Alkenes

• Why are alkenes described as unsaturated?

  Due to their carbon carbon double bond
• What bonds do alkenes have?


  • a covalent and a π bond
  • makes it susceptible to attack by electrophile
  • area of high electron density
• What is the "restricted rotation" of the C=C bond?

  • an overlap of p orbitals which forms a pi bond
• Isomerism of alkenes?

  • position isomers
  • geometric isomers (E-Z isomerism)
  • E - opposite ends of the double bond
  • Z- the same side of the double bond

  A) GEOMETRIC ISOMERS

  
• How do alkenes react?

  • alkenes are more reactive than alkanes due to their C=C bond
  • the double bond forms an electron rich area in the molecule, which can be attacked by charged reagents, these are called electrophiles
  • most of the reactions alkenes undergo are electrophilic addition
• Physics properties of alkenes

  • VDWs are the only forces that act between molecules
  • the physical properties of alkenes are very similar to those of alkanes
  • insoluble in water
  • MP and BP increase with the chain length increasing
• Combustion of alkenes

  • not used as fuels
  • their reactivity makes them useful for other purposes

  
• Electrophilic addition mechanism

  e.g.

  
• What characterizes asymmetrical alkenes?

  The double bond is not in the middle
• What happens when the double bond in an alkene is not symmetrical?

  Two possible carbocations can be formed
• What is the positive inductive effect?

  Alkyl groups release electrons
• How does the electron releasing effect influence carbocations?

  It stabilizes the positive charge of the carbocation
• How does the number of alkyl groups affect carbocation stability?

  More alkyl groups increase carbocation stability
• What is a tertiary carbocation?

  A carbocation connected to three other carbons
• How does the stability of tertiary, secondary, and primary carbocations compare?

  Tertiary > Secondary > Primary in stability
• From which carbocation does the product usually come?

  From the more stable carbocation
• How do you test for an alkene?

  Turns bromine water from orange- brown to colourless
• E.g of a mechanism

  conc sulfuric acid

  forms ethyl hydrogensulphate

  
• What type of polymerisation are alkenes involved in?

  Addition
• Addition polymers

  • made from monomers with a carbon carbon double bond
  • when monomers polymerise, the double bond opens and the monomers bond together to form a backbone of carbon atoms as shown
• Addition polymerisation


  • shows the double bond opening up
  • must include trailing bonds

  
• How do we modify plastics?

  Plasticisers can be used

  • small molecules that get between the polymer chains and force them apart which allows them to slide across each other
  • for e.g. PVC is rigid enough for use as drainpipes but with the addition of a plasticiser it can also be used in making aprons
• Biodegradability

  • polyalkenes have a long chain saturated alkane molecule as their backbone
  • alkanes have strong, non polar carbon carbon bonds
  • this means that they are unreactive and so cannot biodegrade efficiently
  • making waste disposal difficult
• Low density polyethene

  • made by polymerising ethene at high pressure and high temperature via a free radical mechanism
  • produces a polymer with a certain amount of chain branching
  • the branches chains do not pack together very well and so the product is quite flexible and stretches well
  • used in plastic bags and insulation for electric cables
• High density polyethene

  • made at temperatures and pressures a little greater than room conditions and uses a Ziegler-Natta catalyst
  • results in a polymer with much less chain branching
  • chains pack together well and so density is high
  • uses: milk bottles, buckets (LDP would be very rigid)
• What are some solutions to pollution by plastics?

  1. Mechanical recycling

  • simplest form of recycling
  • the first step is to separate the different types of plastics
  • they are then washed and are ground up into smaller pellets
  • then remoulded and reused

  2. Feedstock recycling

  • plastics are heated to a temperature that will break the polymer bonds and produce monomers
  • thermoplastic polymer
  • soften when eated so it can be re melted and re used
  • this can only be done a limited amount of times as each time the chains break and the chain becomes shorter, which degrades the plastics properties