Haloalkanes

• Haloalkanes


  Organic compounds containing a halogen atom (F, Cl, Br, I) attached to an alkane
• Naming haloalkanes


  Based on original alkane, with a prefix indicating halogen atom: Fluoro for F; Chloro for Cl; Bromo for Br; Iodo for I
• Haloalkanes


  • 1-bromopropane
  • 2-chloro-2-methylbutane
• Haloalkanes


  • Can be classified as primary, secondary or tertiary depending on the number of carbon atoms attached to the C-X functional group
• Nucleophilic substitution reactions


  1. Nucleophile attacks positive carbon atom

  2. Depends on strength of C-X bond

  3. Weaker bond = faster reaction
• Nucleophile


  Electron pair donator e.g. :OH-, :NH3, CN-
• Substitution


  Swapping a halogen atom for another atom or groups of atoms
• Nucleophilic substitution with aqueous hydroxide ions

  1. Haloalkane + KOH -> Alcohol + KX

  2. Mechanism: Nucleophilic substitution
• Aqueous conditions needed


  If solvent changed to ethanol, elimination reaction occurs
• Comparing the rate of hydrolysis reactions

  1. Aqueous silver nitrate added to haloalkane

  2. Halide leaving group forms silver halide precipitate

  3. Faster precipitate = faster substitution reaction
• The rate of these substitution reactions depends on the strength of the C-X bond
• The weaker the C-X bond, the easier it is to break and the faster the reaction
• Many uses of haloalkanes have been stopped due to toxicity and effect on ozone layer
• Uses of haloalkanes


  • Aerosols
  • Refrigerants
  • Air-conditioning
• CFCs


  Chlorofluorocarbons - developed for uses like aerosols, refrigerants, and air-conditioning due to low reactivity, volatility and non-toxicity
• HFCs


  Hydrofluorocarbons - now used for refrigerators and air-conditioners, safer as they do not contain C-Cl bond
• Legislation to ban use of CFCs was supported by chemists who developed alternative chlorine-free compounds
• Ozone layer


  Naturally occurring ozone (O3) layer in upper atmosphere filters out harmful UV radiation
• Ozone formation


  1. UV light causes O2 to split into free radicals

  2. Free radicals form O3
• Ozone depletion


  1. O3 + UV light -> O2 + O

  2. Chlorine radicals catalyse decomposition of O3
• Chlorine radicals from CFCs contribute to formation of hole in ozone layer
• CFCs still a concern as they are still entering atmosphere from disused items and used by some countries