Organohalogen compounds in the environment

• Uses of the organohalogens:

  • organohalogen compounds are molecules that contain at least one halogen atom joined to a carbon chain
  • many practical uses
  • used in many pesiticides
  • rarely found in nature and as they are not broken down naturally in the environment, they have become the focus of some concern
• • general solvents - e.g. CHCl3
  • dry cleaning solvents - e.g. C2H2Cl2, C2HCl3
  • making polymers - e.g. C2H3Cl, C2F4
  • flame retardants - e.g. CF3Br
  • refrigerants - e.g. F2CCl2, HCClF2, HCCl2F
• The ozone layer:

  • found at the outer edge of the stratosphere, at a height that varies from 10-40km above the earth's surface
  • only a tiny fraction of the gases making up the ozone layer is ozone - enough to absorb most of the biologically damaging ultraviolet radiation (UV-B) from the Sun's rays, allowing only a small amount to reach Earth's surface
• • UV-B radiation is the radiation most commonly linked to sunburn and much research has been carried out on its harmful effects
  • feared that continued depletion of the ozone layer will allow more UV-B radiation to reach the Earth's surface
  • bad for living organisms - increased genetic damage and greater risk of skin cancer in humans
• • in the stratosphere, ozone is continually being formed and broken down by the action of ultraviolet (UV) radiation
  • initially very higher energy UV breaks oxygen molecules into oxygen radicals, O:
  • O2 = 2O
• • a steady state is then set up involving O2 and the oxygen radicals in which ozone forms and then breaks down
  • in this steady state, the rate of formation of ozone is the same as the rate at which it is broken down:
  • O2 + O = O3 (reversible)
• human activity, especially the production and use of chlorofluorocarbons (CCs has upset this delicate equilibrium)
• CFCs and the ozone layer:

  • until recently, CFCs and HCFCs were the most common compounds used as refrigerants, in air-conditioning units and as aerosol propellants
  • CFCs are very stable bc of the strength of the carbon-halogen bonds within their molecules
• • 1973 - 2 chemists Frank Sherwood Rowland and Mario Molina began to look at the impact of CFCs on the Earth's atmosphere
  • concluded that CFCs remain stable until they reach the stratosphere
  • here the CFCs begin to break down, forming chlorine radicals, which are thought to catalyse the breakdown of the ozone layer
• How do CFCs deplete the ozone layer:

  • stability of CFCs due to the strength of their carbon-halogen bonds means that CFCs have a long residence time in the troposphere - may take them many years to reach the stratosphere
  • once in the stratosphere UV radiation provides sufficient energy to break a carbon-halogen bond in CFCs by homolytic fission to form radicals
  • the C-Cl bond has the lowest bond enthalpy so this is the bond that breaks
• • as radiation initiates the breakdown, this process is called photodissociation
  • e.g. photodissociation of CF2CCl2:
  • CF2Cl2 = CF2Cl radical + Cl radical
• • The chlorine radical formed, is a very reactive intermediate
  • can react with an ozone molecule
  • breaking down the ozone into oxygen
  • 2 step process: propagation 1 and propagation 2
• • propagation step 2 regenerates a chlorine radical, which can attack and remove another molecule of ozone in propagation step 1
  • 2 propagation steps repeat in a cycle over and over in a chain reaction
  • estimated that a single CFC molecule can promote the breakdown of 100 000 molecules of ozone
• • propagation step 1: Cl radical + O3 = ClO radical + O2
  • propagation step 2: ClO radical + O = Cl radical + O2
  • overall: O3 + O = 2O2
• Are CFCs responsible for all ozone-depleting reactions:

  • no
  • other radicals can also catalyse the breakdown of ozone
  • nitrogen oxide radicals are formed naturally during lightning strikes and also as a result of aircraft travel in the stratosphere
• • nitrogen oxide radicals cause the breakdown of ozone by a mechanism similar to that involving chlorine radicals:
  • propagation step 1: NO radical + O3 = NO2 radical + O2
  • propagation step 2: NO2 radical + O = NO radical + O2
  • overall: O3 + O = 2O2
  • overall equation the same as for chlorine radicals - radicals act as a catalyst for the process
• 1987 - Montreal Protocol signed:

  • introduced steps for the complete removal of CFCs in all but a limited number of products where no suitable alternative can be found
  • research into alts for refrigeration and air con units have led to development of coolants that use hydrocarbons, ammonia or even CO2
  • for sprays, some companies have produced pump-action spray dispensers to replace products in aerosol form
  • where aerosols are still being used - propellant is most likely to be a hydrocarbon such as butane
• • brominated flame retardants (BFRs) are organobromine compounds commonly used by the electronics industry in components such as circuit boards, outer coverings and cables to reduce the flammability of the product
  • organobromine compounds are currently under close scrutiny as scientists expect they are toxins that may interfere with the effective function of the human endocrine system
• • both Apple and Dell claim to have reduced or eliminated environmentally damaging flame retardants from products and to have stopped using the organohalogen polymer, PVC