Alkenes

• what is an alkene?


  unsaturated hydrocarbon

  the carbon carbon double bond makes them more reactive than alkanes, because of the high concentration of electrons (high electron density) between the 2 carbon atoms
• describe the structure of ethene

  ethene is a planar molecule

  makes the angle between each bond is roughly 120 degrees

  there’s no rotation about the double bond due to the double bond
• What does a carbon carbon double bond contain?

  One sigma bond

  One pi bond

  Pi bonds have high electron density

  They are vulnerable to be attacked by electrophiles
• define stereoisomerisms

  stereoisomers have the same structural formula but have a different spatial arrangement of atoms
• why do E-Z isomers exist?

  due to the restricted rotation about the carbon carbon double bond

  when there are 2 different groups/atoms attached to both ends of the double bond
• define electrophile

  Electron pair acceptor
• reactions with alkenes

  very reactive

  the carbon carbon double bond forms an electron rich area, which can easily be attacked by electrophiles

  because alkenes are unsaturated, they undergo addition reactions
• describe electrophillic addition reactions - with hydrogen halides

  bromine is more electronegative than hhydrogen, so molecule is polar

  the electrophile is the slightly positive hydrogen

  the slightly positive hydrogen is attracted to the C=C bond because of the high electron density

  one of the pairs of electrons from C=C bond forms a bond with the hydrogen to form a positive ion - a carbocation

  the bond in HBr breaks heterolytically - both pairs of electrons go to bromine, leaving a bromine ion

  bromine ion attaches to positively charged carbon of the carbocation

  
• What is the change in functional group during an electrophillic reaction?

  alkene ——> haloalkane/dihaloalkane

  conditions: room temperature

  reagents: electrophile (H), slightly negative bromine

  bond breaks heterolytically between HBr
• When might 2 products form?

  An asymmetrical alkene has a double bond that’s not in the middle, causing there to be 2 possible products (major and minor)

  The major product ____ is formed from a primary/secondary/tertiary carbocation, which is more stable than the primary/secondary/tertiary carbocation from which the minor product ____ forms ; due to the greater positive inductive effect
• what is markovnikov’s rule?

  helps us to predict the product when a hydrogen halide is added on to an asymmetrical alkene

  states that when hydrogen halides add onto alkenes, the hydrogen atoms add to the carbon atoms already have the most H atoms
• electrophillic addition reactions with sulfuric acid

  concentrated sulfuric acid adds across the double bond

  Occurs at room temperature

  It is exothermic
• Explain why alkene polymers are unreactive

  there are no polar bonds/it is non-polar
• why would it be good for some polymers to be biodegradable

  to prevent build up of waste
• what do plasticisers do?

  small molecules added to plastics that get between the polymer chains forcing them apart and allowing them to slide over each other

  eg. PVC is rigid, but if you added a plasticiser, it is flexible
• define polymer

  very large molecules that are build up from small molecules called monomers
• define addition polymers

  made from monomer(s) with a double bond

  when monomers polymerise, the double bond and the monomers bond together to form a backbone of carbon atoms
• what type of reaction is decolourising bromine solution?

  electrophillic addition reaction
• how can waste polymers be processed?

  • separation and recycling - waste is sorted into different types of polymers and then each type can be recycled by melting/moulding
  • feedstock for cracking - waste polymers can be used as a feedstock for the cracking process, allowing for new plastics to be made
  • combustion for energy production - waste polymers can be incinerated and the heat released can be used to generate electricity
• explain why bromine molecules react with the double bonds in alkenes

  Double bonds have high electron density

  Bromine becomes polar/forms an induced dipole
• explain why HBr react with the double bonds in alkenes

  HBr is a polar molecule

  Br is more electronegative than H

  the slightly positive H is attracted to the high electron density C=C bond