Halogenoalkanes

• Halogenoalkanes


  Alkanes with one or more halogen atoms (F, Cl, Br, I) attached to the carbon chain
• Halogen prefixes


  • Fluoro for F
  • Chloro for Cl
  • Bromo for Br
  • Iodo for I
• Halogenoalkanes


  • 1-bromopropane
  • 2-chloro-2-methylbutane
• Halogenoalkanes


  • 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 of halogenoalkanes

  1. Nucleophile attacks positive carbon

  2. Halogen leaves as X-

  3. New group bonds to carbon
• The rate of nucleophilic substitution reactions depends on the strength of the C-X bond. The weaker the bond, the faster the reaction.</b>
• Bond enthalpy order: C-I < C-Br < C-Cl < C-F
• Iodoalkanes are the fastest to substitute, fluoroalkanes are the slowest
• Aqueous silver nitrate can be used to compare the reactivity of halogenoalkanes
• Nucleophilic substitution with aqueous hydroxide ions

  Halogenoalkane + KOH -> Alcohol + KBr
• Tertiary halogenoalkanes undergo a different mechanism for nucleophilic substitution with hydroxide
• Nucleophilic substitution with cyanide ions

  Halogenoalkane + KCN -> Nitrile + KBr
• Nucleophilic substitution with ammonia

  Halogenoalkane + 2NH3 -> Amine + NH4Br
• Nitriles


  Functional group with C≡N, must be at the end of the carbon chain
• Elimination reaction of halogenoalkanes with alcoholic hydroxide

  Halogenoalkane + KOH -> Alkene + KBr + H2O
• Tertiary halogenoalkanes tend towards elimination, primary towards substitution
• Ozone in upper atmosphere

  • Beneficial as it filters out harmful UV radiation
• Ozone in lower atmosphere

  • Pollutant that contributes to smog formation
• Ozone depletion by CFCs

  1. CFCs -> Cl radicals

  2. Cl radicals catalyse decomposition of ozone
• Legislation banned CFCs, HFCs developed as safer alternatives
• Uses of halogenoalkanes


  • Solvents
  • Refrigerants
  • Pesticides
  • Aerosol propellants
• Many uses of halogenoalkanes stopped due to toxicity and environmental impact