Pearson Edexcel International GCSE Chemistry 4CH1: Organic chemistry – Study Guide

This guide teaches Topic 4, Organic chemistry, of the Pearson Edexcel International GCSE Chemistry (4CH1) specification, Issue 3 (September 2024). It covers points 4.1–4.50 in sub-topics (a) to (h). The qualification is untiered. Points with a “C” reference (bold in the specification) are assessed on Paper 2 only, and this guide labels them Paper 2 only. That includes all of alcohols, carboxylic acids and esters, and condensation polymers.

Useful links: the 4CH1 Chemistry course hub, the printable 4CH1 checklist, the condensed Organic chemistry revision notes and the Organic chemistry practice questions. Check your gaps with the free 4CH1 all-topics diagnostic.

What this topic covers

Spec points	Sub-topic	Paper
4.1–4.6	(a) Introduction: formulae, naming, isomers, reaction types	1 and 2
4.7–4.18	(b) Crude oil: fractions, combustion, pollution, cracking	1 and 2
4.19–4.22	(c) Alkanes	1 and 2
4.23–4.28	(d) Alkenes	1 and 2
4.29C–4.33C	(e) Alcohols	Paper 2 only
4.34C–4.37C	(f) Carboxylic acids	Paper 2 only
4.38C–4.43C	(g) Esters	Paper 2 only
4.44–4.47	(h) Addition polymers	1 and 2
4.48C–4.50C	(h) Condensation polymers	Paper 2 only

(a) Introduction

A hydrocarbon is a compound of hydrogen and carbon only (4.1).

Five kinds of formula (4.2)

Formula	Meaning	Butane
Molecular	Actual number of each atom	C₄H₁₀
Empirical	Simplest whole-number ratio	C₂H₅
General	Formula for the whole series	CₙH₂ₙ₊₂
Structural	Arrangement shown without every bond	CH₃CH₂CH₂CH₃
Displayed	Every atom and every bond drawn	see below

    H   H   H   H
    |   |   |   |
H – C – C – C – C – H
    |   |   |   |
    H   H   H   H

Key terms (4.3)

• Homologous series: a family with the same general formula and functional group, similar chemical properties, and a trend in physical properties. Each member differs from the next by CH₂.
• Functional group: the atom or group of atoms that gives the series its reactions, such as C=C or –OH.
• Isomerism: compounds with the same molecular formula but different structural formulae.

Naming (4.4)

You name compounds with up to six carbon atoms. The stem gives the longest carbon chain: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6). The ending gives the series: -ane, -ene, -anol, -anoic acid. A number shows the position of a double bond or –OH group, counted from the end nearer to it: but-1-ene, propan-1-ol. A branch is named as a side group with its position: 2-methylpropane.

Worked example: isomers of C₄H₁₀ (4.5). Put all four carbons in a line: butane, CH₃CH₂CH₂CH₃. Put three in a line with one branch on the middle carbon: 2-methylpropane, CH₃CH(CH₃)CH₃. Moving the branch to an end carbon just makes a four-carbon chain again, so there are only two isomers. C₅H₁₂ has three: pentane, 2-methylbutane and 2,2-dimethylpropane.

Types of reaction (4.6)

• Substitution: one atom is swapped for another (alkane + halogen).
• Addition: two molecules join to make one (alkene + bromine, alkene + steam, addition polymerisation).
• Combustion: burning in oxygen.

Mechanisms are not required.

(b) Crude oil

Crude oil is a mixture of hydrocarbons (4.7). Fractional distillation (4.8) separates it: the oil is vaporised, and the fractions condense at different heights of a column that is hot at the bottom and cool at the top, according to boiling point.

Fractions and uses (4.9, 4.10)

Fraction (top to bottom of column)	Use
Refinery gases	Bottled gas for heating and cooking
Gasoline	Fuel for cars (petrol)
Kerosene	Fuel for aircraft
Diesel	Fuel for some cars, lorries and trains
Fuel oil	Fuel for ships and power stations
Bitumen	Road surfaces and roofing

Going down the list the molecules get larger, so the colour gets darker, the boiling point gets higher and the viscosity increases (thicker, flows less easily).

Fuels and combustion (4.11, 4.12)

A fuel releases heat energy when burned. Complete combustion in plenty of oxygen gives carbon dioxide and water. Incomplete combustion in limited oxygen gives carbon monoxide and/or carbon (soot), plus water.

Complete:    C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Incomplete:  2C₃H₈ + 7O₂ → 6CO + 8H₂O
             C₃H₈ + 2O₂ → 3C + 4H₂O

Pollutants (4.13–4.16)

• Carbon monoxide is poisonous: it reduces the capacity of the blood to transport oxygen. (References to haemoglobin are not required.) It is colourless and odourless, so it is hard to detect.
• Oxides of nitrogen: the temperature in a car engine is high enough for nitrogen and oxygen from the air to react: N₂ + O₂ → 2NO, then 2NO + O₂ → 2NO₂.
• Sulfur dioxide: some fuels contain sulfur impurities, which burn: S + O₂ → SO₂.
• Acid rain: sulfur dioxide and oxides of nitrogen dissolve in rainwater and react to form acids, so the rain becomes acidic.

Cracking (4.17, 4.18)

Catalytic cracking breaks long-chain alkanes into shorter-chain alkanes and alkenes. Conditions: a silica or alumina catalyst and a temperature of 600–700 °C.

C₁₂H₂₆ → C₈H₁₈ + 2C₂H₄

Check the equation by counting: 12 carbons and 26 hydrogens on each side.

Why crack? Fractional distillation produces more of the long-chain fractions (such as fuel oil) than is needed, and less of the short-chain fractions (such as gasoline) than is needed. Cracking converts the surplus long chains into the shorter chains that are in demand.

(c) Alkanes

• General formula CₙH₂ₙ₊₂ (4.19).
• Saturated: only single C–C bonds, so no more hydrogen can be added (4.20).
• Up to five carbons (4.21): methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀, pentane C₅H₁₂. Draw each as a straight chain with every carbon forming four bonds.

Reaction with halogens (4.22). In ultraviolet light, an alkane reacts with a halogen by substitution. One hydrogen is replaced by a halogen atom (mono-substitution), and a hydrogen halide forms.

CH₄ + Cl₂ → CH₃Cl + HCl        (chloromethane)
C₂H₆ + Br₂ → C₂H₅Br + HBr      (bromoethane)

(d) Alkenes

• Functional group C=C (4.23); general formula CₙH₂ₙ (4.24).
• Unsaturated: they contain a C=C double bond, so other atoms can be added across it (4.25).
• Up to four carbons (4.26): ethene CH₂=CH₂, propene CH₂=CHCH₃, but-1-ene CH₂=CHCH₂CH₃, but-2-ene CH₃CH=CHCH₃. Cis/trans (E/Z) is not required.

Reaction with bromine (4.27) is addition; the product is a dibromoalkane:

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br      (1,2-dibromoethane)
CH₂=CHCH₃ + Br₂ → CH₂BrCHBrCH₃   (1,2-dibromopropane)

This is the basis of the bromine water test (4.28): an alkene decolourises orange bromine water; an alkane does not.

(e) Alcohols – Paper 2 only

Functional group –OH (4.29C). Names and structural formulae (4.30C): methanol CH₃OH, ethanol CH₃CH₂OH, propan-1-ol CH₃CH₂CH₂OH, butan-1-ol CH₃CH₂CH₂CH₂OH. “Propanol” and “butanol” are acceptable.

Oxidation of ethanol (4.31C)

Method	Product
Burning in air or oxygen	CO₂ + H₂O: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Oxygen in the air with microbes (microbial oxidation)	Ethanoic acid: C₂H₅OH + O₂ → CH₃COOH + H₂O
Heating with potassium dichromate(VI) in dilute sulfuric acid	Ethanoic acid (the dichromate turns from orange to green)

Making ethanol (4.32C, 4.33C)

• Hydration of ethene: C₂H₄ + H₂O → C₂H₅OH. Steam, phosphoric acid catalyst, about 300 °C, about 60–70 atm.
• Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Enzymes in yeast, about 30 °C, absence of air.

Reasons for the fermentation conditions: without air, the ethanol is not oxidised to ethanoic acid. At about 30 °C the enzymes work at a good rate; at low temperature the reaction is slow, and at high temperature the enzymes are denatured.

(f) Carboxylic acids – Paper 2 only

Functional group –COOH (4.34C). Up to four carbons (4.35C): methanoic acid HCOOH, ethanoic acid CH₃COOH, propanoic acid CH₃CH₂COOH, butanoic acid CH₃CH₂CH₂COOH. The carbon in –COOH counts in the chain. Displayed formulae of ethanol and ethanoic acid:

    H   H                 H   O
    |   |                 |   ‖
H – C – C – O – H     H – C – C – O – H
    |   |                 |
    H   H                 H

Aqueous solutions react like other acids (4.36C), forming salts called ethanoates, propanoates and so on:

Metal:      2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂           (fizzing; magnesium ethanoate)
Carbonate:  2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂  (fizzing; sodium ethanoate)

Vinegar is an aqueous solution containing ethanoic acid (4.37C).

(g) Esters – Paper 2 only

Functional group –COO– (4.38C). Ethanol and ethanoic acid react, with an acid catalyst, to form ethyl ethanoate and water (4.39C, 4.40C):

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

    H   O       H   H
    |   ‖       |   |
H – C – C – O – C – C – H         ethyl ethanoate
    |           |   |
    H           H   H

Naming (4.41C). The alcohol gives the first word (-yl); the acid gives the second word (-oate).

Worked example. Methanol and propanoic acid: first word methyl, second word propanoate. The acid part keeps its C=O: CH₃CH₂COOCH₃, methyl propanoate. Going backwards, propyl methanoate HCOOCH₂CH₂CH₃ comes from methanoic acid and propan-1-ol.

Esters are volatile with distinctive smells, used as food flavourings and in perfumes (4.42C).

Practical (4.43C). Warm ethanol and ethanoic acid with a few drops of concentrated sulfuric acid in a water bath. Pour the mixture into sodium carbonate solution to neutralise leftover acid, and smell the ester carefully by wafting.

(h) Synthetic polymers

An addition polymer is made by joining many small molecules called monomers (4.44). The C=C opens and the monomers link into a chain.

Monomer	Repeat unit (4.45)
Ethene CH₂=CH₂	–[CH₂–CH₂]ₙ– poly(ethene)
Propene CH₂=CHCH₃	–[CH₂–CH(CH₃)]ₙ– poly(propene)
Chloroethene CH₂=CHCl	–[CH₂–CHCl]ₙ– poly(chloroethene)
Tetrafluoroethene CF₂=CF₂	–[CF₂–CF₂]ₙ– (poly)tetrafluoroethene

Monomer from repeat unit (4.46): take the two carbons in the brackets, remove the extension bonds, and put a C=C between them. The side groups stay where they were.

Disposal problems (4.47): addition polymers are inert and do not biodegrade, so they last for a very long time in landfill. Burning them can produce toxic gases.

Condensation polymers – Paper 2 only

A dicarboxylic acid and a diol react to make a polyester and water (4.48C). Ethanedioic acid HOOC–COOH and ethanediol HO–CH₂–CH₂–OH give (4.49C):

n HOOC–COOH + n HO–CH₂CH₂–OH → –[OC–COO–CH₂CH₂–O]ₙ– + 2n H₂O

Each ester link forms by losing one H₂O. Some polyesters, called biopolyesters, are biodegradable (4.50C).

Common errors

• Writing the empirical formula when asked for the molecular formula.
• Drawing the “isomer” of butane as a bent chain: it is the same molecule.
• Stating “hydrocarbon” for a compound that contains oxygen.
• Saying the nitrogen in oxides of nitrogen comes from the fuel.
• Leaving the ⇌ or the catalyst out of esterification.
• Naming the ester the wrong way round (ethanoyl ethyl).
• Drawing a polymer repeat unit that still has C=C, or no extension bonds.

Next steps

Try the Organic chemistry practice questions, then use the revision notes. The previous topic is Physical chemistry.

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.