11.4Organic Chemistry
Alkanes — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 11 parts Lesson 4 of 10 1 quiz
Lesson summary
11.4 Alkanes — Notes — IGCSE Chemistry notes: Describe the bonding, properties and reactions of alkanes, including combustion and substitution with chlorine.Alkanes — saturated hydrocarbons with only single covalent carbon–carbon bonds.- General formula — CnH2n+2.NameMolecular fo…
- Occurrence
- Appearance
- Physical properties
- Reactivity
- Flammability
- Solubility
Part 1 of 9Bonding and formulae
Alkanes
- Alkanes saturated hydrocarbons with only single covalent carbon–carbon bonds.
- General formula
- CnH2n+2.
| Name | Molecular formula |
|---|---|
| Methane | CH4 |
| Ethane | C2H6 |
| Propane | C3H8 |
| Butane | C4H10 |
| Pentane | C5H12 |
Core
- Core name and draw the displayed formula of methane and ethane.
Bonding
- Bonding alkanes are covalent because they consist of non-metal atoms, and each carbon forms four single covalent bonds.
Occurrence
- Occurrence alkanes are the main component of crude oil and natural gas.
Part 2 of 9Properties
Appearance
- Appearance alkanes are colourless.
Physical properties
- Physical properties they change gradually as the number of carbon atoms increases.
Reactivity
- Reactivity they are generally unreactive, but they undergo combustion, can be cracked and react with chlorine in substitution reactions.
Flammability
- Flammability it decreases with chain length, so short alkanes ignite most easily.
Solubility
- Solubility they are insoluble in water but soluble in organic solvents.
Conductivity
- Conductivity they do not conduct electricity because they have no ions or free electrons.
They are unreactive towards acids, alkalis and oxidising agents.
Part 3 of 9Intermolecular forces and boiling point
Within a molecule
- Within a molecule the covalent bonds are strong and do not break when an alkane boils.
Between molecules
- Between molecules the forces are weak and increase with chain length, so the boiling point and viscosity increase.
Branching
- Branching branched isomers have lower boiling points than straight-chain isomers because they pack less closely.
- Methylpropane boils at −12 °C and butane at −1 °C.
Polarity
- C–H bonds are nearly non-polar, so alkanes do not dissolve in polar water.
| Alkane | Formula | State at 25 °C | Boiling point / °C |
|---|---|---|---|
| Methane | CH4 | Gas | −162 |
| Ethane | C2H6 | Gas | −89 |
| Propane | C3H8 | Gas | −42 |
| Butane | C4H10 | Gas | −1 |
| Pentane | C5H12 | Liquid | 36 |
| Octane | C8H18 | Liquid | 126 |
Part 4 of 9Combustion
Complete combustion
- Complete combustion alkane + oxygen → carbon dioxide + water.
CH4 + 2O2 → CO2 + 2H2O.
C2H6 + 3½O2 → 2CO2 + 3H2O.
C3H8 + 5O2 → 3CO2 + 4H2O.
C4H10 + 6½O2 → 4CO2 + 5H2O.
Incomplete combustion
- Incomplete combustion in limited oxygen produces carbon monoxide and soot as well as water.
Part 5 of 9Uses of alkanes
Methane
- Methane domestic heating and cooking, and power stations.
Propane and butane (LPG)
- Propane and butane (LPG) portable fuels, for example for camping.
Petrol and diesel
- Petrol and diesel mixtures of alkanes that power vehicles.
Long-chain alkanes
- Long-chain alkanes lubricants and waxes.
Short alkanes
- Short alkanes raw materials (feedstocks) for the chemical industry.
Part 6 of 9Substitution with chlorine (Extended)
1Substitution reaction
- Substitution reaction one atom or group of atoms is replaced by another atom or group of atoms.
2Photochemical reaction
- Photochemical reaction alkanes react with chlorine in the presence of ultraviolet light.
3
The UV light provides the activation energy, Ea.
4
Sunlight is a source of UV radiation.
5Change
- Change a hydrogen atom is replaced by a chlorine atom.
6Monosubstitution
- CH4 + Cl2 → CH3Cl + HCl, giving chloromethane and hydrogen chloride.
7Further substitution
- Further substitution more than one hydrogen atom can be substituted, depending on the amount of UV radiation.
8
No reaction happens in the dark at room temperature.
9
Alkanes react similarly with bromine, and the organic products are halogenoalkanes.
10
Ethane reacts similarly, C2H6 + Cl2 → C2H5Cl + HCl.
Part 7 of 9Further substitution and drawing products
- Excess chlorine gives a mixture of products.
| Product | Formula |
|---|---|
| Chloromethane | CH3Cl |
| Dichloromethane | CH2Cl2 |
| Trichloromethane | CHCl3 |
| Tetrachloromethane | CCl4 |
1
Draw the displayed formula of the alkane.
2
Remove one hydrogen atom.
3
Replace it with the halogen atom, keeping four bonds on carbon.
4
Add the hydrogen halide, such as HCl, as the second product.
Part 8 of 9Alkanes compared with alkenes
| Feature | Alkanes | Alkenes |
|---|---|---|
| Bonding | Single bonds only | Contains C=C |
| Reactivity | Low | High |
| With halogens | Substitution in UV light | Addition |
| With bromine water | No change | Decolourises |
Part 9 of 9More from the specification
More from the specification
Industrial catalytic cracking using steam
- Industrial catalytic cracking using steam mixing vaporized hydrocarbon fractions with superheated steam cracks long-chain alkanes into reactive alkenes.
General Formula and Chemical Inertness of Alkanes
- Alkanes adhere to the general formula CnH2n+2 and are generally chemically inert due to strong, non-polar C-C and C-H single bonds, participating primarily in combustion and substitution reactions.
Photochemical Substitution by Halogens
- In the presence of ultraviolet (UV) radiation, alkanes react with halogens via photochemical free-radical substitution: CH4 + Cl2 → CH3Cl + HCl.
- (such as chlorine or bromine)
Physical Trends Along the Alkane Series
- Boiling points and liquid viscosities increase progressively with chain length because larger electron clouds induce stronger London dispersion intermolecular attractions.
Carboxylic acid
- A compound containing –COOH with a name ending in -oic acid.
Fossil fuel
- A fuel formed from the remains of living things, such as coal, natural gas and petroleum.
Homologous series
- A family of compounds with the same functional group and general formula.
Functional group
- The atom or group of atoms that determines the reactions of a series.
Fraction
- A group of hydrocarbons with similar chain length and boiling points.
Alcohol
- A compound containing –OH with a name ending in -ol.
Catalyst
- A substance that speeds up a reaction and is not used up.
Distillation
- Separating a liquid from a solution by boiling and condensing the vapour.
Key terms
| Term | Meaning |
|---|---|
| Alkane | Saturated hydrocarbon with only single bonds. |
| Saturated | Contains only single carbon–carbon bonds. |
| Substitution | One atom or group is replaced by another. |
| Photochemical reaction | A reaction started by light. |
| Complete combustion | Burning in excess oxygen to give CO2 and H2O. |
| Halogenoalkane | an alkane in which hydrogen atoms are replaced by halogen atoms. |
| Intermolecular forces | weak attractions between molecules that increase with chain length. |
| Monosubstitution | replacement of one hydrogen atom by a halogen atom. |
Avoid these mistakes
1
Saying alkanes are unsaturated — they are saturated
2
Saying alkanes react with bromine water — they do not, and bromine water stays orange
3
Forgetting that substitution needs UV light and gives hydrogen chloride as a second product — it is not addition
4
Writing carbon monoxide as a product of complete combustion — it forms only with limited oxygen
5
Saying alkanes dissolve in water or are very reactive — they are insoluble and generally unreactive
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