3.04 Alkenes

• What is an alkene

  Unsaturated hydrocarbons with a C=C double bond
• What is the general formula of an alkene?

  CnH2n
• Why is there no rotation around the C=C double bond?

  Due to the pi orbital - electron density above and below the single bond, which holds carbon atoms in place
• Are alkenes more or less reactive than alkanes?

  More reactive.

  • Due to high electron density of double bond and the fact that the pi bond is slightly easier to break
• What intermolecular forces of attraction do alkenes have?

  Onlt VDW forces due to non-polar bonds
• Are alkenes soluble in water?

  No.

  • Due to non-polar bonds, and the fact that hydrogen bonds are stronger than VDW forces
• Name and describe the 3 kinds of isomers alkenes can have.

  • Chain isomers - branched chains
  • Position isomers - C=C on diff. carbon atom
  • Geometric E-Z isomers (Z = highest atomic number chains are on the same side of the double bond, E = when they are on the opposite sides)
• What is an electrophile?

  Electron-deficient atoms/ions which accept a pair of electrons from the C=C bond
• What is the most stable kind of carbocation intermediate?

  Alkyl groups have a positive inductive effect (pushes its electrons towards the positive carbocation), so the most stable carbocation is the one bonded to the most carbon atoms i.e a tertiary carbocation
• Major products will be formed from which kinds of carbocations?

  Tertiary, or typically the most stable available
• What conditions are needed for the electrophilic addition of H2O to an alkene?

  Acid catalyst, usually phosphoric acid
• What are the products of the reaction of an alkene + water?

  An alcohol
• Draw a mechanism for the addition of water to ethene.


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• What conditions are needed for the electrophilic addition of a hydrogen halide to an alkene?

  Room temperature
• Draw a mechanism for the reaction of HBr and ethene.

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• What conditions are needed for the electrophilic addition of a halogen molecule to an alkene?

  Room temperature and organic solvent
• How does a molecule with a non-polar bond react as if it is an electrophile?

  C=C double bond with high electron density induces a temporary dipole in the halogen molecule therefore the delta-positive atom is attracted to the double-bond
• Draw a mechanism for the reaction between bromine and ethene.

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• Draw a mechanism for the reaction of sulfuric acid with ethene.

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• How would you turn the product from sulfuric acid+ethene into an alcohol, and how does this show that sulfuric acid catalyses the addition of water to an alkene?

  • Add water
  • H2SO4 reforms, showing it catalyses the hydration of alkenes
• What is an addition polymer?

  Many monomers bonded together via rearrangement of bonds without the loss of any atom/molecule
• What are monomers? What form do they usually take?

  • Molecules which combine to form a polymer
  • Usually have a C=C bond which breaks to leave a repeating pattern
• Draw how you would represent the polymerisation of ethene.

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• Give 3 uses of poly(chloroethene)/PVC

  • Drainpipes
  • Vinyl
  • Aprons
• Give two examples of plasticisers

  • Esters
  • Phthalates
• What are plasticisers?

  Small molecules which get between polymer chains to force them apart, and allow them to slide over one another
• How do the physical properties of PVC change due to plasticiser? What applications does this lead to?

  • PVC with plasticiser becomes flexible, used for aprons
  • Without plasticiser, PVC is rigid, used for drainpipes
• Why do things containing mainly C-C and C-H bonds not decompose easily?


  Bonds are non-polar so are not attacked by enzymes
• Why is a lack of biodegradability in compounds with C-C/C-H bonds a problem?

  Disposal is very problematic
• What is mechanical recycling?

  Where plastics are separated into different types: washed, ground down, melted and remoulded
• What is mechanical recycling used for?

  Soft drink bottles into fleeces
• What is feedstock recycling?

  Plastics heated to a temperature which break polymer bonds, leaving original monomers that can be made into new plastics
• What is feedstock recycling used for?

  Making totally new plastics
• What is a problem with recycling?

  Each time thermosoftening plastics are melted and remoulded, their properties degrade, so they can only be remoulded a limited number of times