A level Chemistry · Edexcel 9CH0 · Topic 6B
Alkanes
Key points
- Alkanes (CₙH₂ₙ₊₂) are saturated hydrocarbons from crude oil, separated by fractional distillation.
- Cracking breaks long chains into shorter alkanes and alkenes; reforming makes branched and cyclic hydrocarbons for better fuels.
- Complete combustion gives CO₂ and H₂O; incomplete combustion gives toxic CO and soot. Other pollutants: NOₓ and SO₂; catalytic converters remove CO, NOₓ and unburnt hydrocarbons.
- Biofuels are renewable but raise questions about land use and whether they are truly carbon neutral.
- Free-radical substitution with Cl₂ or Br₂ in UV light: initiation (Cl₂ → 2Cl•), propagation (Cl• + CH₄ → HCl + •CH₃; •CH₃ + Cl₂ → CH₃Cl + Cl•), termination (two radicals combine).
- The method gives a mixture of products because further substitution happens. Radicals are shown with a single dot and arise from homolytic fission.
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Alkanes are the main compounds in crude oil and our most widely used fuels. They are fairly unreactive, but they burn well and react with halogens in ultraviolet light by a radical chain reaction, and their use raises important questions…
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