Haloalkanes and ozone depletion

• When haloalkanes react, the C-X bond breaks and the halide ion (X-) acts as the leaving group
• There are two factors that determine how readily the C-X bond reacts:

  • C-X bond polarity: the higher the difference in electronegativity, the faster the reaction. This suggests a C-F bond should react fastest
  • C-X bond enthalpy: this influences the strength of the bond. This suggests a C-I bond should react the fastest
• Both bond enthalpy and bond polarity play roles in the reactivity of a haloalkane, but bond enthalpy tends to have the most pronounced effect
• Halogenoalkanes are also known as haloalkanes and alkyl halides
• A haloalkane is an alkane in which one or more of the hydrogen atoms have been replaced by a halogen
• A primary haloalkane has one alkyl group attached to the carbon with the halogen atom CH2RX
• Generally haloalkanes are liquids or gases under standard conditions
• There is an increase in boiling point down the group of haloalkanes
• Ozone O3 is naturally formed from oxygen via photochemical reactions
• Ozone prevents harmful ultraviolet radiation from entering the atmosphere
• O + O2 <-> O3
• Chlorofluorocarbons are haloalkanes that contain both fluorine and chlorine but no hydrogen
• Chlorine free radicals act as catalysts during the free radical substitution of decomposition of ozone to oxygen
• What is the initiation of free radical substitution of dichlorodifluoromethane?

  CCl2F2 -UV-> C*ClF2 + Cl*
• What are the propagation steps of free radical substitution of dichlorodifluoromethane?

  Cl* + O3 -> ClO* + O2 , ClO* + O3 -> Cl* + 2O2
• Hydrofluorocarbons (HCFs) have been developed to replace CFCs in many applications
• CFCs have been used as refrigerants, propellants and solvents
• UV light causes bonds in CFCs to form free radicals, which causes ozone to decompose into oxygen
• Bond enthalpy determines which free radicals are most likely to form. This means in CFCs, chlorine free radicals are most likely to form
• Replacements for CFCs must contain bonds with high bond enthalpies that can't produce Cl*