alkenes

• Alkenes are ALWAYS planar around the C=C bond and has bond angle 120
• Due to the sigma and pi bonds, the molecule is "fixed" in place, and so stereoisomerism can arise

  
• Due to the high electron density around the C=C bond, alkenes are susceptible to attack from electrophiles (electron pair ACCEPTOR) in electrophilic addition reactions.

  
  

  The stability of the carbon atoms surrounding the C=C bond determines where the electrophiles attack. The MORE stable intermediate will form the major product

  
  

  Order of stability:

  Tertiary > Secondary > Primary
• Reaction of bromine with alkenes:

  Functional group change: Alkene -> Haloalkane

  Reagent: Bromine

  Conditions: Aqueous (NOT in UV light) and at room temperature

  This is also why bromine water becomes colourless when an alkene is added to it

  
• Reaction of hydrogen bromide with alkenes:

  Functional group change: Alkene -> haloalkane

  Reagent: Hydrogen bromide (HBr) or hydrogen chloride (HCl)

  Conditions: Room temperature

  H-Br is a polar molecule due to bromine being more electronegative than the hydrogen atom. This means that the hydrogen is first attacked by the C=C bond. The halogen bonds to the MOST STABLE carbon atom (the one with the fewest hydrogen atoms attached) to form the major product

  
• Reaction of hydrogen bromide with alkenes:

  
  

  Functional group change: Alkene -> haloalkane

  Reagent: Hydrogen bromide (HBr) or hydrogen chloride (HCl)

  Conditions: Room temperature

  
  

  H-Br is a polar molecule due to bromine being more electronegative than the hydrogen atom. This means that the hydrogen is first attacked by the C=C bond. The halogen bonds to the MOST STABLE carbon atom (the one with the fewest hydrogen atoms attached) to form the major product

  
• Reaction of sulfuric acid with alkenes:

  
  

  Stage 1: Electrophilic addition

  
  

  Functional group change: Alkene ->Alkyl hydrogensulfate

  Reagent: Concentrated sulfuric acid (electrophile)

  Conditions: Room temperature

  
  

  Stage 2: Hydrolysis

  
  

  Functional group change: Alkyl hydrogensulfate -> alcohol

  Reagent: Water

  Conditions: Warm mixture

  
  

  It is a 2 step reaction in total- write the sulfuric acid in its structural form

  
• Alkenes can be hydrated to become alcohols. This is an industrial process that requires PHOSPHORIC ACID. It is a 1 step reaction which DIRECTLY turns the alkene into an alcohol, unlike when sulfuric acid is used (which is a 2 step reaction). The reaction creates NO waste products, and so it has a high atom economy. This means that industrially, it is a more favourable reaction. As well as this, filtering and separating products would be much easier

  Conditions:

  • High temperature (300-600C)
  • High pressure (100 kPa)
  • CONCENTRATED phosphoric acid as a catalyst

  
• Addition polymerisation is a process in which ALKENES are converted into polymers. This is done by breaking the C=C bond in an alkene (which is called a monomer), where another monomer is added onto the end of the chain. Polyalkenes are very unreactive due to very strong C-C bonds and C-H bonds.

  Pure polyalkenes are very brittle, however plasticisers can be added to change the properties of the polymers. Adding a plasticiser causes the polymer to become more elastic and flexible, as the intermolecular forces between chains of polymers are weakened