Properties of alkanes and alkenes, complete and incomplete combustion, substitution and addition reactions, halogenation, hydration, hydrogenation, distinguishing alkanes from alkenes, uses of hydrocarbons, and biogas.
Alkanes and alkenes are both hydrocarbons — compounds of carbon and hydrogen only — but they differ in one fundamental way: alkanes have only single bonds between carbon atoms, while alkenes contain at least one carbon-carbon double bond. This structural difference drives almost every difference in their reactivity.
Alkanes are saturated hydrocarbons — every carbon atom forms only single bonds, so no more atoms can be added without breaking existing bonds.
General formula:
| Name | Molecular formula | Condensed formula |
|---|---|---|
| Methane | CH₄ | CH₄ |
| Ethane | C₂H₆ | CH₃CH₃ |
| Propane | C₃H₈ | CH₃CH₂CH₃ |
| Butane | C₄H₁₀ | CH₃CH₂CH₂CH₃ |
| Pentane | C₅H₁₂ | CH₃(CH₂)₃CH₃ |
| Hexane | C₆H₁₄ | CH₃(CH₂)₄CH₃ |
Physical properties change gradually along the series: boiling point, density, and viscosity all increase as chain length grows, because longer molecules have stronger intermolecular forces.
Complete combustion (excess oxygen): produces carbon dioxide and water. The flame is blue.
Incomplete combustion (limited oxygen): produces carbon monoxide (CO) and/or carbon (soot) instead of CO₂. The flame is yellow and sooty.
Carbon monoxide is colourless, odourless, and extremely toxic. It binds irreversibly to haemoglobin, preventing oxygen transport in blood. It is a leading cause of accidental poisoning in enclosed spaces with poorly ventilated gas appliances.
In the presence of ultraviolet (UV) light, alkanes undergo substitution reactions with halogens. One hydrogen atom is replaced by a halogen atom:
Chloromethane is produced, plus hydrogen chloride. The reaction can continue to give CH₂Cl₂, CHCl₃, and CCl₄ if excess chlorine is present. UV light initiates the reaction; it does not occur in the dark.
This is called a substitution reaction because a Cl atom is substituted for an H atom — the number of bonds to carbon does not change.
Alkenes are unsaturated hydrocarbons — the C=C double bond means the molecule can add atoms across the double bond without breaking the carbon skeleton.
General formula:
| Name | Molecular formula | Condensed formula |
|---|---|---|
| Ethene | C₂H₄ | CH₂=CH₂ |
| Propene | C₃H₆ | CH₃CH=CH₂ |
| But-1-ene | C₄H₈ | CH₂=CHCH₂CH₃ |
| But-2-ene | C₄H₈ | CH₃CH=CHCH₃ |
The C=C double bond opens to allow two new atoms or groups to add — one to each carbon. These are called addition reactions.
Alkenes react with hydrogen gas in the presence of a nickel catalyst to form alkanes. This converts liquid unsaturated oils into solid saturated fats (used in margarine manufacture):
Alkenes decolourise bromine water by addition across the double bond:
The orange/brown bromine solution turns colourless — this is the standard test for unsaturation.
Alkenes react with steam in the presence of a phosphoric acid catalyst to form alcohols. This is the industrial method for making ethanol:
Under high pressure and with a catalyst, many alkene monomers link together to form long-chain polymers. The double bond opens and each monomer adds to the chain — covered in detail on the polymers page.
Because alkanes are saturated and alkenes are unsaturated, they can be distinguished using two chemical tests:
| Test | Alkane result | Alkene result |
|---|---|---|
| Add bromine water | No decolourisation — orange remains | Decolourises — turns colourless |
| Add acidified potassium manganate(VII) | No change — purple remains | Decolourises — turns colourless |
The syllabus states that burning is not an acceptable test to distinguish alkanes from alkenes. Both burn, and although alkenes tend to produce smokier flames, this is not a reliable or acceptable answer. Use the bromine water or acidified KMnO₄ test.
Saturated means only C–C single bonds — no more atoms can add without breaking. Unsaturated means at least one C=C double bond — atoms can add across it. The prefix "un-" signals that the molecule is not full of hydrogen atoms.
The relationship between structure and reactivity runs through everything in this topic:
| Feature | Alkanes | Alkenes |
|---|---|---|
| Bonding | Only C–C and C–H single bonds | Contains C=C double bond |
| Saturation | Saturated | Unsaturated |
| Typical reaction | Substitution (with halogens, UV light) | Addition (with H₂, Br₂, H₂O) |
| Reactivity | Relatively low | Higher than alkanes |
| Main use | Fuels and solvents | Making polymers; feedstocks |
When organic matter such as animal manure or plant waste decomposes anaerobically (without oxygen), bacteria produce a mixture of gases dominated by methane. This biogas can be collected and burned as a fuel. It is a renewable energy source and helps manage agricultural waste.