4. alkenes

• alkenes are unsaturated hydrocarbons (contain one or more double bond) produced in large quantities when crude oil thermally cracked
• ethene is the starting material for large range of products including: polyethene , PVC , polystyrene
• ethene has a trigonal planar shape with bond angle 120
• C=C double covalent bond consists of one sigma (σ) bond and one pi (π) bond.
• π bonds are exposed and have high electron density = vulnerable to attack by species which ‘like’ electrons called electrophiles.
• pi bonds presence means bond cannot rotate - restricted rotation
• • p orbitals of two carbon atoms overlap with each other, a π bond is formed ( contains two electrons)
  • The two orbitals that form the π bond lie above and below the plane of the two carbon atoms to maximise bond overlap

  
• isomers that involve the double bond:

  • positional isomers (double bond moves)
  • geometrical isomers (e-z stereoisomers)
• geometrical isomerism is a form of stereoisomerisms - same structural formula but bonds arranged differently in space, ONLY occurs around the c=c
• physical properties similar to alkanes as double bond doesn't greatly effect physical properties

  • only van der waals forces between alkene molecules
  • mp and bp increase with number carbon atoms
  • insoluble in water
• double bond means alkenes are more reactive than alkanes because c=c electron rich area that can be attacked by positively charged reagents - "electrophiles"
• most reactions of alkenes are called electrophilic additions
• ‘Markownikoff’s Rule’ In most cases, hydrogen will be added to the carbon with the most hydrogens already attached to it.
• addition polymers are made from a monomer with carbon carbon double bond - when the monomer polymerises the double bond opens and the monomers bond together to form a backbone of carbon atoms
• addition polymers:

  A) poly(propene)

  B) n

  
• properties of poly(alkenes)

  • very unreactive = strong non-polar C-C and C-H bonds

  useful to have but it does mean they are not attacked by enzymes so not BIODEGRADABLE
• • Low density poly(ethene)
  • polymerising ethene at high temp and high pressures via free radical mechanism

  polymer with branched chain so is very flexible and used as plastic bags or electrical cables

  
  

  • High density poly(ethene)
  • polymerising ethene at temps and pressures a little greater than room temp using Zeigler-Natta catalyst

  chains pack together well so very dense used in milk crates and buckets
• polymers can be tailored to make them suitable for many applications:

  • Plasticisers are small molecules that get in between polymer chains to allow them to slide more easily past one another and make polymer more flexible
  • = PVC made rigid enough drain pipes and flexible enough for plastic aprons
• State what you would observe if bromine water was added to poly(chloroethene). Explain this observation.

  • no reaction/ stay orange
  • polymer is saturated so doesn't contain any double bonds