Alkenes

• Have the general formula CnH2n
• They all have one double carbon - carbon double bond that makes them unsaturated
• The high electron density around the double bond makes them very reactive
• What is an electrophile?

  An electron pair acceptor
• Bromine water can be added to test if there are alkenes present in a solution. If alkenes are present the solution will go from orange to colourless.
• Alkene to halogenoalkane:

  • Overall reaction: CH2CH2 + HX ->CH3CH2X
  • Mechanism: electrophilic addition
  • Conditions: double bond / positive charge
  • Reagents: Hydrogen halide or halide2
• What is an alkene?

  unsaturated hydrocarbons that have a c=c double bond
• why id there no rotation about the c=c bond?

  Due to the pi bond - electron density above and below the single bond that holds the carbon atoms in place
• are they 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 they have?

  only van der walls due to non-polar bonds
• are they soluble in water?

  no, non-polar bonds
• what is the most stable kind of carbocation?

  a tertiary carbocation due the the fact that they have more carbon atoms around them that can share their electron density
• major products will be formed from which type of carbocation?

  tertiary
• 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 with water?

  Alcohol
• What is an alkene?


  Unsaturated hydrocarbons with a C=C double bond
• Why is there no rotation about the C=C double bond?


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


  More reactive, due to high electron density of double bond and the fact the pi-bond is slightly easier to break
• What intermolecular forces of attraction do they have?


  Only van der Waals due to non-polar bonds
• Are they soluble in water? Why?

  No, non-polar bonds (van der Waals’ < hydrogen bonding)
• Name and describe the three kinds of isomers alkenes can have?

  Chain isomers (branched chains)

  Position isomers (C=C on different carbon atom)

  Geometric E-Z isomers (Z is when 2 highest atomic number chains are on the same side of the double bond; E is when they're on opposite sides)
• Write an equation for the complete combustion of pent-2-ene?


  CH3CHCHCH2CH3 + 7.5O2 -> 5CO2 + 5H2O
• What is an electrophile?


  Electron deficient atoms/ions which accept a pair of electrons (here, from the C=C double bond)
• What is the most stable kind of carbocation intermediate? Why?


  Alkyl groups have a positive inductive effect, so the most stable carbonation is the one bonded to the most other carbon atoms i.e. A tertiary carbocation
• Major products will be formed from which kinds of carbocations?

  Tertiary (or the most stable avaliable)
• What conditions are needed for the electrophilic addition of H2O to an alkene?


  Acid catalyst, usually phosphoric acid
• What are the product(s) of the reaction of alkene and water?

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

  Room temperature
• 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 a high electron density induces a temporary dipole in the halogen molecule -> δ+ atom attracted to double bond
• How would you turn the product into an alcohol and how does 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 or 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
• Give 3 uses of poly(chloroethene) / PVC?


  Drainpipes

  Vinyl

  Aprons
• Give two examples of plasticisers?

  esters and phthalates
• What are plasticisers?


  Small molecules that 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 a plasticiser? What applications does this lead to?


  PVC with a plasticiser become flexible, used for aprons

  Without a 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 or 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 re-moulded