the formation of halogenoalkanes

• What type of reactions do halogens undergo with alkanes?

  Photochemical reactions
• What is required for photochemical halogenation to occur?

  Ultraviolet (UV) light must be present
• What role does UV light play in photochemical halogenation?

  It provides activation energy to start the reaction
• What type of reaction occurs during photochemical halogenation of alkanes?

  Substitution reaction
• What is replaced in the alkane during photochemical halogenation?

  A hydrogen atom is replaced by a halogen
• What are the steps in the free radical substitution mechanism of photochemical halogenation?

  1. Initiation: UV light produces reactive radicals
  2. Propagation: Radicals react in a chain reaction
  3. Termination: Radicals join to form stable molecules
• What happens when methane reacts with chlorine gas in the presence of UV light?

  Produces chloromethane and hydrogen chloride
• What is the equation for the reaction of methane with chlorine?

  CH4 + Cl2 ➔ CH3Cl + HCl
• What occurs during the initiation stage of the reaction?

  • UV light breaks the Cl-Cl bond
  • Produces two chlorine radicals (Cl•)
• What is the result of the homolytic fission of Cl2?

  Two chlorine radicals (Cl•) are formed
• What happens during the propagation stage of the reaction?

  • Chlorine radical attacks methane
  • Produces methyl radical and HCl
  • Methyl radical attacks chlorine to regenerate Cl•
• What is produced when a chlorine radical attacks a methane molecule?

  A methyl radical and hydrogen chloride
• What is the outcome of the propagation cycle?

  Continues until reagents are used up
• What occurs during the termination stage of the reaction?

  • Two radicals join to form stable molecules
  • Possible combinations include Cl• + CH3• ➔ CH3Cl
• What happens when excess chlorine is present in the reaction?

  Further substitution produces a mixture of products
• What are the products when excess methane is present?

  Predominantly single substitution occurs
• What are chlorofluorocarbons (CFCs)?

  • Halogenoalkanes containing chlorine, fluorine, and carbon
  • No hydrogen atoms remaining
• Give two examples of CFCs.

  Trichlorofluoromethane and chlorotrifluoromethane
• What is the role of ozone (O3) in the atmosphere?

  It absorbs harmful ultraviolet radiation
• How do CFCs threaten the ozone layer?

  They produce reactive chlorine radicals
• What are the propagation reactions that chlorine radicals undergo to damage ozone?

  1. Cl• + O3 ➔ ClO• + O2
  2. ClO• + O3 ➔ 2O2 + Cl•
• What is the overall reaction for ozone depletion caused by chlorine radicals?

  2O3 ➔ 3O2
• What was the global response to the threat posed by CFCs?

  CFCs were globally banned in 1989