Halogens & Alkanes (Edexcel IGCSE Science)

Spec point — 4.22

Halogens & alkanes

• In a substitution reaction, one atom is swapped with another atom
• Alkanes undergo a substitution reaction with halogens in the presence of ultraviolet radiation

The reaction between methane and bromine in the presence of ultraviolet radiation

In the presence of ultraviolet (UV) radiation, methane reacts with bromine in a substitution reaction

• The equation for the reaction is

methane + bromine → bromomethane + hydrogen bromide

CH₄+ Br₂→ CH₃Br + HBr

• Chlorine will also react with alkanes under the same conditions to form chloromethane

methane + chlorine → chloromethane + hydrogen chloride

CH₄+ Cl₂→ CH₃Cl + HCl

• The products belong to a family called halogenoalkanes or haloalkanes
• Halogenoalkanes have many uses a solvents, refrigerants, propellants and pharmaceuticals

Exam Hint CFC stands for chlorofluorocarbon, an important class of halogenoalkanes, previously used in fridges and air conditioners, but gradually being phased out because of the damage they cause to the ozone layer.