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Chlorination

Chemistry: AQA A Level, topic 5.2.4

Chlorination

Chlorination is the first reaction mechanism that you need to learn. This mechanism involves free radicals.

Overall reaction

Overall reaction

  • Products:
  • With excess methane, chloromethane.
  • With excess chlorine, you get multiple reactions and end up with tetrachloromethane.
  • The reaction can be split up into three stages:
  • Initiation.
  • Propagation.
  • Termination.
  • This is characteristic of all free radical reactions.
Initiation

Initiation

  • This step first generates the free radicals.
  • The balanced equation is:
  • Cl2→light2Cl∙Cl_2 \xrightarrow[\text{}]{\text{light}} 2Cl\bullet
Propagation

Propagation

  • There are often multiple propagation steps.
  • These involve a free radical, and generate a new free radical.
  • Relevant equations are:
  • CH4+Cl∙→CH3∙+HClCH_4 + Cl\bullet \rightarrow CH_3\bullet+ HCl
  • CH3∙+Cl2→CH3Cl+Cl∙CH_3\bullet + Cl_2 \rightarrow CH_3Cl + Cl\bullet
Termination

Termination

  • A termination step is one which involves two free radicals forming a covalent bond.
  • It is said to ‘consume’ free radicals.
  • It is the final step in the reaction.
  • Relevant equations are:
  • 2CH3∙→C2H62CH_3\bullet \rightarrow C_2H_6
  • 2Cl∙→Cl22Cl\bullet \rightarrow Cl_2
  • CH3∙+Cl∙→CH3ClCH_3\bullet + Cl\bullet \rightarrow CH_3Cl

Test yourself on Chlorination

  • Which of the following best describes propagation reactions?

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