3.4 Alkenes

• what are akenes?

  unsaturated hydrocarbons with the general formula of CnH2n
• what does unsaturated mean?

  there is at least 1 double covalent bond
• what makes alkenes fairly reactive?

  double bonds have a high electron density
• what is an electrophile?


  electron pair acceptor
• why are alkenes attacked by electrophiles?

  due to their double bond which has a higher density of electrons therefore the electrophile attacks the double bond as it electron deficient-
• how do you test for alkenes?

  - add bromine water which causes a colour change from brown-orange to colourless
• what does reacting hydrogen halides with unsymmetrical alkenes?

  2 products which are determined by the stability of the carbocation intermediate
• what makes an intermediate more stable?

  - more alkyl groups bonded to the carbocation the more stable the intermediate is

  - this is because alkyl groups push electrons towards the positive carbocation stabilising it (positive inductive effect)

  - more stable the carbocation the more likely it will form
• How does alkenes react with H2SO4?

  Cold concentrated sulphuric acid produces alky hydrogen sulphites
• How can alcohols be made from alkyl hydrogen sulphate to reform H2SO4?

  By hydrolysis (adding cold water)
• What are addition polymers?

  Alkene monomers that are joined together to form a long, saturated molecule
• What are the two types of monomers?

  Natural:

  -proteins

  -natural rubber

  Synthetic:

  -poly(ethane)

  -poly(propane)
• How are addition polymers made from alkenes?

  The double bond in the monomer opens to form the polymer
• Why are addition polymers not biodegradable?

  Poly(alkenes) are saturated molecules, normally non-polar and hence are unreactive so don't degrade well in landfill
• How do intermolecular forces govern the properties of polymers?

  - most polyalkenes chains are non-polar so they only have van der waals forces between the chains

  - the longer the chain and the closer they are to each other the more van der waals forces you have

  - polymer chains which are shorted and have a lot of branching tend to be more flexible and weaker

  - polymers with no or very little branching and long tend to be more rigid and stronger

  - some polyalkenes have halogens e.g. chlorine (PVC - common name polyvinyl chloride). They can form strong permanent dipole-dipole forces and so will have different properties to other non-polar polyalkenes
• What are plasticisers?


  - Molecules added to polymers to make them more flexible. They slide between the polymer chains pushing them apart

  - this weakens the intermolecular forces between the chains. The chains can now slide over each other more and makes the polymer easier to bend