15.2 Organohalogen compounds in the environment

Molecules that contain at least one halogen atom joined to a carbon chain

Many pesticides

Rarely found in nature and are not broken down naturally

Found at the outer edge of the stratosphere at height that varies between 10-40km above Earth’s surface

Only small fraction is ozone But enough to absorb most biologically damaging UV-B radiation from sun’s rays

Radiaion most commonly linked to sunburn Increased genetic damage and greater risk of skin cancer

Ozone continually formed and broken down by action of UV radiation Initially very high energy UV braks oxygen molecules into radicals O2->2O Steady state set up involving O2 and radicals. Ozone forms and breaks down Rate of formation same as rate at which it is broken down O2+OO3

Due to the strength of their carbon-halogen bonds within their molecules

Concluded that they remain stable until they reach the stratosphere, where they break down forming chlorine radicals which are thought to catalyse the breakdown of the ozone layer

Can take many years to reach stratosphere UV provides sufficient energy to break carbon-halogen bond by homolytic fission to form radicals This is photodissociation

As radiation initiates the breakdown

UV enough energy to break carbon-halogen bond Chlorine radical forms, very reactive intermediate Reacts with ozone molecule, breaks down ozone into oxygen Repeat in cycle

Formed naturally during lightning strikes and aircraft travel in stratosphere Breakdown of ozone in similar way to chlorine

O3+O->2O2

O3+O->2O2

Signed in 1987 Introduced steps for the complete removal of CFCs in all but limited number of products where no suitable alternative can be found