Chemistry AS ES CI 13.1

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  • Created by: A.B.
  • Created on: 19-02-14 12:07
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  • ES Halogenoalkanes CI 13.1
    • Naming Halogenalkanes
      • The alkane chain is prefixed with the name of the halogen.
        • Alphabetical order
        • Number indicates the position of each
    • Physical Properties of Halogenoalkanes
      • The boiling points increase with a heavier halogen atom (R-> I) or with increasing number of halogen atoms (CCL4 > CH2Cl2)
      • As the halogen introduced is larger or the number of halogen atoms increases, the overall number of electrons increases. Increases the number of ID - ID bonds, more energy needed to break.
    • Bond Enthalpies and Reactivity of Halogenoalkanes
      • The C-Hal bond becomes weaker as the size of the halogen atom increases. This makes the bond easier to break and the compounds more reactive.
      • Although the C - F bond is most polar, fluoroalkanes are relatively unreactive.
        • Bond STRENGTH rather than POLARITY has greatest effect on the reactivity of halogenoalkanes.
          • Fluoro - compounds are very unreactive
          • Chloro - compounds are reasonably stable in the troposphere and can react to produce chlorine radicals that deplete ozone.
          • Bromo - and iodo - compounds are reactive and so are useful as intermediates in chemical synthesis.
    • Reactions of Halogenoalkanes
      • Homolytic fission
        • forming radicals
        • gas phase with high temperatures, or the presence of UV radiation
      • Heterolytic fission
        • dissolved in a polar solvent such as an ethanol/water mixture
        • The C - H bond breaks to give ions. Ig the polar C-Hal bond is broken completely a negative halide ion, leaving a carbocation
      • Substitution reactions
        • The C - Hal bond breaks and the halogen is replaced by another functional group.
          • Since the halogen is replaces by a nucleophile these reactions are called nucleophilic substitution reactions.
    • Preparation of Halogenoalkanes
      • Stages in purification of the chloroalkane:
        • 1. The upper layer containing the chloroalkane is run off into a clear beaker
          • 2. the chloroalkane is shaken with a solution of sodium hydrogen carbonate to remove any acidic impurities
            • 3. The chloroalkane layer is run off for a second time
              • 4. Andhydrous sodium sulfate (a drying agent ) is added to remove any water
                • 5. The chloroalkane is purified by distillation


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