Alkenes

• Alkenes are unsaturated hydrocarbons. Unsaturated hydrocarbons are compounds containing at least one C=C, carbon double bond.
• General formula:

  CnH2n
• Bonding in alkenes involves a double covalent bond, a centre of high electron density.
• E-Z isomerism:

  • Stereoisomers are molecules which have the same molecular formula and structural formula, but a different arrangement in space. Positions differ around the C=C.

  Present functional groups are on...

  • Z isomer= Same side.
  • E isomer= Opposite sides.
• Addition reaction with HBr: Electrophilic addition

  1. HBr is polarised, Br is δ− and H is δ+.
  2. Electron pair in C=C attacks δ+ H.
  3. H-C bond forms, breaking H-Br bond.
  4. Carbocation intermediate forms, δ− Br is attracted to C+.
  5. Br-C bond forms, producing an halogenoalkane.

  A) Electrophile

  B) Carbocation intermediate

  
• Addition reaction with cold, concentrated H2SO4: Electrophilic addition

  1. O-H bond in H2SO4 is polarised, meaning δ+ H and δ- O.
  2. Electron pair in C=C attacks δ+ H.
  3. H-C bond forms, breaking O-H bond, electron pair passed onto δ- O.
  4. Carbocation intermediate forms, electron pair on O attacks C+.
  5. Forming a C-O bond, forming an alcohol.

  A) Alcohol

  B) Electrophile

  C) Carbocation intermediate

  
• Addition reaction with Br2: Electrophilic addition

  1. Br2 is polarised, one Br is δ−, the other is δ+.
  2. Electron pair in C=C attacks δ+ Br, forming a C-Br bond.
  3. The pair is passed onto the δ− Br, Br-Br bond breaks.
  4. Forming a carbonation intermediate, δ− Br attacks C+.
  5. Forming a C-Br bond, producing a dibromoalkane.

  
• Test for alkenes: Addition of bromine

  • Add an unknown solution to a test tube, add bromine water.
  • If alkenes present, the solution will turn from brown/orange to colourless, due to the breaking of the C=C.
• Addition reactions of unsymmetrical alkenes forms major and minor products. The amount of the two products is determined by the stability of the carbocation intermediate, the more alkyl groups bonded= the more stable.

  • 1 alkyl group= Primary carbocation.
  • 2 alkyl groups= Secondary carbocation.
  • 3 alkyl groups= Tertiary carbocation.
  • HBr= 1-bromoalkane and 2-bromoalkane.
  • H2SO4 - Primary and secondary alcohol.
• Examples: Synthetic polymers

  • Poly(ethene) = Used for plastic bags and bottles, film wrapping.
  • Poly(propene) = Used for ropes, clothing and carpets.
  • Poly(phenylethene) = Used for
  • Poly(chlorothene) or PVC = Used as window and door frames. When a plasticiser molecule is added, PVC becomes more flexible, so is used to making wellington boots and raincoats.
• Addition polymers:

  • Polymers are made up on monomer units joined together.
  • Addition polymers are formed from alkenes and substituted alkenes.
  • Addition polymers are unreactive because they do not contain a reactive C=C, carbon double bond, and the main carbon chain in non-polar.
  • Addition polymers are joined by strong covalent bonds with weak van der Waal's intermolecular forces between the polymer molecules.
• Addition polymers:

  A) Monomer

  B) Repeating unit

  
• Addition polymers:

  A) Cl

  B) Cl

  C) Cl

  D) Cl

  E) Cl

  F) Monomer

  G) Repeating unit

  H) Polymer