F322: Haloalkanes

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  • Contain a halogen; F, Cl, Br, I, etc.
  • One or more H's have been replaced by a halogen.
  • General formula: CnH2n+1X (X = halogen).
  • Halo-part of the name comes first (i.e. 2-bromobutane).
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Classification Of Haloalkanes

  • Can be classed as Primary, Secondary or Tertiary.
    Primary = The halogen is attached to a C, which is only attached to one other C.
    Secondary = The halogen is attached to a C, which is attached to two other C's.
    Tertiary = The halogen is attachd to a C, which is attached to three other C's.
        PRIMARY^                 SECONDARY^              TERTIARY^ 
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Reactions: Nucleophilic Substitution

  • "Replacement of a halogen by a nucleophile (a lone pair donor)". (i.e. H2O, NH3, OH-)
  • The C-X bond is polar as the halogen has greater electronegativity.
  • The C atom will be attacked by a species with a lone pair of electrons to donate, and then the nucleophile repleaces the halogen atom.
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Reactions: Hydrolysis (Nuc.Sub.)

  • "Reaction with water that breaks a chemical compound into two chemical compounds".
    Reagent = NaOH (aq)
    Conditions = Reflux, in water solvent.
    Equation = C2H5Cl + NaOH -> C2H5OH + NaCl (Produces an alcohol).
  • Marks: Curly arrows, equation, dipoles, lone pairs.
  • The nucleophile is the OH- ion, which attacks the C+ dipole, the C-X bond breaks and the halogen atom becomes a halide ion. (i.e. Cl-); a new covalent bond forms between the C atom and the nucleophile, the Br is SUBSTITUTED by the OH.
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Rate Of Hydrolysis (Primary)

  • Hydrolysis does not occur at the same rate for all the haloalkanes.
  • The lost X- ion can be identified by using AgNO3(aq) in ethanol.
    Ag+ + X- -> AgX (precipitate)
  • Method:
    -Place a measured sample of each haloalkane into test tubes in water baths of 50 degrees.
    -In another test tube place a solution of ethanol, water and AgNO3; into the same water bath. 
    -Once at equal temperature, add equal volumes of ethanol mixture and haloalkane solution.
    -Time how long it takes for the precipitate to form.
  • Chloro-    White (AgCl)        2.5 mins         
    Bromo-    Cream (AgBr)      1.5 mins          
    Iodo-       Yellow (AgI)         1 min              
  • So Iodo- is the fastest and Chloro- the slowest.
  • This is because the bond enthalpy affects the rate of hydrolysis; as the bond enthalpy decreases it is easier to break the C-X bond; the stronger the bond the slower and harder it is to break the C-X bond. (So C-I bond is weakest).
  • SO bond enthalpy takes precidence over polarity of the halogen. 
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Uses Of Haloalkanes

  • CFC's are useful because they are non-toxic, inert gases, and volatile.
    E.g. Trichlorofluoromethane
  • These are used for refrigeration, aerosols, air conditioning and solvents.
  • Haloalkanes can also be used in to the production of plastics.
    E.g. Chlorethane is used to produce PVC.
    E.g. Tetrfluoroethane makes polyPTFE; used to produce non-stick on pans. 
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Problems With CFC's

  • CFC's are very stable and break down the ozone layer (O3); the CFC's affect the layer of the stratosphere and stops it working as a barrier against UV-B radiation (skin cancer).
  • They are inert but broken down by UV light at the stratosphere.
  • Mechanism:
    Initiation - CFCl3 -> Cl. + .CFCl2
    Propogation - Cl. + O3 -> ClO. + O2
    Termination - Cl. + Cl. -> Cl2
  • Overall = O3 + O -> 2O2

  • Alternatives are more commonly used; such as HFC's, HCFC's and alkanes.
  • These "ozone friendly" alternative break down rapidly before the ozone layer.
    HFC's = refrigent, but still cause the greenhouse affect.
    HCFC's = refrigent, still break down ozone layer at 1/10 rate.
    HC's = aerosol propellent, but flammable.  
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