Organohalogen compounds in the environment

• Organohalogen compounds = molecules that contain at least one halogen atom joined to a carbon chain
• organohalogen compounds are rarely found in nature and are not broken down naturally in the environment making them the focus of some concern
• The ozone layer is found at the outer edge of the stratosphere

  At height of about 10-40km

  Only a tiny fraction of the gases making up the ozone layer is ozone, but this is enough to absorb most of the biologically damaging ultraviolet radiation (UV-B) from the sun's rays allowing only a small amount to reach the earth's surface
• In the stratosphere, ozone is continually being formed and broken down by the action of ultraviolet radiation.

  Initially, very high energy UV breaks oxygen molecules into oxygen radicals $O_2 -> 2O$

  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 (reversible reaction between an O2 and O)

  Human activity, especially the production and use of chlorofluorocarbons (CFCs), has upset this delicate equilibrium
• Until recently, CFCs and HCFCs were the most common compounds used in refrigeration, air conditioning, and as aerosol propellants
• CFCs are very stable because of the strength of the carbon-halogen bonds within their molecules.

  They remain stable until they reach the stratosphere.

  Here the CFCs begin to break down, forming chlorine radicals, which are thought to catalyse the break down of the ozone layer
• The stability of CFCs means that they have a long residence time in the troposphere. It 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 have the lowest enthalphy and so is the bond that breaks.

  This process is called photodissociation because radiation initiates the breakdown
• In photodissociation, the chlorine radical formed is a very reactive intermediate. It can react with an ozone molecule, breaking down the ozone into oxygen:

  PROPAGATION STEP 1 = $Cl. +$$O_3 -> ClO. +$$O_2$

  PROPAGATION STEP 2 = $ClO. +$$O -> Cl. +$$O_2$

  OVERALL EQUATION = $O_3 +$$O -> 2O_2$

  Propagation step 2 regenerates a chlorine radical, which can attack and remove another molecule of ozone in propagation step 1. The two propagation steps repeat in a cycle in a chain reaction.
• 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. +$$O_3 -> NO_2. +$$O_2$

  PROPAGATION STEP 2 = $NO_2. +$$O -> NO. +$$O_2$

  The overall equation is the same as with chlorine radicals, showing that the radicals act as catalysts for the process:

  $O_3 +$$O -> 2O_2$