Chemistry Halogenoalkanes and reactions

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oliviaobs
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147723
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Chemistry Halogenoalkanes and reactions
Updated:
2012-04-15 11:38:16
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Halogenoalkanes reactions
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Halogenoalkanes and reactions
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  1. Background
    • All halogens contain at least one halogen (X) attached to a carbon atom
    • If they contain just one halogen atom and there are no multiple bonds the general formula is CnH2n+1X
    • The C-X bond is polarised which makes the carbon atom electron-deficent and open to attack by nucleophiles (e.g. water OH- ions from KOH)
    • Becuase the halide ion is stable it is substituted easily by the nucleophile
  2. Strength of the C-X bond. Strongest first
    • C-F
    • C-Cl
    • C-Br
    • C-I
  3. Reactivity of the halogenoalkanes. Most reactive first
    • C-I
    • C-Br
    • C-Cl
    • C-F
  4. Reactivity of the halogenoalkanes
    • It depends on the breaking of the C-X bond
    • As the C-F bond is the hardest to break the fluoroalkanes are the least reactive
  5. Types of halogenoalkanes
    • Primary - On the carbon with the halogen there are two hydrogen atoms
    • Secondary - Pn the carbon with the halogen there is one hydrogen
    • Tertiary - On the carbon with the halogen there are no hydrogen
  6. Reactions of the halogenoalkanes
    • The electon-deficient carbon in the carbon-halogen bond can be attacked by nucleophiles with their lone-pair electrons
    • The carbon cannot form more than four bonds so the nucleophile forms a covalent bond with the carbon
    • This causes the halogen to leave as X- (e.g. Br-) ion
  7. Why are fluoroalkanes never used for synthesis of carbon compounds?
    Synthesis relies on the halogen being replaced by another group and therefore it needs the C-F bond to break. This bond is very strong so it would take a long time.
  8. Why are bromoalkanes used in syntheses in preference to iodoalkanes?
    • The C-I bond is too weak so iodoalkanes often react with contaminents before than van be used in a reaction.
    • Bromoalkanes are not so reactive that they are unstable or react too easily
  9. What is a Nucleophile?
    It is a molecule or ion that can donate a pair of electrons. It is often represented as Nu- or Nu:-

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