Lesson 6, Topic 1
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Substitution reactions notes

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  • 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 




  • 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