AQA A-Level Chemistry: Carboxylic acids and derivatives (7405) – Revision Notes

These notes condense section 3.3.9 Carboxylic acids and derivatives of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for exams from June 2016 onwards. They cover 3.3.9.1 Carboxylic acids and esters, 3.3.9.2 Acylation and Required practical 10. The whole section is A-level only: it is examined on A-level Paper 2 (organic chemistry), and Paper 3 can test any content and any practical skill. For full explanations and worked examples, use the study guide.

Test yourself afterwards with the practice questions. The course hub is AQA A-Level Chemistry, and the printable checklist lists every point. The free 10-minute diagnostics show which topics to revise first.

3.3.9.1 Carboxylic acids and esters

Key definitions

• Carboxylic acid: contains –COOH. Suffix -oic acid. The –COOH carbon is carbon 1.
• Ester: contains –COO– between two carbon chains. Named alkyl (from the alcohol) + -oate (from the acid).
• Weak acid: only partially dissociates in water. RCOOH ⇌ RCOO⁻ + H⁺.
• Glycerol: propane-1,2,3-triol, HOCH₂CH(OH)CH₂OH.
• Fats and oils: esters of glycerol with three long-chain carboxylic acids.
• Soap: salts of long-chain carboxylic acids.
• Biodiesel: a mixture of methyl esters of long-chain carboxylic acids.

Ester names at a glance

Formula	Name	Made from
HCOOCH₂CH₃	ethyl methanoate	methanoic acid + ethanol
CH₃COOCH₃	methyl ethanoate	ethanoic acid + methanol
CH₃CH₂COOCH₂CH₂CH₃	propyl propanoate	propanoic acid + propan-1-ol
CH₃CH₂CH₂COOCH₃	methyl butanoate	butanoic acid + methanol

Method: naming an ester

1. Find the C=O carbon.
2. The chain containing it is the acid part: count its carbons, add -oate.
3. The group on the other oxygen is the alcohol part: methyl, ethyl, propyl…
4. Write the alcohol part first, as a separate word.

Reactions table

Reaction	Reagents and conditions	Equation (example)
Acid + carbonate	Na₂CO₃ or NaHCO₃ solution	2HCOOH + Na₂CO₃ → 2HCOONa + CO₂ + H₂O
Esterification	Alcohol, acid catalyst (conc. H₂SO₄), warm	CH₃COOH + CH₃OH ⇌ CH₃COOCH₃ + H₂O
Acid hydrolysis of ester	Dilute acid, heat under reflux	CH₃COOCH₃ + H₂O ⇌ CH₃COOH + CH₃OH
Alkaline hydrolysis of ester	NaOH(aq), heat under reflux	CH₃COOCH₃ + NaOH → CH₃COONa + CH₃OH
Soap making	Fat or oil + NaOH(aq), heat	Fat + 3NaOH → 3 soap + glycerol
Biodiesel	Vegetable oil + methanol, catalyst	Oil + 3CH₃OH → 3 methyl esters + glycerol

Observation with a carbonate: effervescence; the gas (CO₂) turns limewater cloudy.

Must-know distinctions

	Acid hydrolysis	Alkaline hydrolysis
Extent	Reversible, incomplete	Goes to completion
Acid product	Carboxylic acid	Carboxylate salt
Why	Equilibrium	Carboxylate ion does not react with the alcohol

	Soap	Biodiesel
Reagent with the fat or oil	NaOH (alkali)	Methanol + catalyst
Main product	Sodium salts of long-chain acids	Methyl esters of long-chain acids
By-product	Glycerol	Glycerol
Ratio	1 fat → 3 salt	1 oil → 3 methyl ester

Uses of esters

Solvents, plasticisers, perfumes and food flavourings (the four the specification lists).

3.3.9.2 Acylation

Structures

• Acyl chloride: R–COCl. Ethanoyl chloride, CH₃COCl.
• Acid anhydride: R–CO–O–CO–R. Ethanoic anhydride, (CH₃CO)₂O.
• Amide: R–CONH₂ (ethanamide, CH₃CONH₂) or R–CONHR′ after a primary amine (N-ethylethanamide, CH₃CONHCH₂CH₃).

Product finder

Nucleophile	Product	By-product with acyl chloride	By-product with anhydride
H₂O	Carboxylic acid	HCl	Carboxylic acid
ROH	Ester	HCl	Carboxylic acid
NH₃ (2 mol)	Amide	NH₄Cl	Ammonium carboxylate
RNH₂ (2 mol)	N-substituted amide	RNH₃⁺Cl⁻	Alkylammonium carboxylate

Worked reminder: CH₃CH₂COCl + 2CH₃CH₂NH₂ → CH₃CH₂CONHCH₂CH₃ + CH₃CH₂NH₃Cl. Product: N-ethylpropanamide.

Method: nucleophilic addition–elimination (acyl chloride)

1. Lone pair on the nucleophile (O of water or alcohol; N of ammonia or amine) attacks the δ+ carbonyl C.
2. Curly arrow from the C=O bond to O. Intermediate: O⁻ on the old carbonyl O, + on the attacking O or N.
3. Curly arrow from O⁻ back to re-form C=O.
4. Curly arrow from the C–Cl bond to Cl; Cl⁻ leaves.
5. H⁺ is lost from the attacking atom (to Cl⁻, or to a second NH₃ or amine molecule).

Name it in full: nucleophilic addition–elimination.

Worked reminder with ammonia (ethanoyl chloride):

NH3 lone pair → C(δ+);  C=O → O
Intermediate: CH3–C(O⁻)(Cl)–N⁺H3
O⁻ → C=O re-forms;  C–Cl → Cl⁻ leaves
second NH3 removes H⁺ from N⁺H3 → NH4⁺
Overall: CH3COCl + 2NH3 → CH3CONH2 + NH4Cl

Why the carbonyl carbon is attacked: O and Cl are both more electronegative than C, so the carbonyl carbon carries a δ+ charge and attracts the lone pair of a nucleophile.

Observations to recall

Mixture	What you see
Ethanoyl chloride + water	Vigorous reaction; misty fumes of hydrogen chloride
Carboxylic acid + NaHCO₃(aq)	Effervescence; gas turns limewater cloudy
Ester formed in a test tube	Sweet, fruity smell

Aspirin: why ethanoic anhydride

• By-product is ethanoic acid, not corrosive, toxic hydrogen chloride gas.
• Reacts less vigorously, so it is easier to control.
• Hydrolysed less readily by water, so it is easier to store and handle.

Equation: HOC₆H₄COOH + (CH₃CO)₂O → CH₃COOC₆H₄COOH + CH₃COOH.

Links to other topics

• Carboxylic acids come from oxidising primary alcohols under reflux: see alcohols.
• The carbonate test and the broad O–H infrared absorption: see organic analysis.
• Partial dissociation and Ka for weak acids: see acids and bases.

Required practical 10 in steps

Pure organic solid (for example aspirin)

1. Filter the crude solid under reduced pressure.
2. Dissolve in the minimum volume of hot solvent.
3. Hot-filter if insoluble impurities are present.
4. Cool to crystallise; soluble impurities stay in solution.
5. Filter under reduced pressure, wash with a little cold solvent, dry.
6. Melting point: pure gives a sharp value matching the data value; impure melts lower over a range.

Pure organic liquid (for example an ester)

1. Reflux reactants with the catalyst; distil off the crude product.
2. Shake with NaHCO₃(aq) in a separating funnel to remove acid; release CO₂ pressure.
3. Run off and discard the aqueous layer.
4. Dry with an anhydrous salt; decant or filter.
5. Redistil and collect the fraction at the product’s boiling point.

Quick self-test

Use Ar values H = 1.0, C = 12.0, N = 14.0, O = 16.0, Cl = 35.5.

1. Name CH₃CH₂CH₂COOCH₂CH₃.
2. Give the formula of the ester made from methanoic acid and propan-1-ol.
3. How many moles of CO₂ form when 0.10 mol of ethanoic acid reacts with excess sodium carbonate?
4. Why is a carboxylic acid described as weak?
5. State the products of heating methyl ethanoate under reflux with sodium hydroxide solution.
6. How many moles of NaOH are needed to hydrolyse 0.0200 mol of a triglyceride completely?
7. Give the reagent used with a vegetable oil to make biodiesel.
8. Write an equation for ethanoic anhydride with ethanol.
9. Name the product of ethanoyl chloride with methylamine.
10. What is the maximum mass of ethanamide from 0.050 mol of ethanoyl chloride with excess ammonia?
11. Calculate the percentage atom economy for CH₃COCl + CH₃CH₂OH → CH₃COOCH₂CH₃ + HCl.
12. Why do you use the minimum volume of hot solvent in recrystallisation?

Answers

1. Ethyl butanoate.
2. HCOOCH₂CH₂CH₃ (propyl methanoate).
3. 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O, so 0.050 mol.
4. It only partially dissociates in water.
5. Sodium ethanoate, CH₃COONa, and methanol.
6. 0.0600 mol (three ester groups per triglyceride).
7. Methanol (with a catalyst).
8. (CH₃CO)₂O + CH₃CH₂OH → CH₃COOCH₂CH₃ + CH₃COOH.
9. N-methylethanamide, CH₃CONHCH₃.
10. 0.050 × 59.0 = 2.95 g.
11. 88.0 ÷ (78.5 + 46.0) × 100 = 70.7%.
12. So the solution is saturated when cool and as much product as possible crystallises out.

Where marks are usually lost

• Writing the ester name in the wrong order (acid part first).
• Saying alkaline hydrolysis gives a carboxylic acid; it gives the carboxylate salt.
• Leaving out “acid catalyst” or “reversible” when describing esterification.
• Using a 1 : 1 ratio between a fat and its soap or methyl ester instead of 1 : 3.
• Forgetting the second NH₃ or amine molecule, so the equation gives free HCl.
• Arrows that start at an atom instead of a lone pair or bond in the mechanism.
• Missing the + and – charges on the tetrahedral intermediate.
• Saying ethanoic anhydride is preferred because of higher atom economy. For aspirin, the ethanoyl chloride route has the higher atom economy; the anhydride wins on safety and control.
• Describing melting point as “pure if it melts” rather than sharp and matching the data value.
• Not releasing pressure from the separating funnel when NaHCO₃ produces CO₂.

Official syllabus

AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards. Published by AQA. These notes cover section 3.3.9 and Required practical 10.