AS Chemistry A OCR - Alkenes

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  • Created by: Shipreck
  • Created on: 11-02-16 10:54

Properties of Alkenes.

  • Alkenes are unsaturated (Contain at least one multiple bond) hydrocarbons (contain only hydrogen and carbon)
  • Alkenes contain a C=C double bond.
    • Double bond is made up of a π bond and a σ bond.
      • π bond is the sideways overlap of adjacent P orbitals above and below the bonding atoms.
      • π bonds have restricted rotation.
      • σ bond is the overlap of orbitals directly between the two nuclei. 
  • Alkenes are trigonal planer around one of the carbons of the double bond.
    • 3 bonding regions, 4 bonding pairs, no lone pairs.
    • Bond angle of 120 degrees.
  • Alkenes are relatively reactive.
    • π bond has low bond enthalpy.
    • double bond has high electron density that attracts electrophiles. (electron pair acceptors)
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  • Stereoisomerism
    • Compounds with the same structural formula, but a different arrangement in space.
  • E/Z isomerism.
    • Only happens when there are  2 different species on each of the carbon atoms in a double bond.
    • The restricted rotation means that if two bonds are facing opposite, they can't spin to allign.
    • E is when the two species with the highest total atomic number are facing opposite directions.
    • Z is when the two species with the highest total atomic number are facing the same direction.
  • Cis/Trans Isomerism.
    • When there is one of the same species on each of the carbons of the double bond.
    • Cis is when they are on the same side.
    • Trans is when they are on the opposite side. 
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Electrophilic Addition Reaction

  • Addition of Dihydrogen (H2).
    • Reagents: H2
    • Conditions: Presence of Nickel catalyst
    • Product: Alkane
  • Addition of Halogen (Cl2, Br2, etc)
    • Reagents: Dihalogen
    • Conditions: None
    • Product: Dihaloalkane
    • Use: Bromine Water (aqueous Br2) is used to check if a sample is unsaturated.
      • Colour Change: Orange/Brown to Colourless.
  • Addition of Hydrogen Halide (HBr, HCl, etc)
    • Reagents: Hydrogen Halide
    • Conditions: None
    • Product: Haloalkane
  • Addition of H2O
    • Reagents: H2O
    • Conditions: H3PO4 Catalyst, high temperature
    • Product: Alcohol
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Electrophilic Addition Reaction Mechanism

  • Halogen
    • Br2 is not polar. 
    • Br2 becomes polar because of electron repulsion by the double bond.
    • One atom of Br is bonded to the Alkene and the other becomes a negative ion.
    • The negative ion bonds to the carbocation.

  • Hydrogen Halide
    • HBr is polar
    • The slightly positive H atom is attracted to the double bond and reacts with it. 
    • The slightly negative Br atom becomes an negative Ion. 
    • The negative Br ion bonds with the carbocation.


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Markownikoff's rule.

  • Types of stability
    • Primary 
      • The carbon that has the positive charge is connected to one other carbon.
      • The least stable.
    • Secondary
      • The carbon that has the positive charge is connected to two other carbons.
    • Tertiary
      • The carbon that has the positive charge is connected to three other carbons.
      • The most stable.
  • Markownikoff’s rule is that the major organic product of a reaction will be the one with the most stable carbocation step.
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  • Monomer
    • A single alkene that will be made into a polymer.
  • Polymer
    • Lots of monomers repeating by the double bond.
  • Repeating unit
    • What one molecule of an alkene looks like as part of the polymer.

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Recycling Polymers

  • The benifits of cheap oil based polymers is counteracted by the problems they cause the enviroment in landfills.
  • Combustion
    • Burnt to create energy.
  • Used as a feedstock to create other plastic items.
    • The plastic is cleaned, melted down, and recasted into a new item.
  • Removal of toxic waste products from incinerated halogenated plastics.
    • A base or carbonate is added to the incinerator chimney.
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