2.8 haloalkanes

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  1. chloroalkanes can be made form alkanes in a reaction known as
    free radical substitution
  2. methane + chlorine -->

    CH4 + Cl2
    chloromethane + hydrochloric acid

    CH3Cl + HCl
  3. a substitution reaction is one in which
    one atom or group of atoms on the organic molecule is replaced by another
  4. in the methane and chlorine reaction the H atom on the alkane is replaced by
    a Cl atom
  5. since all the carbon atoms in alkanes are attached to four atoms it isn't possible to add another species to an alkane unless
    another species leaves . thus addition reactions aren't possible and alkanes can undergo substitution only
  6. a free radical is a species
    which contains an unpaired electron . free radicals are very reactive species and will bond with almost anything they come into contact with
  7. free radicals are represented by
    a dot next to the atom containing an unpaired electron . they are caused by homolytic fission of covalent bond
  8. homolytic fission is the
    breaking of a covalent bond in such a way that one electron goes to each atom
  9. free radical substitution reactions proceed in three stages :
    initiation , propagation and then termination
  10. chloroalkanes and flurochloroalkanes can be used as
    solvents .
  11. one type in particular , known as chloroflurocarbons (CFC'S) , are widely used in
    aerosols and fridges
  12. CFC's are
    • haloalkanes containing chlorine and fluorine atoms but not hydrogen atoms 
    • e.g. CCL2For CClF3
  13. the small CFC's are gases and
    can escape into the atmosphere
  14. usually CFC's are very unreactive . however
    in the upper atmosphere the C-Cl bonds can undergo homolytic fission if exposed to uv light
  15. ozone (O3) is a
    naturally occurring substance found in the upper atmosphere . it plays an important role in absorbing uv radiation from the sun and preventing it from getting to the earths surface
  16. however if CFC's find their way to the upper atmosphere
    and the uv light breaks them down to chlorine radicals , these radicals can act as catalysts in the destruction of the ozone layer
  17. the process of destruction of the ozone layer can
    repeat itself indefinitely meaning even small quantities of chlorine radicals can significantly destroy the ozone layer
  18. this process has resulted in
    the formation of a hole in the ozone layer
  19. as a result of this
    chemists have supported legislation to ban CFC's completely and they have been replaced in fridges and aerosols and fridges by alternative chlorine free compounds
  20. the hole in the ozone is slowly
    mending itself
  21. the C-X bond is ... and the carbon atom is .... . therefor haloalkanes can react with ....
    • polar
    • δ+ 
    • nucleophiles
  22. a nucleophile is a species with
    a lone pair of electrons which it can use to bond with an electropositive carbon atom on an organic molceule
  23. the nucleophile generally
    replaces the halogen atom on the molecule . thus halaolkanes can undergo nucleophilic substitution reactions
  24. haloalkanes can alos undergo
    elimination reactions
  25. an elimination reaction is one in which
    the organic molecules lose two species from adjacent carbon atoms without replacement , resulting in the formation of a double bond between two carbon atoms
  26. the three nucleophiles most commonly used in nucleophilic substitutions of haloalkanes are
    hydroxide ions , OH- , cyanide ions , CN- , and ammonia , NH3
  27. haloalkanes react with hydroxide ions when
    boiled under reflux conditions with aqueous sodium hydroxide or aqueos potassium hydroxide
  28. if haloalkanes are boiled with an ethanolic solution of potassium hydoxide instead of with an aqueous solution ,
    they will undergo elimination of HX molecule to give an alkene
  29. sodium hydroxide isn't used because
    it is sparingly soluble in ethanol
  30. the reaction works best if
    distillation apparatus is used since the alkene product is volatile
  31. the rate of substitution or elimination of a haloalkane depends on
    the ease with which the C-X bond can b broken . this depends on the strength of the C-X bond , which in turn depends on the length of the bond
  32. since the C-F bond is very short
    it is very strong and difficult to break . thus fluroalkanes react very slowly
  33. the C-Cl bond is longer and weaker than the   C-F bond and the C-X bonds become progressively .... and .... descending the group
    • longer 
    • weaker
  34. thus the ..... is the longest and weakest and easiest to break thus ........ react the most quickly
    • C-I
    • iodoalkanes
  35. thus the rates of reactions decrease in the order
    iodoalkanes > bromoalkanes > chloroalkanes > fluroalkanes
  36. the relative importance of substitution and elimination  depends on several factors
    • structure of the haloalkane 
    • base strength of the nucleophile 
    • reaction conditions
  37. primary haloalkanes (RCH2X) give predominately
    substitution products
  38. tertiary haloalkanes (R3CX) generally favour
  39. with secondary haloakanes (R2CHX)
    both substitution and elimination take place at the same time (concurrently)
  40. draw primary , secondary and tertiary haloalkanes
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  41. the likelihood of elimination increases as the
    base strength of the nucleophile increases
  42. higher reaction temperatures lead to a greater proportion of
  43. in the reaction of 2-bromopropane with potassium hydroxide both elimination and substitution occur together :
    • elimination is favoured by hot ethanolic conditions 
    • substitution is favoured by warm aqueous condition
  44. haloalkane --> alcohol
    • reagent : NaOH(aq) or KOH(aq) 
    • conditions : warm under reflux 
    • mechanism : nucleophilic substitution 
    • role of hydroxide ion : nucleophile
  45. haloalkane --> nitrile
    • reagent : KCN in aqueous ethanol
    • conditions : boil under reflux 
    • mechanism : nucleophilic substitution
  46. haloalkane --> amine
    • reagent : ammonia in ethanol in sealed tube 
    • conditions : heat 
    • mechanism : nucleophilic substitution
  47. haloalkane --> alkene
    • reagent : KOH in ethanol
    • conditions : heat 
    • equations :hydroxide ion + haloalkane ---> alkene + halide ion + water 
    • mechanism : elimination 
    • role of hydroxide ion : base

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2.8 haloalkanes
2014-05-27 07:03:37
chemistry unit two

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