Halogenoalkanes
- Haloalkanes are saturated organic compounds that contain at least one halogen atom, e.g. F, Cl, Br, or I.
- The C-X bond has a permanent dipole due to the large difference in electronegativity between the carbon and halogen atoms. With the electrons closer to the halogen atom.
- The δ- on the carbon atom makes it easily attacked by electron-rich nucleophiles (an electron pair donor that is attracted to electron deficient regions).
- Nucleophilic substitution occurs with hydroxide ions, resulting in hydrolysis of the haloalkane:
- The OH- ions can be generated by using an aqueous metal hydroxide solution (e.g. KOH) or by using water in the presence of AgNO3 and ethanol
- Going down Group 7, the electronegativity of the halogen atoms decreases, so the polarity of the C-X bond also decreases. We would expect fluoroalkanes to be the most reactive, however iodoalkanes are the most reactive because bond enthalpy decreases down the group. C-I bond is the longest and weakest
Ozone Depletion
- In the Earth’s atmosphere, there is a layer of ozone (O3) which protects us from the harmful UV radiation produced by the sun
- Chlorofluorocarbons (CFCs) are a type of organic compound that contain chlorine and fluorine atoms. They can diffuse through layers of the atmosphere where they are exposed to UV radiation, which causes homolytic fission, forming radicals
- The chlorine radials produced catalyse the decomposition of ozone
Cl• + O3 → ClO• + O2
ClO• + O3 → 2O2 + Cl•
Overall: 2O3 → 3O2
ClO• + O3 → 2O2 + Cl•
Overall: 2O3 → 3O2
- The chlorine radicals are regenerated and act as a catalyst in the breakdown of ozone molecules into oxygen
- Holes in the ozone layer increase the likelihood of skin cancers and sunburns