alkene

• alkenes are unsaturated hydrocarbons with the general formula CnH2n
• alkenes are quite reactive as the double bonds have high electron density
• cycloalkenes are ring structures with two fewer hydrogens than their straight chain version of the same molecule
• electrophile is electron pair acceptor, they are deficient in electrons and are attracted to the double bond
• addition of bromine tests for alkenes

  if alkene is present goes from brown-orange to colourless

  this is because bromine is an electrophile and adds to the alkene to form dibromoalkane (colourless)
• addition of bromine (alkene test) steps

  1. Br2 is polarised as the electrons in the double bond repels electrons in Br2
  2. electron pair in the double bond is attracted to the +bromine and forms a bond, breaking the br-br bond
  3. a carbocation intermediate forms and Br- is attracted to C+
  4. colourless 1,2-dibromoethane forms
• addition of hydrogen halides

  1. Hbr is polarised
  2. an electron pair in the double bond is attracted to H+ and forms a bond, this breaks h-br bond
  3. a carbocation intermediate forms and br- is attracted to c+
  4. bromoethane forms
• reacting hydrogen halides with unsymmetrical alkenes produces two different products

  the amount of two products is determined by the stability of the carbocation intermediate
• the more alklyl groups bonded to the carbocation, the more stable the intermediate is

  this is because alkyl groups push electrons towards the positive carbocation, stabilising it

  more stable = more likely to form
• alkenes react with cold, concentrated sulfuric acid to form alkyl hydrogen sulfates
• sulfuric acid is used as a catalyst in making an alcohol from an alkene
• alcohols can be made alkyl hydrogen sulfates with sulfuric acid being reformed

  add cold water to warm ethyl hydrogen sulfate will form ethanol

  this is hydrolysis
• alkene monomers join to form addition polymers
• polymers can be:

  natural like proteins or natural rubber

  synthetic like polyethane and polypropene
• common polymer examples are polyethane, nylon, teflon
• polyalkenes are saturated and non polar, therefore unreactive and do not degrade well in landfill
• most polyalkenes are non polar so only have VDW forces

  longer and closer chains have more VDW forces
• shorter, branche polymer chains are usually more flexible and weaker
• long polymers with little to no branching are usually more strong and rigid
• some polyalkenes have halogens like PVC (polyvinyl chloride)

  they can form stronger permanent dipole-dipole forces

  they will have different properties to non polar polyalkenes
• plasticisers are added to polymers to change their properties and make them more flexible
• plasticisers slide between polymer chains to push them apart

  weakens intermolecular forces between chains

  chains now slide over eachother so polymer can bend
• PVC is hard and brittle so used in drain pipes

  PVC with plasticiser is flexible and used in electrical cable insulation