Edexcel International A Level Chemistry: Carboxylic acids and their derivatives (YCH11) – Revision Notes

For full explanations and longer worked examples, use the carboxylic acids and derivatives study guide. Once the ideas make sense, use this page to drill the facts. Aldehyde oxidation and LiAlH₄ reduction also appear in the carbonyl compounds study guide for 15B.

Outcomes 15.9 to 15.16 (parts 15C and 15D of Topic 15) of the Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry specification (YCH11), Issue 1, September 2017, are covered. This is International A2 (IA2) content; Pearson assesses it in Unit 4: Rates, Equilibria and Further Organic Chemistry. Mark off what you know on the YCH11 checklist, then try the carboxylic acids practice questions. The YCH11 course hub lists the other units. Take a free diagnostic to pick your next topic.

The four families

Compound type	Functional group	Suffix	Named example
Carboxylic acid	–COOH	-oic acid	CH₃CH₂CH(CH₃)COOH, 2-methylbutanoic acid
Acyl chloride	–COCl	-oyl chloride	(CH₃)₂CHCOCl, 2-methylpropanoyl chloride
Ester	–COO–	alkyl -oate	CH₃CH₂COO(CH₂)₄CH₃, pentyl propanoate
Dicarboxylic acid	two –COOH	-dioic acid	HOOCCH₂CH₂COOH, butanedioic acid

15.9 and 15.13 Naming in steps

Acids and acyl chlorides

1. Find the longest chain that includes the C=O carbon. That carbon is C1 and takes no number.
2. Add the ending: -oic acid or -oyl chloride.
3. Number branches from C1.

Esters

1. Split the ester at the single-bonded oxygen that is joined to the C=O carbon.
2. The side holding the C=O carbon is the acid part: -oate.
3. The other side is the alkyl group from the alcohol: -yl.
4. Write the alkyl word first.

Worked reminder: HCOOCH₂CH₂CH₂CH₂CH₂CH₃. The acid part has one carbon (methanoate); the other chain has six (hexyl). Name: hexyl methanoate, from methanoic acid and hexan-1-ol.

Three kinds of formula

• Structural: enough to show the arrangement, e.g. CH₃CH₂COCl.
• Displayed: every atom and every bond drawn, including the O–H in an acid.
• Skeletal: zigzag of carbon lines; O, Cl and the H of O–H written in; H on carbon omitted.

15.10 Hydrogen bonding

• Acids hydrogen bond to each other through O–H and the C=O oxygen, often as dimers.
• Acids have higher boiling temperatures than alcohols, aldehydes, ketones or esters of similar Mr.
• Esters, aldehydes and ketones have no O–H, so they cannot hydrogen bond to each other. They still accept hydrogen bonds from water.
• Short-chain acids are miscible with water. Solubility falls as the non-polar chain gets longer.

Method for any “explain the boiling temperature” answer:

1. Name the intermolecular forces in each substance.
2. Say which is strongest and why (O–H present or not).
3. Link the strength to the energy needed to separate molecules.
4. If the Mr values differ, comment on London forces too.

15.11 Making carboxylic acids

Starting material	Reagents and conditions	Equation
Primary alcohol	excess K₂Cr₂O₇ with dilute H₂SO₄, heat under reflux	RCH₂OH + 2[O] → RCOOH + H₂O
Aldehyde	same	RCHO + [O] → RCOOH
Nitrile	dilute HCl, heat under reflux	RCN + 2H₂O + HCl → RCOOH + NH₄Cl

• The dichromate turns from orange to green.
• Distil instead of reflux and you collect the aldehyde, not the acid.
• A nitrile keeps the same number of carbons when it is hydrolysed: propanenitrile, CH₃CH₂CN, gives propanoic acid, CH₃CH₂COOH. Alkaline hydrolysis gives the salt plus NH₃; acidify afterwards to release the acid.

15.12 Reactions of carboxylic acids

Reagent	Product	Observation or note
LiAlH₄ in dry ether	primary alcohol	RCOOH + 4[H] → RCH₂OH + H₂O
NaOH(aq)	salt + water	neutralisation, no visible change
Na₂CO₃ or NaHCO₃	salt + water + CO₂	effervescence; CO₂ turns limewater milky
PCl₅ (dry)	acyl chloride + POCl₃ + HCl	steamy fumes
alcohol + conc. H₂SO₄, warm	ester + water	reversible; fruity smell

Equation pattern with a carbonate: 2RCOOH + Na₂CO₃ → 2RCOONa + H₂O + CO₂.

15.14 Acyl chlorides

Reacts with	Product	By-product
water	carboxylic acid	HCl
alcohol	ester	HCl
concentrated ammonia (2 mol)	amide, RCONH₂	NH₄Cl
primary amine R′NH₂ (2 mol)	N-substituted amide, RCONHR′	R′NH₃⁺Cl⁻

Worked reminder: CH₃CH₂COCl + 2CH₃CH₂CH₂NH₂ → CH₃CH₂CONHCH₂CH₂CH₃ + CH₃CH₂CH₂NH₃⁺Cl⁻. The organic product is N-propylpropanamide.

Why acyl chlorides are more reactive: oxygen and chlorine both withdraw electrons from the C=O carbon, so it is strongly δ+ and attracts nucleophiles. Reactions are fast at room temperature and are not reversible.

15.15 Ester hydrolysis

	Acidic	Alkaline
Reagent	dilute HCl or H₂SO₄	NaOH(aq)
Conditions	heat under reflux	heat under reflux
Products	carboxylic acid + alcohol	carboxylate salt + alcohol
Extent	reversible, equilibrium	goes to completion
To get the free acid	already present	add strong acid afterwards

Worked reminder: CH₃CH₂COO(CH₂)₄CH₃ + NaOH → CH₃CH₂COONa + CH₃(CH₂)₄OH (sodium propanoate and pentan-1-ol).

15.16 Polyesters

• Condensation polymerisation: bifunctional monomers join and a small molecule (H₂O, or HCl from a diacyl dichloride) is lost at every link.
• Polyester = diol + dicarboxylic acid, linked by –COO– groups.
• Terylene: ethane-1,2-diol + benzene-1,4-dicarboxylic acid. Repeat unit –OC–C₆H₄–COO–CH₂CH₂–O–.

Method for drawing a repeat unit:

1. Draw each monomer with its functional groups at the ends.
2. Take OH from each COOH and H from each alcohol OH.
3. Join the C=O carbon to the diol oxygen.
4. Show one of each monomer residue, with open bonds at both ends.

To go backwards, cut each ester link between C=O carbon and O, then add OH to the carbon and H to the oxygen.

Must-know distinctions

• Addition vs condensation polymer: addition polymers come from C=C monomers and lose nothing; polyesters lose a small molecule at each link.
• [O] vs [H]: 2[O] to oxidise a primary alcohol to an acid; 4[H] to reduce an acid to a primary alcohol.
• PCl₅ test: any –OH (alcohol or acid) gives steamy fumes. Use NaHCO₃ fizzing to pick out the acid.
• Acid vs acyl chloride esterification: the acid route is reversible and needs a catalyst; the acyl chloride route is fast and complete but releases HCl.
• Amide vs ammonium salt: one NH₃ becomes part of the amide; the other becomes NH₄Cl.

Quick self-test

1. Name CH₃CH₂CH(CH₃)COOH.
2. Use these boiling temperatures: ethanoic acid 118 °C, propan-1-ol 97 °C, methyl methanoate 32 °C. All three have Mr 60.0. Explain the order.
3. Give the reagents and conditions to make propanoic acid from propan-1-ol.
4. Name the product when hexanoic acid reacts with LiAlH₄ in dry ether.
5. Write an equation for methanoic acid reacting with sodium hydrogencarbonate.
6. 0.0400 mol of methanoic acid reacts with excess sodium carbonate. Calculate the volume of CO₂ at room temperature and pressure. (Molar volume 24.0 dm³ mol⁻¹)
7. Name HCOOCH₂CH(CH₃)₂ and name the acid and alcohol it comes from.
8. Name the organic product of pentanoyl chloride with methylamine.
9. State why alkaline hydrolysis of an ester goes to completion.
10. Name the monomers of a polyester with repeat unit –OCH₂CH₂OOC(CH₂)₂CO–.
11. Outline a three-step route from propan-1-ol to propyl propanoate that avoids an equilibrium in the last step.
12. Calculate the volume of 0.500 mol dm⁻³ NaOH that neutralises 2.59 g of propanoic acid. (Mr 74.0)

Answers

1. 2-methylbutanoic acid.
2. Methyl methanoate has no O–H, so only London forces and permanent dipole–dipole forces act. Propan-1-ol hydrogen bonds through its O–H. Ethanoic acid forms hydrogen-bonded dimers using O–H and C=O, so it needs the most energy to boil.
3. Excess potassium dichromate(VI) acidified with dilute sulfuric acid, heated under reflux.
4. Hexan-1-ol.
5. HCOOH + NaHCO₃ → HCOONa + H₂O + CO₂.
6. 2HCOOH + Na₂CO₃ → 2HCOONa + H₂O + CO₂. n(CO₂) = 0.0400 ÷ 2 = 0.0200 mol; volume = 0.0200 × 24.0 = 0.480 dm³.
7. 2-methylpropyl methanoate, from methanoic acid and 2-methylpropan-1-ol.
8. N-methylpentanamide, CH₃(CH₂)₃CONHCH₃.
9. The hydroxide turns the acid into a carboxylate ion. That ion cannot re-form the ester, so no back reaction takes place.
10. Ethane-1,2-diol and butanedioic acid.
11. Oxidise some propan-1-ol to propanoic acid (excess acidified dichromate, reflux). Convert the acid to propanoyl chloride with PCl₅. React the acyl chloride with more propan-1-ol at room temperature.
12. n = 2.59 ÷ 74.0 = 0.0350 mol, ratio 1 : 1; volume = 0.0350 ÷ 0.500 = 0.0700 dm³ = 70.0 cm³.

Where marks are usually lost

• Numbering the C=O carbon in a name, or counting it as a side chain.
• Writing an ester name with the acid part first, or drawing the ester the wrong way round.
• Saying “aldehydes and ketones hydrogen bond to each other”; they only accept hydrogen bonds from water.
• Missing “under reflux” or “excess oxidising agent” when the target is the acid.
• Adding a carbon when a nitrile is hydrolysed: the acid has the same number of carbons as the nitrile.
• Using aqueous LiAlH₄ or leaving out “dry ether”.
• Balancing a carbonate equation with one acid molecule instead of two.
• Writing HCl instead of NH₄Cl as the by-product with ammonia.
• Giving the acid, not the salt, as the product of alkaline hydrolysis.
• Drawing a polyester repeat unit with a full monomer and both end groups still attached.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry (XCH11/YCH11) specification, Issue 1, September 2017, Pearson Education Limited – Unit 4, Topic 15, parts 15C Carboxylic acids and 15D Carboxylic acid derivatives, outcomes 15.9 to 15.16.