Environmental concerns of haloalkanes

What is an organohalogen compound?

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

What is a use of an organohalogen compound?

Pesticides

Ozone molecule (dot and cross)

What is the ozone layer?

Layer of gasses at the outer edge of the stratosphere

What does the ozone layer do?

Absorbs UV-B radiation from the suns rays

What does UV-B radiation do?

Causes sunburn + skin cancer

Formation of ozone

1. O₂ —> 2O

• high energy UV breaks down oxygen molecules into oxygen radicals

2. O₂ + O —> O₃

• formed at the same rate as it is broken down

How is the ozone layer depleted?

by CFCs ad HCFCs

What is a CFC and HCFC?

CFC = chlorofluorocarbons

HCFC = hydrochlorfluorocarbon

What are CFCs used for?

• refrigeration

• AC units

• aerosol propellants

Why were CFCs used?

• very stable

• strong carbon-halogen bonds

• non-flammable

• easily converted between solid and liquid states

CFC alternatives

• HCFCs - do not contain chlorine

• natural refrigerants e.g. ammonia, CO₂ and hydrocarbons

How do CFCs deplete the ozone layer?

1. UV radiation provides energy to break carbon-halogen bond

• homolytic fission forms radicals

• C-Cl bond = weakest so breaks first

• Photodissociation = CF₂Cl₂ —> CF₂C* + Cl*

2. Cl* radical reacts with ozone molecules

• propagation 1: Cl* + O₃ —>ClO* + O₂

• propagation 2: ClO* + O → Cl* + O₂

• overall: O₃ + O —> 2O₂

Other radical depletion of the ozone layer

Nitrogen oxide radicals

• formed during lightning strikes

• result of aircraft travel

Propagation 1: NO* + O₃ —> NO₂* + O₂

Propagation 2: NO₂* + O₂ —> NO* + O₂

Overall: O₃ + O —> 2O₂