Halogenalkanes and Green Chemistry

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Halogenalkanes

  • Halogenalkanes are made by substituting the -OH group in an alcohol for a halogen atom.
  • This can be done using hydrogen halides (HX) or using phosphorus halides
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Halogenalkanes (cont)

  • When using hydrogen halides to form halogenalkanes, the general formula is: ROH + HX = RX + H20
  • The way in which we get each HX that we use for a specific halide is different, and depends on the halide.
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Halogenalkanes (cont)

  • Alcohol + hydrochloric acid gives a chloroalkane + water.
  • However, this only works well with tertiary alcohols, because the reaction rate is too slow with primary or secondary alcohols (use other method instead).

         (http://www.chemguide.co.uk/organicprops/haloalkanes/padding.gif)(http://www.chemguide.co.uk/organicprops/haloalkanes/rohhcleqn.gif)

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Halogenalkanes (cont)

  • Alcohol + hydrogen bromide gives bromoalkane + water.
  • However, we do not use hydrobromic acid to give HBr.
  • Instead, we treat the alcohol with a mixture of potassium bromide and sulphuric acid. This produces HBr which reacts with the alcohol. 
  • The first step does not appear in the overall formation of bromoalkane equation. 

                 (http://www.chemguide.co.uk/organicprops/haloalkanes/rohhbreqn.gif)

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Halogenalkanes (cont)

  • Alcohol + hydrogen iodide gives iodoalkane + water.
  • The alcohol is treated with a mixture of potassium iodide and phosphoric (V) acid, which produces the HI that reacts with the alcohol.
  • Phosphoric (V) acid is used instead of conc sulphuric acid because sulphuric acid oxidises the iodide ions and so produces hardly any hygrogen iodide

                 (http://www.chemguide.co.uk/organicprops/haloalkanes/rohhieqn.gif)

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Halogenalkanes (cont)

  • Alcohol + phosphorus (III) chloride (liquid) gives chloroalkane + H3PO3.
  • (http://www.chemguide.co.uk/organicprops/haloalkanes/padding.gif)(http://www.chemguide.co.uk/organicprops/haloalkanes/rohpcl3eqn.gif) 
  • Alcohol + phosphorus (V) chloride (solid) gives chloroalkane + HCl + POCl3.
  • This is a violent reaction producing steamy fumes of HCl, which  is very visible.
  • This is why we use  phosphorus (V) chloride as a test for the -OH group. 
  •      (http://www.chemguide.co.uk/organicprops/haloalkanes/rohpcl5eqn.gif)
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Halogenalkanes (cont)

  • Bromoalkanes and iodoalkanes are made using the same steps with phosphorus. 
  • The alcohol is heated with the halogen and red phosphorus under reflux.
  • The halogen reacts with the red phosphorus to produce the phosphorus (III) halide.
  • The phosphorus (III) halide then reacts with the alcohol to give the halogenalkane and H3PO3.
  • (http://www.chemguide.co.uk/organicprops/haloalkanes/padding.gif)(http://www.chemguide.co.uk/organicprops/haloalkanes/pbr3eqn.gif)

    (http://www.chemguide.co.uk/organicprops/haloalkanes/rohpbr3eqn.gif)

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Halogenalkanes (cont)

  • When we make bromoethane (from KBr, H2SO4 and ethanol), there will be impurities such as HBr, Bromine, sulphur dioxide and unreacted ethanol.
  • These impurities are removed in 5 stages.
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Halogenalkanes (cont)

  • Pour the impure bromoethane into a separating funnel, add some water and shake. A bromoethane layer will form on the bottom with an aqeuous layer on top. Discard the aqueous layer, which contains all the soluble impurities.
  • Add sodium carbonate/hydrogencarbonate solution to the bromoethane, which will react with any acidic impurites.Separate and remove the bromoethane layer as above. 
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Halogenalkanes (cont)

  • Wash the bromoethane with water in the separating funnel to remove inorganic impurities (excess sodium carbonate). Transfer remaining lower bromoethane layer to a dry test tube.
  • Add anydrous calcium chloride to the tube, shake well and leave to stand. This removes any excess water and ethanol.
  • Fractionally distil the bromoethane, collecting at 35'C.
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Halogenalkanes (cont)

                    (http://www.chemguide.co.uk/organicprops/haloalkanes/purify1.gif)

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Halogenalkanes (cont)

  • When a halogenalkane reacts with sodium/potassium hydroxide, a substitution or an elimination reaction will occur.
  • A subsitution reaction forms an alcohol + sodium/potassium halide.
  • An elimination reaction forms an alkene + sodium/potassium halide + water.
  • If the halogenalkane is primary it is more likely to undergo substitution, if it is tertiary, it is far more likely to be an elimination reaction (almost exclusively). 
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Halogenalkanes (cont)

  • The proportion of water to ethanol in the KOH/NaOH effects which reaction will occur.
  • Water encourages substitution, ethanol encourages elimination
  • Higher temperatures favour elimination.
  • Concentrated KOH/NaOH favours elimination, dilute favours substitution.
  • Both elimination and substitution require heat and reflux
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Halogenalkanes (cont)

substitution:(http://www.chemguide.co.uk/organicprops/haloalkanes/padding.gif)(http://www.chemguide.co.uk/organicprops/haloalkanes/substprop.gif) 

Elimination: (http://www.chemguide.co.uk/organicprops/haloalkanes/elimprop.gif)

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Halogenalkanes (cont)

  • Halogenalkane + ammonia gives amine + ammonium halide.
  • The ammonia is concentrated and dissolved in ethanol.
  • The reactants are heated with pressure, no reflux because the ammonia would escape through the condenser as a gas
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Halogenalkanes (cont)

  • The reactivity of a halogenalkane depends on the halogen present, and the number of methyl groups attached to the carbon attached to the halogen (primary, secondary or tertiary).
  • The weakest carbon-halogen bond breaks more easily and so is more reactive. 
  • The further the two atoms are from eachother (atomic radii), the weaker the bond will be. So C-I is the weakest bond.
  • Tertiary is the most reactive, because more methyl groups weaken the carbon-halogen bond. 
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Halogenalkanes (cont)

  • Halogenalkanes are generally unreactive, have low boiling points and are non-flammable, so they are used as fire-retardants and refrigerants.
  • Chlorofluorocarbons (CFCs) were found to deplete the ozone layer in the atmosphere, allowing harmful UV radiation to reach the earth surface, increasing the risk of skin cancer and eye cateracts
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