OCR A Level Chemistry A: Carboxylic acids and esters (H432) – Revision Notes

These revision notes condense section 6.1.3, Carboxylic acids and esters, of the OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017. They cover every learning outcome, 6.1.3(a) to (f), in Module 6, Organic chemistry and analysis. H432 is the full A level. The Carboxylic acids and esters study guide explains each idea in full, with longer worked examples.

Once these notes feel secure, test yourself with the Carboxylic acids and esters practice questions. Every H432 topic is linked from the OCR chemistry hub, and the printable checklist lets you tick off each outcome. For a short check across the course, use the free diagnostics.

Key definitions

Term	Meaning	Example
Carboxylic acid	contains –COOH	hexanoic acid, CH₃(CH₂)₄COOH
Carboxylate salt	contains RCOO⁻ with a metal ion	potassium hexanoate, CH₃(CH₂)₄COOK
Ester	contains –COOR’	methyl hexanoate
Acid anhydride	(RCO)₂O	ethanoic anhydride, (CH₃CO)₂O
Acyl chloride	contains –COCl	butanoyl chloride, CH₃CH₂CH₂COCl
Primary amide	contains –CONH₂	butanamide
Secondary amide	contains –CONHR’	N-propylbutanamide
Hydrolysis	splitting a compound by reaction with water	ester → acid + alcohol

6.1.3(a): Solubility in water

• The –COOH group hydrogen bonds to water: the acid’s O–H hydrogen to a water oxygen lone pair, and water’s hydrogen to lone pairs on the acid’s oxygens.
• Short-chain acids (methanoic to butanoic) are miscible with water.
• Solubility falls as the chain lengthens: the non-polar chain cannot hydrogen bond with water.

Model answer: “The –COOH group forms hydrogen bonds with water molecules: the δ+ H of O–H is attracted to a lone pair on a water oxygen. As the carbon chain gets longer, more of the molecule is non-polar and cannot hydrogen bond, so solubility decreases.”

6.1.3(b): Acid reactions in aqueous conditions

Carboxylic acids are weak acids: RCOOH ⇌ RCOO⁻ + H⁺. They still react as acids.

Reagent	Products	Example equation
Metal	salt + hydrogen	2HCOOH + Zn → (HCOO)₂Zn + H₂
Carbonate	salt + water + carbon dioxide	2CH₃COOH + K₂CO₃ → 2CH₃COOK + H₂O + CO₂
Metal oxide	salt + water	2CH₃CH₂COOH + MgO → (CH₃CH₂COO)₂Mg + H₂O
Alkali	salt + water	CH₃(CH₂)₄COOH + NaOH → CH₃(CH₂)₄COONa + H₂O

Must-know distinction: carboxylic acid vs phenol (PAG7, 6.3.1(c))

Test	Carboxylic acid	Phenol
NaOH(aq)	reacts, salt forms	reacts, salt forms
Na₂CO₃(aq)	effervescence, CO₂	no reaction

Comparing the acidity of different carboxylic acids is not required. The full set of PAG7 functional-group tests is in Chromatography and qualitative analysis.

Method in steps: balancing an acid equation

1. Write the salt. Each RCOO⁻ carries a 1− charge, so a 2+ metal needs two: (RCOO)₂Mg, (RCOO)₂Ca.
2. Count the RCOO⁻ groups in the salt; that is the number of RCOOH on the left.
3. Add H₂ (metal), H₂O (oxide or alkali) or H₂O + CO₂ (carbonate).
4. In ionic equations keep RCOOH whole, because it is a weak acid.

PAG7 test-tube method for –COOH

1. Add sodium carbonate solution to a small sample.
2. Effervescence means a gas is given off.
3. Bubble the gas through limewater; it turns milky, confirming CO₂ and a carboxylic acid.

6.1.3(c): Making esters

Naming in steps

1. Find the alcohol. Its chain gives the first word: methanol → methyl, propan-1-ol → propyl, propan-2-ol → propan-2-yl.
2. Find the acid. Change -oic acid to -oate: hexanoic acid → hexanoate.
3. Join them: methyl hexanoate.

Two routes in this outcome

	(i) Carboxylic acid + alcohol	(ii) Acid anhydride + alcohol
Conditions	heat with conc. H₂SO₄ catalyst	warm
Reversible?	yes, equilibrium	no
Other product	H₂O	RCOOH
Yield	limited by equilibrium	higher

Worked reminder (ii): (CH₃CO)₂O + CH₃OH → CH₃COOCH₃ + CH₃COOH, giving methyl ethanoate and ethanoic acid.

6.1.3(d): Hydrolysis of esters

	(i) Hot aqueous acid	(ii) Hot aqueous alkali
Reagent	dilute H₂SO₄ or HCl, reflux	NaOH(aq) or KOH(aq), reflux
Products	carboxylic acid + alcohol	carboxylate salt + alcohol
Goes to completion?	no: reversible equilibrium	yes

Worked reminder (ii): HCOOCH₂CH₂CH₃ + NaOH → HCOONa + CH₃CH₂CH₂OH, giving sodium methanoate and propan-1-ol.

Method in steps: working back from hydrolysis products

1. The salt or acid tells you the R–COO part.
2. The alcohol tells you the R’ part.
3. Write RCOOR’ and check the molecular formula adds up.

6.1.3(e): Acyl chlorides from SOCl₂

RCOOH + SOCl₂ → RCOCl + SO₂ + HCl

• Both by-products are gases, so the acyl chloride is easy to separate.
• Conditions must be dry, because acyl chlorides react with water.

Worked reminder: CH₃CH₂COOH + SOCl₂ → CH₃CH₂COCl + SO₂ + HCl, giving propanoyl chloride.

6.1.3(f): Acyl chlorides in synthesis

All of these happen readily at room temperature and are not reversible. Every one gives HCl, which ends up as an ammonium salt when ammonia or an amine is used.

Reacts with	Product type	General equation
Water	carboxylic acid	RCOCl + H₂O → RCOOH + HCl
Alcohol	ester	RCOCl + R’OH → RCOOR’ + HCl
Phenol	ester	RCOCl + C₆H₅OH → RCOOC₆H₅ + HCl
Ammonia	primary amide	RCOCl + 2NH₃ → RCONH₂ + NH₄Cl
Primary amine	secondary amide	RCOCl + 2R’NH₂ → RCONHR’ + R’NH₃⁺Cl⁻

Must-know distinctions

• Phenol is not readily esterified by carboxylic acids. Use an acyl chloride to make a phenyl ester.
• Ammonia gives a primary amide; a primary amine gives a secondary amide.
• Two moles of NH₃ or amine per mole of acyl chloride: one to react, one to take up the HCl.

Observations worth quoting

Reaction	What you see
Carboxylic acid + magnesium	effervescence; the metal dissolves
Carboxylic acid + carbonate	effervescence; the gas turns limewater milky
Carboxylic acid + copper(II) oxide, warm	black solid dissolves; blue solution forms
Carboxylic acid + alkali	no visible change (follow with an indicator)
Acyl chloride + water	vigorous reaction; misty fumes of HCl
Phenol + sodium carbonate	no effervescence

Reaction map

• Carboxylic acid → carboxylate salt: metal, carbonate, metal oxide or alkali.
• Carboxylic acid → ester: alcohol, conc. H₂SO₄, heat.
• Carboxylic acid → acyl chloride: SOCl₂.
• Acid anhydride → ester: alcohol, warm.
• Acyl chloride → carboxylic acid, ester, primary amide or secondary amide: water, alcohol or phenol, ammonia, primary amine.
• Ester → carboxylic acid + alcohol: hot dilute acid.
• Ester → carboxylate salt + alcohol: hot aqueous alkali.

All routes to an ester at a glance

Starting compound	Reagent	Conditions	By-product
RCOOH	R’OH	heat, conc. H₂SO₄	H₂O (equilibrium)
(RCO)₂O	R’OH	warm	RCOOH
RCOCl	R’OH or phenol	room temperature	HCl

Quick self-test

1. Name the ester formed from methanol and hexanoic acid.

2. Butyl ethanoate is hydrolysed in hot dilute acid. Name the two organic products.

3. Write the equation for propanoic acid reacting with calcium carbonate.

4. Excess potassium carbonate is added to 0.0450 mol of ethanoic acid. Taking one mole of gas as 24.0 dm³, find the volume of carbon dioxide released, in cm³.

5. Explain why heating an ester with dilute acid never gives a 100% yield of carboxylic acid.

6. Give the products when ethyl methanoate is heated with NaOH(aq).

7. Name both inorganic products when a carboxylic acid reacts with SOCl₂.

8. Give the products of ethanoyl chloride with excess propylamine.

9. Which reagent distinguishes hexanoic acid from phenol, and what do you see with each?

10. Give the products when ethanoic anhydride reacts with propan-2-ol.

11. Explain why ethanoic acid mixes with water but decanoic acid barely dissolves.

12. Name the organic product of propanoyl chloride with excess ammonia, and state its type of amide.

Answers

1. Methyl hexanoate.

2. Ethanoic acid and butan-1-ol.

3. 2CH₃CH₂COOH + CaCO₃ → (CH₃CH₂COO)₂Ca + H₂O + CO₂

4. 2CH₃COOH + K₂CO₃ → 2CH₃COOK + H₂O + CO₂, so n(CO₂) = 0.0450 ÷ 2 = 0.0225 mol; 0.0225 × 24.0 = 0.540 dm³ = 540 cm³.

5. Acid hydrolysis is reversible, so the mixture reaches equilibrium with some ester and water left.

6. Sodium methanoate, HCOONa, and ethanol.

7. Sulfur dioxide, SO₂, and hydrogen chloride, HCl.

8. N-propylethanamide, CH₃CONHCH₂CH₂CH₃, and propylammonium chloride, CH₃CH₂CH₂NH₃⁺Cl⁻.

9. Sodium carbonate solution: hexanoic acid fizzes (CO₂); phenol shows no reaction.

10. Propan-2-yl ethanoate, CH₃COOCH(CH₃)₂, and ethanoic acid.

11. Both –COOH groups hydrogen bond to water. In decanoic acid the long non-polar chain cannot hydrogen bond to water, so the molecule as a whole is barely soluble.

12. Propanamide, CH₃CH₂CONH₂, a primary amide.

Where marks are usually lost

• Reversing an ester name: the alcohol gives the alkyl word, the acid gives the -oate word.
• Showing water as the second product when an acid anhydride or acyl chloride makes an ester.
• Giving the carboxylic acid instead of the carboxylate salt for alkaline hydrolysis.
• Not mentioning equilibrium or reversibility when asked why acid-catalysed esterification or acid hydrolysis is incomplete.
• Writing one NH₃ or one amine in an acyl chloride equation, leaving free HCl alongside a base.
• Calling the product from a primary amine a primary amide.
• Saying phenol reacts with sodium carbonate.
• Describing hydrogen bonding vaguely (“it is polar”) without naming the δ+ H and the oxygen lone pair.
• Forgetting the 2 : 1 acid-to-carbonate ratio, and so doubling the CO₂ volume.

Official syllabus

OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017, section 6.1.3 Carboxylic acids and esters, learning outcomes (a) to (f). Published by OCR.