• Halogens react with alkanes in photochemical reactions.
• Photochemical reactions are started by light. This reaction requires ultraviolet light to start

A hydrogen atom is substituted (replaced) by chlorine or bromine. This is a radical substitution reaction

• Sunlight provides the energy to break the Cl-Cl bond, this is photodissociation

• The bond splits equally and each atom keeps one electron, this is homolytic fission
• The atom becomes a highly reactive radical, Cl•, because of its unpaired electron

• This equation can be represented using curly arrows to show the movement of electrons when forming the free radicals

• Cl• attacks the methane molecule

• The new ethyl radical • CH₃, can attack another Cl₂ molecule

• The new Cl•, can attack another CH₄ molecule, and so on, until all the Cl₂ of CH₄ molecules are used up

• If two free radicals join together, they make a stable molecule
• There are many possible termination reactions

Desired product

Trace impurity

• Some of the products made in this way will be trace impurities in the final sample

• Further substitution is possible for example the producing dichloromethane, trichloromethane and  tetrachloromethane rather than the desired product chloromethane 

Dichloromethane

Trichloroethane

Tetrachloroethane

• A mixture of products form, both by further substitution and through termination steps

• The likelihood of further reactions being produced can be reduced by adding an excess of methane

• The mixture of products can be separated by fractional distillation since their boiling temperatures will be different
• This has an economic cost in industrial processes