Part 3. The chemistry of HALOGENOALKANES and OZONE

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

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Part 3.9 OZONE depletion and halogenoalkanes (haloalkanes)

The chemistry of ozone depletion - cause and prevention

An introduction and general description of the problem and solution

Abbreviations used: CFC/CFCs = chlorofluorocarbon; HCFC/HCFCs = hydrochlorofluorocarbon; HFC/HFCs = hydrofluorocarbon

is a typical CFC molecule CCl₂F₂, dichlorodifluoromethane, known commercially as CFC-12.

These kinds of molecules 'where' used in aerosol sprays as propellant gases and refrigerant gases.

Unfortunately, when CFCs get into the atmosphere at ground level, because they are chemically inert, they diffuse up into the upper atmosphere where they are decomposed by ultraviolet (uv) radiation producing chlorine radicals.

(chlorine atoms are free radicals with an unpaired electron)

The chlorine radicals decompose (destroy) ozone via free radical chain reactions and it is estimated that one chlorine atom can lead to the decomposition of 100, 000 ozone molecules in a catalytic cycle.

In the Antarctic winter the chlorine radicals build up on small ice crystals in the air high up in the upper atmosphere.

When these ice crystals melt in the spring sun, the chlorine radicals are released causing massive depletion of the ozone layer - in fact they 'did' cause a large hole in the (uv protecting) polar ozone layer of the southern hemisphere.

Fortunately, CFCs are now banned and new refrigerant coolants and aerosol propellant gases have been developed that contain hydrogen atoms - known as HCFCs and HFCs

Examples: a HCFC CHClF₂ chlorodifluoromethane and a HFC difluoromethane CH₂F₂

HCFCs and HFCs are more reactive and are decomposed at lower altitudes, before they can diffuse up into the ozone layer and HFCs obviously cannot generate chlorine radicals.

However, they still have the disadvantage of being powerful greenhouse gases!

Other ozone and CFC notes on the detailed free radical chemistry involved have been already written up in great on The CFC chemistry of ozone depletion and how this environmental problem was solved