15.2 Organohalogen compounds in the environment

• CFCs have been used as propellants in aerosols, as refrigerants and in air-conditioning
• CFCs were used because of their low reactivity, high volatility and because they are non-toxic
• Ozone helps to filter out harmful UV rays
• Overexposure to UV radiation can cause skin cancer, cataracts and damage to crops
• In the upper atmosphere, UV radiation provides the energy to form radicals which then break down ozone
• In the presence of UV, C-Cl bonds in the CFC molecules are broken by homolytic fission to form radicals
• Radicals are species with an unpaired electron
• CFCs can escape into the Earth's atmosphere where they slowly diffuse into the upper atmosphere
• Propagation of ozone

  Cl. + O3 -> ClO. + O2

  ClO. + O -> Cl. + O2

  overall reaction:

  O3 + O -> 2O2
• Nitrogen oxide radicals are made when nitrogen and oxygen react together at high temperatures and pressures
• Propagation of nitrogen oxides

  NO. + O3 -> NO2. + O2

  NO2. + O -> NO. + O2