Haloalkanes

• Haloalkanes are saturated organic compounds that contain carbon atoms and at least one halogen atom
• Halogenoalkanes are insoluble in water because C-H bonds are non-polar and not compensated for enough by C-X bond polarity
• Halogenoalkanes have a polar bond as halogen has a higher electronegativity than carbon (halogen is δ-, carbon is δ+)
• Halogenoalkanes have permanent dipole-dipole and London forces of attraction due to C-X bond polarity creating permanent dipoles
• Halogenoalkanes have higher boiling points with an increase in carbon chain length and when the halogen is further down Group 7
• The mass of a haloalkane is greater than that of an alkane of the same chain length because the mass of the halogen is greater than the mass of hydrogen
• The most important factor in determining halogen reactivity is the strength of the carbon-halogen bond
• Bond polarity suggests that C-F would be the most reactive as it has the most polar bond
• Bond enthalpies suggest that C-I would be the most reactive as it has the lowest bond enthalpy
• A primary halogen is when the halogen atom is present at the end of the chain
• A nucleophile is an electron pair donor
• Examples of nucleophiles include: :OH-, :CN-, :NH3
• Nucleophilic substitution is a reaction where a nucleophile donates a lone pair of electrons to a δ+ carbon atom, causing a δ− atom to leave the molecule
• Hydrolysis is a reaction where water is a reactant
• Water often produces hydroxide ions for hydrolysis
• Water undergoes heterolytic fission to produce OH-
• CFCs are chlorine-fluoro-carbons, haloalkanes containing carbon, fluorine, and chlorine only (no hydrogen)
• The problem with CFCs is that although unreactive under normal conditions, they catalyze the breakdown of ozone in the atmosphere via free radical substitution
• The main function of the ozone layer is to provide protection from harmful UV radiation
• Ozone does not play a protection role in all layers of the atmosphere; in the troposphere, it contributes towards photochemical smog
• CFCs break down the ozone layer via free radical substitution
• The overall decomposition of ozone into oxygen is represented by the equation: 2O3 → 3O2
• Free radical substitution equations showing how Cl free radicals catalyze the breakdown of O3:

  • Cl2 → 2Cl• (in presence of UV light)
  • Cl• + O3 → ClO• + O2
  • ClO• + O3 → 2O2 + Cl•
  • Overall: 2O3 → 3O2