The formation of haloalkanes

alkane(g) + halogen(l) -- ultraviolet light --> haloalkane(l) + HX(g)
1. free-radical substitution reaction
2. ultraviolet starts the reaction
if we look at CH4(g)+Cl2(g)-->CH3Cl(g)+HCl(g)
1. chain reaction
  1. initiation
    1. breaking of the Cl-Cl bond, forms two Cl atoms
      1. Cl molecules absorbs the energy of a single quatum of ultraviolet light
        so bond breaks
        as atoms are the same, one electro goes to each
        Cl-Cl --ultraviolet--> 2Cl.
        free-radicals are highly reactive
        dot shows unpaired electrons
        C-H bond does not break, as needs more energy
  2. termination
    1. this is where the free radicals are removed
      1. can happen in 3 ways
        Cl. +Cl.--> Cl2
        get chlorine
        .CH3 + .CH3 --> C2H6
        get ethane
        Cl.+ .CH3 --> CH3Cl
        get chloronmethane
  3. propagation
    1. two stages
      1. first stage
        Cl. free radical takes a hydrogen atom from methane to form HCL, leaving a methyl free radical .CH3
        Cl. +CH4 --> HCl + .CH3
      2. second stage
        methyl free radical is also very reactive
        reacts with a Cl molecule
        produces another Cl. free radical and chlormethane
        .CH3 + Cl2 ---> CH3Cl + Cl.
    2. effect of these two steps is to produce
      1. HCl
      2. Chloromethane
      3. Cl.
other products of the chain reaction
1. dichloromethane
  1. made at propagation
    1. if a chlorine radical reacts with some chloromethane that as already been formed
      1. CH3Cl + Cl. --> .CH2Cl + HCl
        followed by
        .CH3Cl + Cl2 --> CH2Cl2 + Cl.
2. with longer chain alkanes
  1. there will be many isomers formed
    1. because Cl. can replace any of the hydrogen atoms
Why are chain reactions important
1. CFC's are destroying the ozone layer
  1. ozone = O3
    1. it decomposes oxygen
      1. ozone is important because
        it protects the earth from the harmful exposure of too many UV rays
        too much UV can cause skin cancer
2. Cl free radicals are formed from CFC's
  1. these attacks ozone molecules
    1. Cl. + O3 --> ClO. + O2
      1. ClO. + O3 --> 2O2 + Cl.
        add two equations together, the Cl free radical is terminated
        acts as a catalyst in the breakdown of ozone to oxygen
        2O3 ---> 3O2

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The formation of haloalkanes

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	Created by jasmin.sahota almost 11 years ago