1.8Unit 1. Structure, Bonding and Introduction to Organic Chemistry

Alkanes — IAL Edexcel Modular (Chemistry) Revision Notes & Topical Questions

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1.8 Alkanes — IAL EDEXCEL MODULAR (CHEMISTRY) notes: Describe the structure, reactivity and combustion of alkanes, their production and use as fuels, the pollutants they cause, alternative fuels, and the free-radical substitution mechanism.Alkanes — saturated hydrocarbons with ge…

Cycloalkanes
Boiling points
Strong bonds
Low polarity
Thermal cracking
Catalytic cracking

Section 1 of 12Structure and properties of alkanes

Structure and properties of alkanes

Alkanes

Alkanes saturated hydrocarbons with general formula C_nH_2n+2
Hydrocarbons contain hydrogen and carbon only.
Alkanes contain only single bonds and no functional group, and they are aliphatic.

Shape

Shape tetrahedral around each carbon with bond angles of 109.5°
Alkanes can be linear, branched or cyclic.

Cycloalkanes

Cycloalkanes general formula C_nH_2n, named cyclo + alkane
They are saturated even though they share the alkene formula.

Boiling points

Boiling points rise with chain length, and branching lowers them
Branched molecules have less surface contact, so London forces are weaker.

Section 2 of 12Why alkanes are unreactive

Strong bonds

Strong bonds the C–C bond enthalpy is 316 kJ mol^-1 and the C–H bond enthalpy is 411 kJ mol^-1
C–H is stronger because the hydrogen atom has only one shell, so the bond is shorter.

Low polarity

Low polarity carbon and hydrogen have very similar electronegativities, so the σ bonds are almost non-polar
There are no δ+ or δ− sites for nucleophiles or electrophiles to attack.

Section 3 of 12Alkanes from crude oil

Crude oil

Crude oil a mixture of alkanes, cycloalkanes and arenes
It is separated at a refinery by fractional distillation into fractions based on boiling point.

Cracking

Cracking heavy fractions are broken into shorter, more useful alkanes and alkenes
Cracking gives a shorter alkane plus at least one alkene, for example decane gives octane and ethene.
C₁₂H₂₆ → C₈H₁₈ + 2C₂H₄.

Thermal cracking

Thermal cracking very high temperature and high pressure (up to 70 atm), giving a high proportion of alkenes
- (up to about 1000 °C)

Catalytic cracking

Catalytic cracking about 600–700 °C and slight pressure with a zeolite or aluminium oxide catalyst
It gives mainly branched alkanes and aromatic hydrocarbons for petrol.

Section 4 of 12Reforming

Knocking

Knocking straight-chain alkanes ignite too early in the cylinder, which lowers engine efficiency

Reforming

Reforming straight-chain alkanes are converted over a platinum catalyst into branched and cyclic hydrocarbons that burn more smoothly
Octane forms 2,5-dimethylhexane.
C₆H₁₄ → C₆H₁₂ (cyclohexane) + H₂.

Section 5 of 12Combustion of alkanes

Complete combustion

Complete combustion excess oxygen oxidises all the carbon and hydrogen to CO₂ and H₂O
CH₄ + 2O₂ → CO₂ + 2H₂O.

Incomplete combustion

Incomplete combustion limited oxygen forms carbon monoxide, and soot (carbon) forms when oxygen is lower still
2CH₄ + 3O₂ → 2CO + 4H₂O.
It often occurs inside car engines.

Carbon dioxide

Carbon dioxide a greenhouse gas that contributes to climate change

Section 6 of 12Pollutants from burning fossil fuels

Pollutant	Source	Effect
Carbon monoxide	Incomplete combustion	Toxic and odourless; binds to haemoglobin so it cannot carry oxygen
Particulates (soot)	Incomplete combustion	Respiratory problems and pollution
Sulfur dioxide and trioxide	Combustion of sulfur impurities	Form H₂SO₃ and H₂SO₄, causing acid rain
Oxides of nitrogen	N₂ and O₂ reacting at engine temperatures: N₂ + O₂ → 2NO	Form nitric acid in rain; react with unburnt hydrocarbons to make photochemical smog (PAN)
Unburnt hydrocarbons (VOCs)	Incomplete burning of fuel	Form photochemical smog with nitrogen oxides

Acid rain

Acid rain damages aquatic life, crops and forests, corrodes metal structures and attacks carbonate rocks
SO₂ + H₂O → H₂SO₃ and SO₃ + H₂O → H₂SO₄.
4NO₂ + 2H₂O + O₂ → 4HNO₃.

Catalytic converters

Catalytic converters a honeycomb coated with platinum, rhodium and palladium gives a large surface area
2CO + 2NO → 2CO₂ + N₂.
Unburnt hydrocarbons are oxidised to CO₂ and H₂O.

Section 7 of 12Greenhouse effect and alternative fuels

1Greenhouse effect

Greenhouse effect the Earth's surface re-emits absorbed sunlight as infrared, and CO₂, CH₄ and water vapour absorb and trap it
Extra CO₂ from burning fossil fuels causes the enhanced greenhouse effect and global warming.

2Reasons for alternative fuels

Reasons for alternative fuels fossil fuels are non-renewable and their combustion causes pollution and climate change

3Biofuels

Biofuels renewable fuels from living matter
Biodiesel is made by refining fats and oils.
Bioethanol is made by fermentation of sugars.
Biogas is released when organic waste breaks down.

4Carbon neutral

Carbon neutral no net change in atmospheric CO₂

Plants absorb CO₂ as they grow and release it when burned.
Biofuels are not fully carbon neutral because of energy used for growing, harvesting, processing and transport.

5Other considerations for biofuels

Other considerations for biofuels land use competes with food crops, and yield per unit area and conversion costs matter

6Hydrogen

Hydrogen burns to form only water: 2H₂ + O₂ → 2H₂O
Its low density and explosive nature make storage difficult.
Production is costly and often uses fossil fuels.

Section 8 of 12Free radicals and bond fission

Homolytic fission

Homolytic fission each atom takes one electron from the bond, forming two free radicals
A free radical has an unpaired electron and is shown with a single-headed (fish-hook) arrow.

Heterolytic fission

Heterolytic fission the more electronegative atom takes both electrons, forming a negative and a positive ion
The negative ion is a nucleophile and the positive ion is an electrophile.

Section 9 of 12Free-radical substitution mechanism

Overall
- alkane + halogen → halogenoalkane + hydrogen halide, needing ultraviolet light

Initiation: UV light breaks the halogen bond homolytically, Cl₂ → 2Cl•.

Propagation 1: Cl• + CH₄ → HCl + •CH₃.

Propagation 2: •CH₃ + Cl₂ → CH₃Cl + Cl•, regenerating the radical so the chain continues.

Termination: two radicals combine, for example Cl• + •CH₃ → CH₃Cl or 2•CH₃ → C₂H₆.

Free-radical substitution mechanism

Evidence for UV

Evidence for UV an alkane and bromine stay orange in the dark but decolourise in sunlight

Limitations

Limitations the reaction gives a mixture of products
Excess halogen causes further substitution to CH₂Cl₂, CHCl₃ and CCl₄.
Termination forming ethane is evidence for •CH₃ radicals and gives an impurity.
Substitution at different carbons gives isomers, so it is poor for making one pure product.
Excess alkane favours monosubstitution.

Substitution at a middle carbon

Substitution at a middle carbon with propane, Br• removes H from C2 to give CH₃C•HCH₃
This reacts with Br₂ to give 2-bromopropane and a new Br•.
The radical dot is placed on the carbon that lost the hydrogen.

Section 10 of 12More from the specification

Tetrahedral bond angles of carbon atoms
- sp3 hybridized carbon atoms form four symmetrical single sigma bonds separated by 109.5 degree angles.

Section 11 of 12Key terms

Term	Meaning
Alkane	A saturated hydrocarbon with only single bonds, general formula C_nH₂n+2.
Cracking	Breaking long hydrocarbons into shorter alkanes and alkenes.
Reforming	Converting straight-chain alkanes into branched and cyclic hydrocarbons using a platinum catalyst.
Free radical	A species with an unpaired electron.
Homolytic fission	Bond breaking in which each atom takes one electron.
Initiation	The step that forms radicals from a molecule using UV light.
Propagation	Chain steps in which a radical reacts and a new radical is formed.
Termination	A step in which two radicals combine and the chain stops.
Carbon neutral	Having no net effect on the amount of carbon dioxide in the atmosphere.

Section 12 of 12Exam technique

Using full arrows for single-electron movement — homolytic fission uses single-headed arrows

Putting a radical dot on the wrong carbon in propagation steps — it goes on the carbon that lost the hydrogen

Saying free-radical substitution is a clean route to one product — it gives mixtures

Forgetting that UV light is required — without it the alkane and halogen do not react

Calling biofuels completely carbon neutral — energy used in growing, processing and transport adds CO₂

Last updated: 15 October 2026