Section 1 of 5The Substitution Reaction
1
Ultraviolet light supplies the energy needed to break the halogen–halogen bond and start the reaction.
2
The halogen atoms formed then react with alkane molecules, replacing hydrogen atoms one at a time.
3
Methane reacts with bromine to give bromomethane and hydrogen bromide, with one hydrogen replaced by bromine.
4
Chlorine reacts with methane in the same way, forming chloromethane and hydrogen chloride.
Section 2 of 5Equations and Products
Equations and Products
Products
- Halogenoalkanes are the organic products, with one or more halogen atoms in place of hydrogen.
Uses
- Halogenoalkanes are used as solvents, refrigerants, propellants and in pharmaceuticals.
CFCs
- Chlorofluorocarbons were used in fridges and air conditioners but damage the ozone layer, so they are being phased out.
Light
- The reaction proceeds only in the presence of ultraviolet radiation, which starts the substitution.
| Reaction | Equation | Product family |
|---|---|---|
| Methane + bromine | CH₄ + Br₂ → CH₃Br + HBr | Bromoalkane (halogenoalkane) |
| Methane + chlorine | CH₄ + Cl₂ → CH₃Cl + HCl | Chloroalkane (halogenoalkane) |
| Ethane + chlorine | C₂H₆ + Cl₂ → C₂H₅Cl + HCl | Chloroalkane (halogenoalkane) |
Section 3 of 5Conditions and Products
Reagents
- The halogen must be used as a molecule such as Cl₂ or Br₂, and it reacts with the alkane under ultraviolet light.
Hydrogen halide
- The small by-product is HCl or HBr, which is a hydrogen halide formed as hydrogen atoms are displaced.
Organic product
- The organic product always contains the halogen atom in place of a hydrogen atom from the alkane.
Ozone layer
- Halogenoalkanes containing chlorine can damage the ozone layer when released, which is why CFCs are being phased out.
Section 4 of 5Worked Example
Question Complete and balance the equation CH₄ + Cl₂ → ?, and name the organic product.
- Replace one hydrogen in CH₄ with chlorine to give CH₃Cl
- The other product is hydrogen chloride, HCl
- Check the atomsC 1 = 1; H 4 = 3 + 1; Cl 2 = 1 + 1
Answer
- CH₄ + Cl₂ → CH₃Cl + HCl.
- The organic product is chloromethane.
Section 5 of 5Exam Checklist & Tips
4.3.2 – Key Points for the Exam
- Substitution A reaction in which one atom or group is replaced by another.
- Halogenoalkane An organic compound in which one or more hydrogen atoms are replaced by halogen atoms.
- Halogen A Group 7 element, such as chlorine or bromine, that forms diatomic molecules.
- Ultraviolet radiation Electromagnetic radiation of higher frequency than visible light, which starts the reaction.
- CFC A chlorofluorocarbon, a halogenoalkane that damages the ozone layer.
Common Mistakes
- Omitting ultraviolet light from the conditions for the reaction of alkanes with halogens.
- Calling the reaction of alkanes with halogens an addition reaction.
- Writing HCl or HBr as the organic product; the organic product is the halogenoalkane.
- Stating that CFCs are safe for the ozone layer; they damage it and are being phased out.