Alkanes are the main hydrocarbons in crude oil and natural gas. Their strong, single C−C and C−H bonds make them unreactive towards most chemicals, which leaves just two reactions to learn: burning, and substitution with chlorine in ultraviolet light.
Alkanes are saturated hydrocarbons with the general formula CnH2n+2; every bond is a single covalent bond. They are generally unreactive, except for combustion and substitution by chlorine. In complete combustion an alkane burns in plenty of oxygen to give carbon dioxide and water, releasing a lot of energy, which is why alkanes are used as fuels. In substitution a hydrogen atom is replaced by a chlorine atom: CH4 + Cl2 → CH3Cl + HCl. (Extended) This is a photochemical reaction: ultraviolet light provides the activation energy.
Alkanes do not react with acids, alkalis or bromine water in the dark. Their one big everyday reaction is combustion.
Balancing tip: balance C first (CO2), then H (H2O), then count the O atoms on the right and halve for O2. If you get a half, double everything.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which word describes alkanes?
What are the products of the complete combustion of methane?
What condition is needed for methane to react with chlorine?
What type of reaction happens between ethane and chlorine?
Mix methane and chlorine in the simulator and shine ultraviolet light on them. Watch one hydrogen atom at a time swap for a chlorine atom. You do not need the step-by-step mechanism at IGCSE, only the reactants, conditions and products.
| Methane + chlorine | Ethane + chlorine | |
|---|---|---|
| condition | ultraviolet light | ultraviolet light |
| equation | CH4 + Cl2 → CH3Cl + HCl | C2H6 + Cl2 → C2H5Cl + HCl |
| organic product | chloromethane | chloroethane |
| other product | hydrogen chloride | hydrogen chloride |
Extended. The reaction is photochemical: in the dark nothing happens, because ultraviolet light supplies the activation energy. If chlorine is in excess, further substitution gives CH2Cl2, CHCl3 and CCl4, but exam questions are limited to monosubstitution, one Cl replacing one H.
Substitution makes two products: do not forget the HCl. Alkanes do not undergo addition, because they have no C=C bond to open. The condition is ultraviolet light, not heat with a catalyst, and the chlorine replaces a hydrogen, never a carbon.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Which equation shows the reaction of methane with chlorine in ultraviolet light?
Which is the balanced equation for the complete combustion of propane?
(Extended) Why is the reaction of methane with chlorine described as photochemical?
Why do alkanes not decolourise bromine water?
This skill is now recorded on your profile. Keep the chain going.