 marks)
Building confidence• preparation
Question
Starting from bromoethane, show how to make propanoic acid (two steps).
Step-by-step solution
1. Step 1
  Step 1: KCN in ethanol, heat → propanenitrile (adds a C).
  $\text{CH}_3\text{CH}_2\text{Br} + \text{CN}^- \rightarrow \text{CH}_3\text{CH}_2\text{CN} + \text{Br}^-$
2. Step 2
  Step 2: reflux with dilute HCl (hydrolysis).
  $\text{CH}_3\text{CH}_2\text{CN} + 2\text{H}_2\text{O} + \text{HCl} \rightarrow \text{CH}_3\text{CH}_2\text{COOH} + \text{NH}_4\text{Cl}$
Answer
Bromoethane → propanenitrile (KCN/ethanol) → propanoic acid (hydrolysis).
Examiner tip
KCN adds one carbon — note propanoic acid (C3) from bromoethane (C2).
5Effect of structure on acidity (4 marks)
Stretch• acidity
Question
Explain why chloroethanoic acid (ClCH₂COOH) is a stronger acid than ethanoic acid, and predict where dichloroethanoic acid lies. (4)
Step-by-step solution
1. Step 1
  Cl is electron-withdrawing (−I effect), pulling electron density away from the –COO⁻.
2. Step 2
  This spreads/stabilises the negative charge on the carboxylate anion.
3. Step 3
  A more stable anion means the acid dissociates more readily → stronger acid.
4. Step 4
  Two chlorines (Cl₂CHCOOH) withdraw even more, so it is stronger still.
Answer
Cl's −I effect stabilises the carboxylate anion → stronger acid; dichloroethanoic is stronger again.
Examiner tip
Acid strength is about ANION stability — electron-withdrawing groups stabilise it (stronger acid).
6Reduction and formation routes (3 marks)
Stretch• reduction, preparation
Question
Give (a) the reagent and product when ethanoic acid is reduced, and (b) two ways to make ethanoic acid from named starting materials.
Step-by-step solution
1. Step 1
  (a) LiAlH₄ (in dry ether) reduces –COOH to a primary alcohol → ethanol.
  $\text{CH}_3\text{COOH} \xrightarrow{\text{LiAlH}_4} \text{CH}_3\text{CH}_2\text{OH}$
2. Step 2
  (b) Route 1: oxidise ethanol (or ethanal) with K₂Cr₂O₇/H₂SO₄ under reflux.
3. Step 3
  Route 2: hydrolyse a nitrile (e.g. CH₃CN + acid) → ethanoic acid.
Answer
(a) LiAlH₄ → ethanol; (b) oxidise ethanol/ethanal (reflux), or hydrolyse a nitrile.
Examiner tip
Use LiAlH₄ (not NaBH₄) for acids; oxidation of a 1° alcohol needs REFLUX to reach the acid.

Key Formulae — Carboxylic acids

The formulae you need to memorise for carboxylic acids on the Cambridge International A Level 9701 paper, with every variable defined in plain English and a note on when to use it.

General formula of carboxylic acids
$C_nH_{2n}O_2$
$n$
number of carbon atoms (including the COOH carbon)
When to use
Deducing a carboxylic acid's formula (the –COOH carbon is counted).

Key Definitions and Keywords — Carboxylic acids

Definitions to memorise and the exact keywords mark schemes credit for carboxylic acids answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9701 sitting.

Carboxylic acid
Examiner keyword
A compound containing the –COOH group; a weak acid that reacts with metals, alkalis and carbonates.
Weak acid (carboxylic)
Examiner keyword
Only partially dissociates in water (CH₃COOH ⇌ CH₃COO⁻ + H⁺) — most molecules stay un-ionised.
Carbonate (CO₂) test
Examiner keyword
Effervescence (CO₂) on adding a carbonate confirms a carboxylic acid and distinguishes it from a phenol or alcohol.
Effect of structure on acidity
Examiner keyword
Electron-withdrawing groups (e.g. –Cl, −I effect) stabilise the carboxylate anion → stronger acid; electron-donating alkyl groups weaken it.
Esterification
Examiner keyword
Reversible condensation of a carboxylic acid with an alcohol (conc H₂SO₄ catalyst) to give an ester + water.

Common Mistakes and Misconceptions — Carboxylic acids

The traps other students keep falling into on carboxylic acids questions — taken from recent Cambridge International A Level 9701 examiner reports and mark schemes — and how to avoid them.

✕Saying NaBH₄ reduces carboxylic acids.
9701 Examiner Reports 2022-2024
Why it happens
Confusing with carbonyl reduction.
How to avoid it
Use LiAlH₄ for carboxylic acids; NaBH₄ only reduces aldehydes/ketones.
✕Claiming phenols react with carbonates to give CO₂.
Why it happens
Treating phenol like a carboxylic acid.
How to avoid it
Phenols are too weak; only carboxylic acids release CO₂.
✕Treating carboxylic acids as strong acids.
Why it happens
Seeing the word 'acid'.
How to avoid it
They are WEAK acids (partially dissociated), e.g. ethanoic acid.
✕Saying electron-donating groups strengthen a carboxylic acid.
Why it happens
Reversing the inductive argument.
How to avoid it
Electron-WITHDRAWING groups stabilise the anion (stronger acid); donating groups weaken it.

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Carboxylic acids — frequently asked questions

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Carboxylic acids

Short Study Notes in the form of Slides

Short Study Notes — Carboxylic acids

Detailed Notes

What you’ll learn

How it’s examined

1Reactions with carbonate and alkali (3 marks)

2Reaction with a metal (2 marks)

3Distinguishing acid from phenol (3 marks)

4Carboxylic acid from a nitrile (3 marks)

5Effect of structure on acidity (4 marks)

6Reduction and formation routes (3 marks)

General formula of carboxylic acids

Carboxylic acid

Weak acid (carboxylic)

Carbonate (CO₂) test

Effect of structure on acidity

Esterification

✕Saying NaBH₄ reduces carboxylic acids.

✕Claiming phenols react with carbonates to give CO₂.

✕Treating carboxylic acids as strong acids.

✕Saying electron-donating groups strengthen a carboxylic acid.

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Carboxylic acids — frequently asked questions