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 

• 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. 

-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.
  
  

• 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 

• 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.

• 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. 

• Problems:
  -Non-biodigradable.
  -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.