Haloalkanes

• Haloalkanes


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


  • Primary: One carbon attached to the carbon atom adjoining the halogen
  • Secondary: Two carbons attached to the carbon atom adjoining the halogen
  • Tertiary: 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
• Strength of C-X bond


  Determines rate of substitution reaction - the weaker the bond, the easier it is to break and the faster the reaction
• Bond enthalpy order: C-F > C-Cl > C-Br > C-I
• Nucleophilic substitution mechanism


  1. Nucleophile attacks positive carbon atom

  2. Carbon has small positive charge due to electronegativity difference

  3. Curly arrows show movement of electron pairs
• Nucleophilic substitution with aqueous hydroxide ions


  1. Reagent: potassium (or sodium) hydroxide

  2. Conditions: In aqueous solution; Heat under reflux

  3. Mechanism: Nucleophilic substitution

  4. Type of reagent: Nucleophile, OH-

  5. Change in functional group: haloalkane alcohol
• Hydrolysis rate of haloalkanes


  Depends on strength of C-X bond - the weaker the bond, the faster the hydrolysis
• Aqueous silver nitrate test can compare reactivity of haloalkanes
• CFC's were developed for use as aerosols, refrigerants, and in air-conditioning due their low reactivity, volatility and non-toxicity
• CFC's are now banned due to toxicity and detrimental effect on ozone layer
• HFCs (Hydro fluoro carbons)


  Safer alternatives to CFC's as they do not contain the C-Cl bond
• Ozone formation


  1. UV light causes O2 to split into free radicals

  2. Free radicals combine with O2 to form ozone
• Ozone depletion


  1. UV light causes ozone to split into O2 and free radicals

  2. Chlorine and nitrogen radicals catalyse the breakdown of ozone
• Ozone layer filters out harmful UV radiation
• Legislation to ban CFC's was supported by chemists who developed alternative chlorine-free compounds