Pearson Edexcel International GCSE Chemistry 4CH1: Organic chemistry – Revision Notes

Condensed for the final weeks. For full explanations and worked examples, use the Organic chemistry study guide, then test yourself with the Organic chemistry practice questions.

These notes cover Topic 4 of the Pearson Edexcel International GCSE Chemistry (4CH1) specification, Issue 3 (September 2024): points 4.1–4.50, sub-topics (a) Introduction to (h) Synthetic polymers. The qualification is untiered. Points with a C reference are Paper 2 only: alcohols, carboxylic acids, esters and condensation polymers. Course links: 4CH1 Chemistry hub, printable checklist and the free all-topics diagnostic.

(a) Introduction (4.1–4.6)

Term	Definition
Hydrocarbon	Compound of hydrogen and carbon only
Homologous series	Same general formula and functional group, similar chemical properties, trend in physical properties; members differ by CH₂
Functional group	Atom or group of atoms that determines the reactions of the series
Isomers	Same molecular formula, different structural formula
Substitution	One atom replaced by another
Addition	Two molecules join to form one

Formula types for propene: molecular C₃H₆; empirical CH₂; general CₙH₂ₙ; structural CH₂=CHCH₃; displayed shows every bond.

Naming stems: meth 1, eth 2, prop 3, but 4, pent 5, hex 6. Endings: -ane, -ene, -anol, -anoic acid, -yl …-oate (esters). Number the position of C=C or –OH from the nearer end.

Method in steps: drawing isomers from a molecular formula (4.5)

1. Draw the longest straight chain.
2. Shorten the chain by one carbon and add a methyl branch to an inner carbon.
3. Repeat until no new arrangement is possible.
4. Check: a bent chain or a branch on an end carbon is not a new isomer.

(b) Crude oil (4.7–4.18)

Fractions

Fraction	Use	Trend going down
Refinery gases	Bottled gas: heating, cooking	↓ darker colour
Gasoline	Petrol for cars	↓ higher boiling point
Kerosene	Aircraft fuel	↓ more viscous
Diesel	Some cars, lorries, trains
Fuel oil	Ships, power stations
Bitumen	Roads, roofing

Combustion and pollution

Product	Where it comes from	Problem
CO₂ + H₂O	Complete combustion (plenty of O₂)	–
CO	Incomplete combustion	Poisonous: reduces the blood’s capacity to carry oxygen
C (soot)	Incomplete combustion	–
NO, NO₂	N₂ and O₂ from air reacting at high temperature in engines	Acid rain
SO₂	Sulfur impurities in the fuel burning	Acid rain

A fuel is a substance that releases heat energy when burned.

Method in steps: balancing a complete combustion equation

1. Balance carbon: the number of CO₂ equals the number of C atoms in the fuel.
2. Balance hydrogen: the number of H₂O is half the number of H atoms.
3. Count oxygen atoms on the right and halve to get O₂. If that gives a half, double everything.

Reminder: butane gives 4CO₂ + 5H₂O, which needs 13 O atoms, so 6½O₂. Doubled: 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O.

Cracking

• Long-chain alkane → shorter-chain alkane + alkene.
• Silica or alumina catalyst, 600–700 °C.
• Why: supply of long-chain fractions is greater than demand; demand for short-chain fractions (e.g. gasoline) is greater than supply.

Method in steps: balancing a cracking equation

1. Write the alkane on the left and the products given on the right.
2. Carbons left over go into the alkene (C₂H₄ units if the product is ethene).
3. Check both C and H balance.

Reminder: C₁₆H₃₄ → C₁₀H₂₂ + C₆H₁₂. Carbons 16 = 10 + 6; hydrogens 34 = 22 + 12.

(c) and (d) Alkanes and alkenes (4.19–4.28)

	Alkanes	Alkenes
General formula	CₙH₂ₙ₊₂	CₙH₂ₙ
Bonding	Single C–C only: saturated	Contains C=C: unsaturated
Names you must know	Methane to pentane (unbranched)	Ethene, propene, but-1-ene, but-2-ene
Typical reaction	Substitution with a halogen in UV light	Addition with bromine
Example	C₃H₈ + Cl₂ → C₃H₇Cl + HCl	C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane)
Bromine water	Stays orange	Decolourised (orange → colourless)

Only mono-substitution is needed for alkanes. No mechanisms, no cis/trans.

(e) Alcohols (4.29C–4.33C) – Paper 2 only

• Functional group –OH. Methanol CH₃OH, ethanol C₂H₅OH, propan-1-ol C₃H₇OH, butan-1-ol C₄H₉OH.

Oxidising ethanol:

• Burning (complete combustion): C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.
• Microbial oxidation by oxygen in air → ethanoic acid.
• Heating with potassium dichromate(VI) in dilute sulfuric acid → ethanoic acid.

Making ethanol:

	Hydration of ethene	Fermentation of glucose
Equation	C₂H₄ + H₂O → C₂H₅OH	C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Catalyst	Phosphoric acid	Enzymes in yeast
Temperature	About 300 °C	About 30 °C (optimum)
Pressure / other	About 60–70 atm	Absence of air

Why these fermentation conditions: no air so ethanol is not oxidised to ethanoic acid; about 30 °C because colder is slow and hotter denatures the enzymes.

(f) Carboxylic acids (4.34C–4.37C) – Paper 2 only

• Functional group –COOH. Methanoic HCOOH, ethanoic CH₃COOH, propanoic C₂H₅COOH, butanoic C₃H₇COOH.
• Acid + metal → salt + hydrogen.
• Acid + metal carbonate → salt + water + carbon dioxide.
• Salts are named -oate: sodium ethanoate, magnesium ethanoate.
• Vinegar = aqueous solution containing ethanoic acid.

(g) Esters (4.38C–4.43C) – Paper 2 only

• Functional group –COO–.
• Alcohol + carboxylic acid ⇌ ester + water, with an acid catalyst.
• Ethanol + ethanoic acid → ethyl ethanoate, CH₃COOC₂H₅.
• Volatile, distinctive smells: food flavourings and perfumes.
• Practical (4.43C): warm the alcohol and acid with a few drops of concentrated sulfuric acid in a water bath; pour into sodium carbonate solution to remove leftover acid; waft to smell the ester.

Method in steps: naming an ester

1. Alcohol → first word, ending -yl (ethanol → ethyl).
2. Acid → second word, ending -oate (propanoic acid → propanoate).
3. In the formula, the acid part comes first: CH₃CH₂COO– then the alkyl group from the alcohol.

(h) Synthetic polymers (4.44–4.50)

• Addition polymer: many monomers joined; the C=C opens. Repeat unit has single bonds and extension bonds through the brackets.
• Know: poly(ethene), poly(propene), poly(chloroethene), (poly)tetrafluoroethene.
• Monomer from repeat unit: take the two backbone carbons, put C=C between them, keep the side groups.
• Disposal: inert, do not biodegrade; burning gives toxic gases.
• (Paper 2 only) Condensation polymer: dicarboxylic acid + diol → polyester + water. Ethanedioic acid + ethanediol → –[OC–COO–CH₂CH₂–O]ₙ– + 2n H₂O. Biopolyesters are biodegradable.

Must-know distinctions

• Addition vs condensation polymerisation: addition gives one product only; condensation also gives water.
• Addition vs substitution: alkenes add (one product); alkanes substitute (two products, one is HX).
• Complete vs incomplete combustion: CO₂ versus CO and/or C.
• Nitrogen oxides vs sulfur dioxide: N comes from the air; S comes from the fuel.
• Molecular vs empirical formula: C₆H₁₂ vs CH₂.
• Microbial oxidation vs fermentation: oxygen turns ethanol into ethanoic acid; fermentation needs the absence of air to make ethanol.
• Polymer vs repeat unit: the polymer is the whole chain; the repeat unit is the bracketed section with n outside.

Quick self-test

1. An alkane has Mr = 72. Give its molecular formula and name. (C = 12, H = 1)
2. Give the empirical formula of hexane, C₆H₁₄.
3. Complete: C₁₄H₃₀ → C₁₀H₂₂ + ……
4. Write a balanced equation for the complete combustion of pentane.
5. Name CH₃CH=CHCH₃.
6. Name the product of propene and bromine.
7. State the conditions for catalytic cracking.
8. (Paper 2 only) Name the ester formed from methanol and ethanoic acid, and give its structural formula.
9. (Paper 2 only) Name the alcohol and the acid used to make ethyl propanoate.
10. (Paper 2 only) Ethanoic acid reacts with calcium carbonate. Name all three products.
11. Draw the monomer of the polymer with repeat unit –[CHCl–CHCl]ₙ–.
12. (Paper 2 only) State the catalyst, temperature and pressure for making ethanol from ethene.

Answers

1. 14n + 2 = 72, so n = 5: C₅H₁₂, pentane.
2. Divide by 2: C₃H₇.
3. 2C₂H₄ (C: 14 = 10 + 4; H: 30 = 22 + 8).
4. C₅H₁₂ + 8O₂ → 5CO₂ + 6H₂O.
5. But-2-ene.
6. 1,2-dibromopropane, CH₂BrCHBrCH₃.
7. Silica or alumina catalyst; 600–700 °C.
8. Methyl ethanoate, CH₃COOCH₃.
9. Ethanol and propanoic acid.
10. Calcium ethanoate, water and carbon dioxide.
11. CHCl=CHCl (1,2-dichloroethene): the two carbons joined by a double bond, each with one H and one Cl.
12. Phosphoric acid; about 300 °C; about 60–70 atm.

Where marks are usually lost

• Giving a structural formula with a carbon that has five bonds, or a hydrogen with two.
• Counting a bent chain as a new isomer.
• Writing “bromine water goes clear”: use colourless.
• Forgetting “UV light” for alkane + halogen substitution, or leaving the hydrogen halide out of the products.
• Quoting the cracking catalyst or temperature wrongly (it is silica or alumina, 600–700 °C).
• Explaining the need for cracking without the supply-and-demand comparison.
• Mixing up the hydration and fermentation conditions (phosphoric acid and 300 °C vs yeast and 30 °C).
• Naming esters backwards, or writing the formula with the alcohol part first.
• Leaving the C=C in a repeat unit, or missing the extension bonds.
• Forgetting the water (2n H₂O) in a polyester equation.

Official syllabus

Pearson Edexcel International GCSE in Chemistry (4CH1), Specification, Issue 3, September 2024, published by Pearson Education Limited. Topic 4: Organic chemistry, points 4.1–4.50.