Alkenes

• General formula is CnH2n
• Alkenes are unsaturated hydrocarbons
• C=C double covalent bond consists of one sigma (σ) bond and one pi (π) bond.

  A) Pi bond

  B) Sigma bond

  
• π bonds are exposed and have high electron density.
• Electrophile: an electron pair acceptor
• The double bonds in alkenes are areas with high electron density. This attracts electrophiles and the alkenes undergo addition reactions.
• Addition reaction: a reaction where two molecules react together to produce one
• Reaction of bromine and alkenes

  Alkene -> Dihalogenoalkane

  Reagent: Bromine

  Conditions: Room temperature

  Mechanism: Electrophilic additon
• Reaction of hydrogen bromide with alkenes

  Alkene -> halogenolalkane

  Reagent: HCl or HBr

  Conditions: Room temperature

  Mechanism: Electrophilic addition
• Markownikkoff's rule states that the major product will have the hydrogen in the hydrogen halide bond to the carbon atom already bonded to the highest number of hydrogens.
• Why is the order of stability of carbocation tertiary>secondary>primary


  because the methyl groups on either side of the positive carbon are electron releasing and reduce the charge on the ion which stabilises it.
• Reaction of sulfuric acid with alkenes

  Stage 1

  Alkene -> alkyl hydrogen sulfate

  Reagents: concentrated H2SO4

  Conditions: room temperature

  Mechanism: Electrophilic addition

  Type of reagent: Electrophile, H2SO4

  CH2=CH2 + H2SO4 -> CH3CH2OSO2OH
• Reaction of sulfuric acid with alkenes

  Stage 2

  Alkyl hydrogensulfate ->alcohol

  Reagents: water

  Conditions: warm mixture

  Type of reaction: hydrolysis

  CH3CH2OSO2OH + H2O -> CH3CH2OH + H2SO
• Overall role of sulfuric acid is that of a catalyst (as it is regenerated)
• Hydrolysis

  a reaction where the molecule is split by the addition of water.
• Testing for alkenes with bromine water

  • Bromine water decolourises in the presence of a double bond.
  • This can be used as a test for the presence of an double bond in a molecule.
  • It can be used quantitatively to show the presence of multiple double bonds in compounds like polyunsaturated oils.
• Essential conditions for direct industrial hydration of alkenes to form alcohols

  • High temperature 300 to 600°C
  • High pressure 70 atm
  • Catalyst: concentrated H3PO4
• Poly(alkenes) like alkanes are unreactive due to the strong C-C and C-H bonds
• arrangement of bonds around C double bond

  planar, bond angle 120
• Poly(chloroethene) is a

  • polymer that is water proof
  • electrical insulator
  • doesn’t react with acids
  • In pure form it is a rigid plastic due to the strong intermolecular bonding between polymer chains prevents them moving over each other.
  • In this un-plasticised form it is used make uPVC window frame coverings and guttering.
  • If a plasticiser is added the intermolecular forces are weakened which allows the chains to move more easily, resulting in more flexibility in the polymer.
  • In this form PVC is used to make insulation on electrical wires, and waterproof clothing.