F322: Alkenes



  • Alkenes and cyclohexenes are unsaturated, containing at least one C=C bond.
  • General Formula = CnHn.
  • General Formula of cyclohexenes = CnHn-2.
    E.g. Pentene = C5H10
           Cyclohexene = C6H10 
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  • Alkanes = All sigma bonds, a tetrahedral shape with 109.5 degrees bond angle.
  • Alkenes = Sigma and a PI bond; this pi bond is formed by from the overlap of sigma bonds.
    -These have a bond angle of 120 degrees, and the shape is triginal planar.
  • So alkenes are more reactive than alkanes due to the high electronegativity from the overlap. 
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Reactions Of Alkenes

-All addition reactions so 100% atom economy.

  • Hydrogenation:  
    -Reagent: H2.
    -Conditions: Nickel catalyst, 150 degrees and 1000 kPa.
    -Industrial importance: Making of margarine/vegetable oils. 
  • Bromination:
    -Reagent: Br2 (aq).
    -Conditions: Add dropwise and shakeee.
    -Mechanism: Electrophillic addition.
    -Use: To test for presence of C=C; adding Br and then observing orange to colourless.

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Reactions Of Alkenes (Cont.)

  • Adding HBr:
    -Reagent: HBr.
    -Conditions: Room temperature.
    -Mechanism: Electrophilic addition.
    E.g. C2H4 + HBr -> C2H5Br
  • Hydration: 
    -Reagent: Steam (H2O [g])
    -Conditions: H3PO4 catalyst, high pressure and temperature.
    -Mechanism: Electrophilic addition.
    -Industrial importance: Production of alcohols. 
    E.g. C2H4 + H2O -> C2H5OH 
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Reaction Mechanism

  • The mechanism of electrophilic addition:
    (Electrophile is a species that accepts a pair of electrons)

    Mark points: Dipole (Not shown here -.- )
                       Curly Arrows
                       Charges (Not shown)
                       Lone pairs 
  • Heterolytic bond breaking where a carbocation is formed in an intermediate stage.
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  • The production of a saturated polymer from many unsaturated monomers.
  • Uses of polymers:
    -Polythene: Plastic bags
    -Polychloroethene: Window frames.
    -Polyphenylethene: Packaging and insulation.
    -Polypropene: Food packaging. 
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Polymer Disposal

  • Problems:
    -Toxic fumes produced when burnt.
    -Contain poisonous additives.
  • Disposal Options:
    -Sort and recycle: 1. Sorting using codes, E.g. PET must be removed from HDPE.
                               2. Reclamation where chopped into flakes and melted/remoulded.
    -Combustion: Can provide energy but produce poisonous gases.
    -Cracking into feedstock.
  • Biogradable polymers made from renewable raw materials that are compostable (E.g. Maize).
    -Used in cutlery and compostable biobags. 
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This helps extremely! Thank you!

Joanne Rose


"Add dropwise and shakeee" - that made me laugh! Great notes though :) Thanks

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