Alkanes

· Alkanes are saturated hydrocarbons containing only C–C single bonds and C–H bonds.
· They are generally unreactive because C–H bonds are strong and almost non-polar.
· They do not react readily with polar reagents because there are no electron-rich or electron-deficient sites for attack.
· Main reactions in this topic: combustion, free-radical substitution, hydrogenation to produce alkanes, and cracking.

Producing Alkanes

· Hydrogenation of alkenes: alkene + H₂(g) → alkane.
· Conditions: Pt or Ni catalyst and heat.
· Example: ethene + hydrogen → ethane: C₂H₄ + H₂ → C₂H₆.
· Cracking of longer-chain alkanes can produce shorter alkanes and alkenes.
· Conditions for cracking: heat with Al₂O₃ catalyst.
· Cracking is important because it converts heavier crude oil fractions into more useful lower-Mᵣ alkanes and alkenes.

Cracking breaks large alkane molecules into smaller, more useful hydrocarbons. In CIE questions, remember that cracking usually produces a shorter alkane plus an alkene, using heat and Al₂O₃. Source

Complete and Incomplete Combustion

· Complete combustion occurs when there is excess oxygen.
· Products of complete combustion: CO₂ and H₂O.
· General equation: alkane + oxygen → carbon dioxide + water.
· Example: C₂H₆ + 3½O₂ → 2CO₂ + 3H₂O.
· Use whole-number equations if preferred: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.
· Incomplete combustion occurs when there is limited oxygen.
· Products may include CO, C/soot, and H₂O.
· Carbon monoxide, CO, is dangerous and polluting; soot contributes to smoke and particulates.

This diagram represents complete combustion of methane. It is useful for remembering that complete combustion of alkanes produces CO₂ and H₂O only when oxygen is in excess. Source

Free-Radical Substitution with Chlorine or Bromine

· Alkanes react with Cl₂ or Br₂ by free-radical substitution.
· Conditions: ultraviolet light / UV light.
· This is a substitution reaction because a hydrogen atom is replaced by a halogen atom.
· Example with ethane: C₂H₆ + Cl₂ → C₂H₅Cl + HCl.
· Further substitution can occur, so a mixture of products may form.
· The reaction is controlled by free radicals, which are species with an unpaired electron.

Free-Radical Substitution Mechanism

· Initiation: UV light causes homolytic fission of the halogen molecule.
· Example: Cl₂ → 2Cl·.
· Propagation: radicals react and regenerate new radicals, so the chain reaction continues.
· Example 1: C₂H₆ + Cl· → C₂H₅· + HCl.
· Example 2: C₂H₅· + Cl₂ → C₂H₅Cl + Cl·.
· Termination: two radicals combine, removing radicals from the reaction mixture.
· Examples: Cl· + Cl· → Cl₂, C₂H₅· + Cl· → C₂H₅Cl, C₂H₅· + C₂H₅· → C₄H₁₀.
· Exam tip: always label the steps as initiation, propagation, and termination.

Cracking and Usefulness of Products

· Cracking breaks long-chain alkanes into smaller alkanes and alkenes.
· CIE conditions: heat with Al₂O₃.
· Smaller alkanes are useful as fuels because they burn more easily and are in higher demand.
· Alkenes formed are useful for making polymers and other organic chemicals.
· Example pattern: long-chain alkane → shorter alkane + alkene.
· Always check that cracking equations are balanced.

Environmental Consequences of Alkane Combustion

· Combustion in an internal combustion engine can produce pollutants.
· Carbon monoxide, CO forms during incomplete combustion.
· Oxides of nitrogen, NOₓ, form at high engine temperatures when nitrogen and oxygen in air react.
· Unburnt hydrocarbons can be released when fuel does not burn completely.
· A catalytic converter removes pollutants by converting them into less harmful substances.
· Typical catalytic removal: CO → CO₂, NOₓ → N₂, and unburnt hydrocarbons → CO₂ + H₂O.

A catalytic converter reduces harmful exhaust emissions from internal combustion engines. For CIE, focus on the removal of CO, NOₓ, and unburnt hydrocarbons from alkane combustion products. Source

Why Alkanes Are Generally Unreactive

· C–H bonds are strong, so they require a lot of energy to break.
· C–H bonds are almost non-polar, so polar reagents are not strongly attracted.
· Alkanes have no functional group, so they have limited reaction pathways.
· They usually need high-energy conditions, such as UV light for halogenation or heat for cracking/combustion.