Alkenes

• Alkenes are unsaturated
• What kind of bonding is in an alkene?

  Sigma bonding

  Pi bonding
• What kind of isomerism can occur is alkenes?


  E/Z isomerism occurs due to restricted rotation of C=C bonds
• How is E/Z priority determined?


  Z is when the 2 highest atomic numbers are on the same side

  E is when the 2 highest atomic numbers are on different sides
• The double bond makes alkenes more reactive than alkanes
• Give an example of an electrophile

  A H+ ion. Electrophiles are electron pair acceptors
• Alkenes can undergo addition reaction as they are unsaturated
• Describe the mechanism of electrophilic addition reactions


  1 The electrophile is attracted to the double bond

  2 Electrophiles are positively charged and accept a pair of electrons from the electron dense area of the pi bond

  3 A positive ion (a carbocation) is formed

  4 A negatively charged ion forms a bond with the carbocation

  
• What determines which products are major/minor?


  The stability of the intermediate carbocation
• What makes a tertiary carbocation more stable than a primary one?


  The positive inductive effect felt by the surrounding electrons by the carbocation makes the carbocation more energetically stable and therefore it has a lower activation energy and so reacts more easily
• What can be used to test for unsaturation?


  Bromine water - orange to colourless
• Draw out the electrophilic addition reaction between ethene and conc sufuric acid


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• What are polymers?


  Repeatedly bonded monomers - alkenes
• Addition polymers are unreactive
• Why are addition polymer unreactive?

  Because they are saturated and have strong C-H bonds
• Explain the intermolecular forces in polyalkenes (branched and straight-chain)


  Van der Waals forces hold the molecules together.

  In straight chain polymers the molecules pack close togther and are held by strong VDW forces so are rigid.

  In branched polymers the irregularity of the shape means VDW forces will not be as strong as molecules cannot be as close together - this makes them more flexible
• What are the purpose of plasticisers?


  Plasticisers space out the chain of polymers making them less dense and therefore have less attraction between molecules causing them to be more flexible.

  They are used to make PVC cable
• Give some examples of the uses of poly(chlorethene)


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  • Guttering
  • Plastic windows
  • Electrical cable insulation
  • Sheet flooring
  • Footwear