halogenoalkanes

Preparation & Purification of a HALOGENOALKANE

Step One: Preparation

React an ALCOHOL with CONCENTRATED HYDROCHLORIC ACID (HCl)
this is a SUBSTITUTION reaction: Cl replaces the OH group in the alcohol.  

eg. (CH₃)₃C-OH + HCl →   (CH₃)₃C-Cl + H₂O

This should be carried out in a separating funnel; keep releasing the pressure, because the product is volatile- it evaporates easily, which raises the pressure in the funnel)

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Step Two: Separation

The halogenoalkane produced is INSOLUBLE, so it can be separated from the aqueous reaction mixture using a SEPARATING FUNNEL after it has been allowed to separate into layers. 
Run off the AQUEOUS lower layer, leaving the IMPURE HALOGENOALKANE.  

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Step Three: Purification


In order to get a pure halogenoalkane, 3 things must be removed: excess acid, water and alcohol

a) Shake the halogenoalkane with NaHCO₃(aq) (SODIUM HYDROGENCARBONATE SOLUTION), to remove any ACIDIC IMPURITIES

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b) Shake the halogenoalkane with WATER to remove SOLUBLE IONIC IMPURITIES. 

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c) Remove traces of WATER by DRYING with ANHYDROUS SODIUM SULPHATE (a drying agent)

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d) DISTIL the solution: collect any liquid that distils over the BOILING POINT of the DESIRED PRODUCT. This separates the HALOGENOALKANE from other volatile liquids (such as unreacted alcohol), confirming its identity. 

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AS Flash Cards: HalogenoalkAnes
  • Halogenoalkanes are formed when a hydrocarbon has one or more hydrogen atoms substituted for halogen atoms.
  • Halogenoalkanes may be hydrolysed to form alcohols, using hot sodium hydroxide solution.
  • The hydroxide ion acts as a nucleophile when reacting with a halogenoalkane. A nucleophile is a lone pair electron donor.
  • The C–I bond is weaker than the C–Br and C–Cl bond, and so it breaks faster when attacked by nucleophiles.

USES

  • Chloroethene and tetrafluoroethene are used to make the polymers PVC and PTFE (Teflon) respectively.
  • CFCs (chlorofluorocarbons) were manufactured for use in aerosols and refrigerants because of their low reactivity and high volatility.
  • CFCs have caused the ozone layer to break down, leading to an increased intensity in damaging ultraviolet radiation on Earth.
  • Biodegradable CFCs and HCFCs are now used as alternatives to CFCs.
Halogenoalkanes

Making Halogenoalkanes

alcohol + hydrogen halide —> halogenoalkane + water

                   

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  • this is a substitution reaction where the halide replaces the -OH
  • reagents for iodohalogenoalkanes are potassium iodide and phosphoric acid H3PO4. 
  • reagents for bromohalogenoalkanes are potassium bromide and sulfuric acid.
  • the the halogenoalkane is collected by distillation under ice-cold water
  • for chlorohalogenoalkanes, you only need to shake for 20 minutes and the liquid chloroalkane will form in a separate layer

    Reactions of Halogenoalkanes (substitution)

    • heat under reflux  in aqueous solution
      halogenoalkane + (KOH or NaOH) —> alcohol + (Potassium/ sodium halide)
    • heat under reflux in aqueous ethanolic solution
      halogenoalkane + KCN —> cyanide + (potassium halide) 
    • heat ammonia in ethanol in a sealed tube
      halogenoalkane + ammonia —>  amine + (Hydrogen halide)

    these reaction happen because the halide atom is more electronegtive than the carbon atom, so the bond between them is polar, causing the C atom to become a  slightly positive electophile that is prone to attack by nucleophiles.

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    Reactions of Halgenoalkanes (elimination)

    • heat under reflux in ethanolic solution
      halogenoalkane + KOH —> alkene + water + (potassium halide)

    Rates of reaction of Halogenoalkanes

    use hot aqueous silver nitrate solution to investigate this. the time taken for the silver halide ppt to first appear indicates the rate of reaction.

    the flouro compounds will react slowest because their bonds are the strongest and so hardest to break due to the big difference in electronegativity. so the reactions of halides become faster down the group.

    also, the more methyl groups you have, the faster the reaction will be because this makes the difference in the electronegativity of the carbon-halogen bond, less significant, so easier to break.

    Uses of Halogenoalkanes

    • used as fire retardants because they are unreactive and do not burn easily.
    • Hydroflourocarbond (HFCs) are used instead of cloroflourocarbons (CFCs) as they do not release free radicals that deplete the ozone layer. the depletion of the ozone layer is harmful as it enables UV radiation to reach the earth which increases the risks of skin cancer and eye cataracts.
      The radical substitution of an alkane with chlorine

      1) Inititation - first, the radicals have to be made. * represents a radical.
      Cl2 (+UV radiation) —> 2Cl*    this is homolytic fission  

      2) Propagation - this is where the radical substitution takes place. 
      CH4 + Cl* —> CH3* + HCl
      CH3* + Cl2 —> CH3Cl + Cl*
      Remember for two step propagations like this, or the decomposition of ozone, the catalyst has to come out the other side for the reactions to keep happening. If it doesn’t…you’ve gone wrong :P

      3) Termination - this is how the reaction stops: the radicals are “mopped up”
      Cl* + Cl* —> Cl2   (ie you turn off UV and it stops breaking down)
      CH3* + CH3* —-> C2H6  (ethane) 
      CH3* + Cl* —> CH3Cl (chloromethane) 

      As you can see, ethane is produced, as well as some other chemicals like CH2Cl2. This kind of radical substitution is difficult to control.