OxfordAQA A-Level Chemistry: Carboxylic acids and derivatives (9620) – Revision Notes

These notes summarise section 3.3.9 (Carboxylic acids and derivatives: subsections 3.3.9.1 and 3.3.9.2, with required practical 10), of OxfordAQA’s International AS and A-level Chemistry specification (9620), Version 5.3, for International AS and A-level exams from May/June 2020. It is International A2 content, so it is International A-level only. Questions on it appear in Unit 4 (Organic 2 and Physical 2), and any section can also appear in Unit 5 (Practical and synoptic). For full explanations and worked examples, read the carboxylic acids and derivatives study guide.

When you have revised, test yourself with the practice questions. Then plan what comes next with the course hub, the printable checklist and the free diagnostics.

What the specification asks for

3.3.9.1 Carboxylic acids and esters

• the structures of carboxylic acids and esters
• carboxylic acids are weak acids but liberate CO₂ from carbonates
• acids and alcohols give esters in the presence of an acid catalyst
• common uses of esters: solvents, plasticisers, perfumes and food flavourings
• oils and fats from plants and animals are esters of propane-1,2,3-triol (glycerol)
• ester hydrolysis in acid or alkaline conditions
• alkaline hydrolysis of oils and fats gives soap and glycerol
• biodiesel: what it is and how it is made

3.3.9.2 Acylation

• the structures of acid anhydrides, acyl chlorides and amides
• the nucleophilic addition–elimination reactions of water, alcohols, ammonia and primary amines with acyl chlorides and acid anhydrides
• outline the mechanism for acyl chlorides with all four nucleophiles
• why industry prefers ethanoic anhydride to ethanoyl chloride when making aspirin
• required practical 10, in which you make a pure organic solid and check its purity

Functional groups at a glance

Family	Group	Name ending	Example
Carboxylic acid	–COOH	-oic acid	CH₃CH₂CH₂CH₂COOH, pentanoic acid
Ester	–COO–	alkyl …-oate	CH₃CH₂CH₂CH₂COOCH₃, methyl pentanoate
Acyl chloride	–COCl	-oyl chloride	(CH₃)₂CHCH₂COCl, 3-methylbutanoyl chloride
Acid anhydride	–CO–O–CO–	-oic anhydride	(CH₃CO)₂O, ethanoic anhydride
Amide	–CONH₂ or –CONHR	-amide	CH₃CONH₂, ethanamide

Naming an ester: method in steps

1. Find the C=O. The carbon chain that contains it is the acid part.
2. Count its carbons, including the C=O carbon, and write the -oate name.
3. The chain on the other oxygen is the alcohol part. Write it as an alkyl group.
4. Put the alkyl word first: alkyl alkanoate.

Worked reminder. HCOOCH₂CH₂CH₂CH₂CH₃: the acid part has one carbon (methanoate) and the other chain has five (pentyl), so it is pentyl methanoate, from methanoic acid and pentan-1-ol.

Reactions of carboxylic acids and esters

Reaction	Reagents and conditions	Products
Acid + carbonate	Any carbonate, room temperature	Salt + CO₂ + H₂O (fizzing)
Esterification	Alcohol, acid catalyst (concentrated H₂SO₄), warm	Ester + water (reversible)
Acid hydrolysis of ester	Dilute acid, heat under reflux	Carboxylic acid + alcohol (reversible)
Alkaline hydrolysis of ester	NaOH(aq) or KOH(aq), heat under reflux	Carboxylate salt + alcohol (complete)
Fat or oil + alkali	NaOH(aq), heat	Soap + glycerol
Vegetable oil + methanol	Catalyst	Biodiesel (methyl esters) + glycerol

Worked reminder: why alkaline hydrolysis goes to completion. The acid forms as its carboxylate ion. The ion does not react with the alcohol, so the reverse reaction cannot happen.

Fats, soap and biodiesel: the 1 : 3 rule

A fat or oil molecule is a triester of glycerol. Every calculation follows the same ratios:

Per 1 mol of fat or oil	Soap making	Biodiesel making
Reagent used	3 mol NaOH	3 mol CH₃OH
Main product	3 mol carboxylate salt (soap)	3 mol methyl ester
Other product	1 mol glycerol	1 mol glycerol

Biodiesel is a mixture of methyl esters because natural oils contain several different long-chain acids.

Acylation: what each reagent gives

Nucleophile	Product family	By-product from acyl chloride	By-product from acid anhydride
Water	Carboxylic acid	HCl	Carboxylic acid
Alcohol	Ester	HCl	Carboxylic acid
Ammonia (2 mol)	Amide (–CONH₂)	NH₄Cl	Ammonium carboxylate
Primary amine (2 mol)	N-substituted amide (–CONHR)	Alkylammonium chloride	Alkylammonium carboxylate

Worked reminder. 3-methylbutanoyl chloride with excess methylamine:

(CH₃)₂CHCH₂COCl + 2CH₃NH₂ → (CH₃)₂CHCH₂CONHCH₃ + CH₃NH₃Cl

The second methylamine molecule removes the H⁺ that would otherwise form HCl.

Nucleophilic addition–elimination: method in steps

1. Show the δ+ on the carbonyl carbon and δ– on O.
2. Curly arrow: nucleophile’s lone pair (on O or N) → carbonyl C.
3. Curly arrow: C=O π bond → carbonyl O.
4. Draw the intermediate: O⁻ on the old carbonyl oxygen, a positive charge on the attacking O or N, Cl still attached.
5. Curly arrow: O⁻ lone pair → re-forms C=O.
6. Curly arrow: C–Cl bond → Cl; a chloride ion leaves.
7. Show loss of H⁺ from the positive O or N (taken by Cl⁻, or by a second NH₃ or amine molecule).

Aspirin and ethanoic anhydride

Ethanoic anhydride acylates the –OH of 2-hydroxybenzoic acid. Industry prefers it to ethanoyl chloride because it is cheaper, less corrosive, less readily hydrolysed (easier to store), reacts less vigorously (easier to control) and gives no hydrogen chloride gas.

Required practical 10 in brief

• Recrystallise: minimum volume of hot solvent → hot filtration (insoluble impurities) → cool → collect crystals by vacuum filtration → rinse with a small volume of cold solvent → dry.
• Test purity: measure the melting point. Pure: sharp, at the data-book value. Impure: lower and over a wider range.

Must-know distinctions

• Weak acid vs reacts with carbonates. Partial dissociation makes it weak; it still releases CO₂.
• Acid vs alkaline hydrolysis. Acid gives the free acid and is reversible. Alkali gives the salt and is complete.
• Soap vs biodiesel. Soap: fat + alkali, product is a salt. Biodiesel: oil + methanol + catalyst, product is a methyl ester.
• Acyl chloride vs acid anhydride. Same products, different by-product: HCl or a carboxylic acid.
• Amide from ammonia vs from a primary amine. –CONH₂ vs –CONHR, named N-alkyl…amide.
• Mechanism scope. You outline the mechanism for acyl chlorides; for anhydrides you need the equations.

Quick self-test

1. Name CH₃CH₂CH₂CH₂COOCH₃ and give the acid and alcohol that form it.
2. Write an equation for butanoic acid reacting with calcium carbonate.
3. Ethyl hexanoate is heated under reflux with KOH(aq). Name the two products.
4. Give the systematic name of glycerol.
5. 0.0175 mol of a fat is completely hydrolysed by NaOH(aq). Calculate the amount of NaOH used and the mass of glycerol formed. (Mr of glycerol = 92.0)
6. State what biodiesel is and the two reagents (besides the oil) used to make it.
7. Write an equation for ethanoyl chloride with excess butylamine, CH₃CH₂CH₂CH₂NH₂, and name the organic product.
8. Write an equation for ethanoic anhydride with pentan-1-ol and name the ester.
9. Name the mechanism of acyl chlorides with water. Which atoms carry charges in the intermediate?
10. A sample of a solid melts from 118 °C to 124 °C. The data-book value is 127 °C. What does this show?
11. Give two reasons, other than cost, why ethanoic anhydride is used to make aspirin.

Answers

1. Methyl pentanoate, from pentanoic acid and methanol.
2. 2CH₃CH₂CH₂COOH + CaCO₃ → (CH₃CH₂CH₂COO)₂Ca + CO₂ + H₂O
3. Potassium hexanoate and ethanol.
4. Propane-1,2,3-triol.
5. NaOH = 3 × 0.0175 = 0.0525 mol. Glycerol = 0.0175 mol, so mass = 0.0175 × 92.0 = 1.61 g.
6. Methyl esters of long-chain carboxylic acids, mixed together; made from the oil using methanol and a catalyst.
7. CH₃COCl + 2CH₃CH₂CH₂CH₂NH₂ → CH₃CONHCH₂CH₂CH₂CH₃ + CH₃CH₂CH₂CH₂NH₃Cl. Product: N-butylethanamide.
8. (CH₃CO)₂O + CH₃(CH₂)₄OH → CH₃COO(CH₂)₄CH₃ + CH₃COOH. Ester: pentyl ethanoate.
9. Nucleophilic addition–elimination. The old carbonyl O is negative (O⁻); the O from water is positive.
10. The sample is impure: it melts below the data-book value and over a wide range.
11. Any two: less corrosive; less readily hydrolysed by moisture; less vigorous, easier-to-control reaction; no hydrogen chloride gas formed.

Where marks are usually lost

• Writing the acid part of an ester name first, such as “hexanoate ethyl” or “hexyl ethanoate” for ethyl hexanoate.
• Using → instead of ⇌ for esterification or acid hydrolysis, or leaving out the acid catalyst.
• Giving the free acid, not the carboxylate salt, as the product of alkaline hydrolysis.
• Forgetting the factor of 3 when going from moles of fat to moles of soap, methyl ester or NaOH.
• Describing biodiesel as “made from methanol” without saying it is a mixture of methyl esters of long-chain acids.
• Writing HCl as a product of an acid anhydride reaction.
• Using one molecule of NH₃ or amine per acyl chloride, leaving HCl next to a base.
• Starting a curly arrow from an atom symbol or a charge instead of a lone pair or a bond.
• Answering “safer” for the aspirin question without saying what the hazard is.
• Saying an impure solid melts “at a different temperature” without “lower” and “over a range”.

Official syllabus

OxfordAQA, International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards: section 3.3.9 Carboxylic acids and derivatives (3.3.9.1 and 3.3.9.2) and required practical 10, International A2 content.