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Transcript of Haloalkanes CFC
Homolytic bond fission can occur if molecules are hit by high energy ultraviolet electromagnetic radiation. Short-chain CFCs have been extensively used in fridges and aerosols This is because they:
are very unreactive under normal conditions
are non toxic
have low flammability
Ozone is being rapidly destroyed by chemical processes. This leads to holes in the important Ozone Layer, such as the Antarctic Ozone Hole and this means more harmful UV Radiation can reach the Earth's surface leading to an increased risk of sunburn and skin cancer. CFCs are to blame. How do CFCs destroy Ozone? Chlorine atoms from CFCs act as a catalyst in the destruction of ozone (cc) image by nuonsolarteam on Flickr What can be done? Many countries are now banning the use of CFCs But, scientists predict it will take many years for the depleted ozone layer to return to its 'original' ozone concentration Scientists are now working towards developing less harmful alternates to CFCs Attempts have been made to develop compounds with a low ozone depletion potential. These alternatives include: Hydrochloroflurocarbons (HCHCs) such as CF3CHCl2 Hydroflurocarbons (HCFs) such as CF3CH2F Hydrocarbons such as butane and propane They break down more quickly in the atmosphere and they have a lower percentage of chlorine so a lower ozone depletion potential than CFCs BUT they could damage ozone if overused They have no chlorine and thus are "ozone safe".
HOWEVER, the safety question on their toxicity is still unsolved. They are cheap and readily available and contain no chlorine. BUT, they are flammable and poisonous. Ozone is a highly reactive and unstable molecule and decomposes into dioxygen when hit by other uv light photons. Then an oxygen atom combines with an oxygen molecule to form ozone. Ozone is formed in the stratosphere by free radical reactions:
A stable oxygen O2 (dioxygen) molecule is split in two by high energy ultraviolet electromagnetic radiation into two oxygen atoms (which are themselves free radicals). Longer-chain CFCs are used as dry cleaning and de-greasing solvents Free Radical Substitutions in Haloalkanes A free radical substitution takes place in three stages and the reactions between any alkane and halogen goes by the same mechanism. For example, methane and chlorine; CH4 + Cl2 --> CH3Cl + HCl Initiation The chlorine molecule absorbs the energy of a single quantum of UV light whose energy is greater than Cl-Cl so the bond breaks.
The bond breaks homolytically resulting in two free radicals which are highly reactive. Cl-Cl ---> 2Cl . The C-H bond requires more energy to break than what is in a quatum of UV radiation so this bond does not break Propagation The chlorine free radical takes a hydrogen atom from methane to form hydrogen chloride and a methyl free radical Cl + CH4 --> HCl + CH3 . . The methyl free radical reacts with a chlorine molecule to produce a chlorine free radical and a molecule of chloromethane CH3 + Cl2 --> CH3Cl + Cl These two steps produce Hydrogen Chloride, Chloromethane and a Chlorine free radical which is ready to react with more methane and repeat the two steps Termination This is where the free radicals are removed and can happen in three ways: Two chlorine free radicals react together to give chlorine Two methyl free radicals react together to give ethane A chlorine free radical and a methyl free radical react together to give chloromethane Cl + Cl --> Cl2 CH3 + CH3 --> C2H6 Cl + CH3 --> CH3Cl . . . . . . BUT Termination can take place Unfortunately, a large amount of CFCs remain in the atmosphere still and it will take a long time for the ozone layer to recover.