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4.3.2Organic Chemistry: Part 1

Halogens and Alkanes — Edexcel International GCSE Science Double Award (Modular) - Chemistry Unit 1 Revision Notes & Topical Questions

5 min read 7 parts Lesson 10 of 12

📚 Edexcel International GCSE Science Double Award (Modular) - Chemistry Unit 1 Hub / Topical Revision
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Part 1 of 5Alkanes and halogens

A substitution reaction
  • A substitution reaction In a substitution reaction, one atom in a molecule is swapped for another atom.
Alkanes are fairly unreactive
  • Alkanes are fairly unreactive Alkanes are saturated, with only single bonds, so they do not undergo addition reactions.
The condition needed
  • The condition needed Alkanes react with halogens in the presence of ultraviolet radiation, which supplies the energy to start the reaction.
    • Without UV light, there is no reaction at room temperature in the dark.

Part 2 of 5Methane and bromine

1Mix the reactants
  • Mix the reactants Methane gas and bromine are brought together.
2Expose to UV
  • Expose to UV Ultraviolet radiation breaks the reaction into motion.
    • The orange-brown colour of bromine fades as it is used up.
3Hydrogen replaced
  • Hydrogen replaced A bromine atom takes the place of one hydrogen atom in methane.
4Products
  • Products methane + bromine → bromomethane + hydrogen bromide: CH4 + Br2 → CH3Br + HBr

Part 3 of 5Methane and chlorine

1
Chlorine reacts with methane under the same conditions, with ultraviolet light present.
2
One hydrogen atom in methane is replaced by a chlorine atom.
3
The other chlorine atom combines with the displaced hydrogen.
4
The equation is methane + chlorine → chloromethane + hydrogen chloride: CH4 + Cl2 → CH3Cl + HCl.

Part 4 of 5Halogenoalkane products

ReactantsOrganic productOther productProduct family
Methane + bromineBromomethane, CH3BrHydrogen bromide, HBrHalogenoalkane
Methane + chlorineChloromethane, CH3ClHydrogen chloride, HClHalogenoalkane

Part 5 of 5Uses and environmental impact

Halogenoalkanes
Everyday uses
  • Everyday uses Halogenoalkanes are used as solvents, refrigerants, propellants in aerosols and in making pharmaceuticals.
CFCs
  • CFCs CFCs are chlorofluorocarbons, a type of halogenoalkane once widely used in fridges and air conditioners.
Ozone damage
  • Ozone damage CFCs damage the ozone layer, so they are being phased out and replaced with safer alternatives.

Key terms

TermMeaning
Substitution reactionA reaction where one atom or group is replaced by another.
HalogenoalkaneAn alkane in which one or more hydrogen atoms have been replaced by halogen atoms; also called a haloalkane.
Ultraviolet radiationHigh-energy light that provides the energy to start halogen and alkane reactions.
SaturatedContaining only single carbon-carbon bonds.
CFCA chlorofluorocarbon, a halogenoalkane that damages the ozone layer.

Avoid these mistakes

1
Describing the methane and halogen reaction as addition. It is substitution, with a second product, hydrogen halide.
2
Leaving out ultraviolet light when giving the conditions.
3
Writing only one product. The equation has a halogenoalkane and a hydrogen halide such as HBr or HCl.
4
Mixing up the product namesCH3Br is bromomethane and CH3Cl is chloromethane.
5
Saying that CFCs are still widely used in fridges. They are being phased out because of ozone damage.

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