Haloalkanes

• Haloalkanes


  Saturated organic compounds that contain carbon atoms and at least one halogen atom
• Haloalkanes are insoluble in water
• Polarity of haloalkanes


  Polar, as halogen has a higher electronegativity than carbon (halogen is δ-, carbon is δ+)
• Intermolecular forces in haloalkanes

  • Permanent dipole-dipole and London forces of attraction
  • C-X bond polarity creates permanent dipoles
• Factors affecting boiling point of haloalkanes

  • Increase in carbon chain length
  • Halogen further down group 7
• Mass of haloalkane vs alkane of same chain length

  Greater, as mass of halogen > mass of H
• Determining halogen reactivity


  The strength of carbon-halogen bond
• Order of reactivity based on bond polarity

  • C-F most reactive as most polar bond
• Order of reactivity based on bond enthalpy

  • C-I most reactive as lowest bond enthalpy
• Primary halogen


  Halogen atom present at the end of the chain
• Nucleophile


  Electron pair donor
• Nucleophiles


  • :OH-
  • :CN-
  • :NH3
• Nucleophilic substitution


  A reaction where a nucleophile donates a lone pair of electrons to δ+ C atom, δ− atom leaves molecule (replaced by nucleophiles)
• Hydrolysis


  A reaction where water is a reactant
• Water often produces hydroxide ions for hydrolysis
• Fission of water to produce OH-

  Heterolytic fission
• CFCs


  Chlorine-fluoro-carbons - haloalkanes containing C, F and Cl only (no H)
• Problem with CFCs


  • Although unreactive under normal conditions, they catalyse the breakdown of ozone in the atmosphere via free radical substitution
• Function of ozone layer

  Provides protection from harmful UV radiation
• Ozone does not play a protection role in all layers of the atmosphere
• How CFCs break down ozone layer


  Free radical substitution
• Equation for overall decomposition of ozone into oxygen (O2)

  2O3 → 3O2
• Free radical substitution equations showing how Cl free radicals catalyse the decomposition of O3


  1. Cl2 → 2Cl• (in presence of UV light)

  2. Cl• + O3→ ClO• + O2

  3. ClO• + O3 → 2O2 + Cl•

  4. Overall: 2O3 → 3O2