haloalkanes

• Haloalkanes


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


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

  2. Substituents are listed alphabetically
• Haloalkane classification


  • Primary
  • Secondary
  • Tertiary
• Primary haloalkane


  One carbon attached to the carbon atom adjoining the halogen
• Secondary haloalkane


  Two carbons attached to the carbon atom adjoining the halogen
• Tertiary haloalkane


  Three carbons attached to the carbon atom adjoining the halogen
• Nucleophile


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


  Swapping a halogen atom for another atom or groups of atoms
• The rate of nucleophilic substitution reactions depends on the strength of the C-X bond. The weaker the bond, the easier it is to break and the faster the reaction.</b>
• Bond enthalpy: C-I 238 kJ/mol, C-Br 276 kJ/mol, C-Cl 338 kJ/mol, C-F 484 kJ/mol
• Nucleophilic substitution with aqueous hydroxide ions

  1. Change in functional group: haloalkane alcohol

  2. Reagent: potassium (or sodium) hydroxide

  3. Conditions: In aqueous solution; Heat under reflux

  4. Mechanism: Nucleophilic substitution

  5. Type of reagent: Nucleophile, OH-
• Aqueous conditions needed for substitution with hydroxide. If solvent is changed to ethanol an elimination reaction occurs
• Comparing the rate of hydrolysis reactions

  1. Water is a poor nucleophile but it can react slowly with haloalkanes in a substitution reaction. Use reflux OR heat for more than 20 minutes

  2. Aqueous silver nitrate is added to a haloalkane and the halide leaving group combines with a silver ion to form a silver halide precipitate

  3. The quicker the precipitate is formed, the faster the substitution reaction and the more reactive the haloalkane
• The rate of nucleophilic substitution reactions depends on the strength of the C-X bond. The weaker the bond, the easier it is to break and the faster the reaction.
• Many uses of haloalkanes have now been stopped due to the toxicity of haloalkanes and also their detrimental effect on the ozone layer in the atmosphere
• Uses of haloalkanes


  • Aerosols
  • Refrigerants
  • Air-conditioning
• Alternatives to haloalkanes


  • HFCs (Hydro fluoro carbons) e.g. CH2FCF3
  • CO2 as a blowing agent for expanded polymers
• Legislation to ban the use of CFCs was supported by chemists and chemists have now developed alternative chlorine-free compounds
• Ozone layer


  Naturally occurring ozone (O3) layer in the upper atmosphere that filters out much of the sun's harmful UV radiation
• Ozone formation


  1. UV light causes an O2 molecule to split into free radicals

  2. O + O2 O3
• Ozone depletion


  1. O3 + ultraviolet light O2 + O

  2. There is a continuous cycle of formation and depletion of ozone
• The frequency of ultra-violet light absorbed equals the frequency of biologically damaging ultra-violet radiation. These reactions therefore filter out harmful UV and allow life to survive on earth.
• UV light can increase risk of skin cancer and increase crop mutation.
• Destruction of Ozone Layer

  1. Chlorine radicals are formed in the upper atmosphere when energy from ultra-violet radiation causes C–Cl bonds in chlorofluorocarbons (CFCs) to break

  2. Cl. + O3 ClO. + O2

  3. ClO. + O. O2 + Cl.

  4. Overall equation: O3 + O. 2 O2
• The chlorine free radical atoms catalyse the decomposition of ozone due to these reactions because they are regenerated. (They provide an alternative route with a lower activation energy)
• CFCs have a long lifetime in the atmosphere and it takes a long time for CFCs to reach upper atmosphere.