Alkenes

• Alkene basics

  • alkene = hydrocarbons with 1 or more carbon-carbon double bonds
  • are unsaturated
  • general formula: CnH2n
  • suffix: -ene & IUPAC: e.g. but-2-ene
  • can form positional or geometrical isomers
  • similar physical properties to alkanes
  • even though enthalpy is higher (C=C have sigma & pi bonds)
  • alkenes more reactive -> C=C forms electron-rich area attacked by positively charged reagents & undergoes addition reactions (mainly electrophilic addition)
• bond hybridisation = process by which atomic orbitals in the same energy level mix together to form new (hybrid) orbitals that are able to form covalent bonds


  C in excited state promotes electron to higher energy level -> atomic orbitals merge to form hybrid orbitals (1s & 3p -> sp^3 orbitals)

  • pi bond = overlap of left-over non-hybridised p orbitals
  • sigma bond = overlap of hybridised atomic orbitals

  to rotate a double bond -> pi bond needs to be broken which takes energy thus forms isomers
• E-Z isomerism

  1. Draw vertical line through C=C to create 2 halves
  2. On LHS, rank which branch has higher priority based on its Mr
  3. higher Mr -> higher ranking
  4. if the highest rank is on the same level on both sides -> same side (Z) isomer else (E) isomer
• Electrophilic addition

  electrophile = lone pair acceptor

  case 1: reagent -> hydrogen halide

  case 2: reagent -> halogens

  • when halogen approaches double bond -> dipole is induced -> temporary dipole
  • -ve charge of double bond repels electrons in halogen

  case 3: reagent: H2SO4

  A) extra electron pair moves to partial +ve

  B) extra lone pair leaves with Br

  C) carbocation forms - doesn't have H to bond to

  
• Asymmetrical alkenes

  • alkyl groups have a tendency to release electrons -> positive inductive effect
  • presence of alkyl group is stabilising
  • Markovinov's rule: when hydrogen halides add on to alkenes, H adds on to C atom which already has the most hydrogens
  • Major product & minor product formed
  • major - makes up majority of the product
  • most stable carbocation -> more alkyl groups bonded to carbon which halogen is bonded to
  • minor product - less stable & makes up minority of product

  A) major product

  B) minor product

  
• Test for alkenes/double bonds

  add a few drops of bromine water

  • if present -> goes from orange/red -> colourless
  • alkenes can also react with water to produce an alcohol
  • steam used with an acid catalyst at the right pressure & temperature
• Addition polymers

  • addition polymerisation = process of adding monomers together using a catalyst to make 1 product only
  • monomer = unsaturated molecules (alkenes) making polymer
  • polymer = addition of monomers together
  • identifying the monomer:
  • choose any 2 adjacent carbons
  • add in the double bond
• addition polymers

  additives like plasticisers can modify properties of polymer

  • small molecules get between polymer chain
  • forces them apart & allows them to slide across each other
  • makes plastic more flexible e.g. PVC for aprons

  polyalkanes have long saturated chain

  • alkenes have strong non-polar C-C & H-H bonds
  • very unreactive
  • means they can't be attached by biological agents
  • not biodegradable
• Low density polyethene (LDPE)

  • formed under high temp, high pressure via free-radical substutition
  • causes branched to form due to randomness of FRS
  • means product is more flexible -> branches can't pack together & has lower density

  High density polyethene (HDPE)

  • formed at temps & pressure > room conditions
  • uses Ziegler-Natta catalyst
  • much less branching
  • chains/branches pack together well -> less flexible & greater density
  • higher melting point
• Recycling

  mechanical recycling (simplest):

  • seperate into different types of plastic
  • wash them & ground into small particles
  • pellets melted & remoulded

  feedstock recycling:

  • plastics heated at temp that breaks polymer bond & make monomers
  • used to make new plastics

  Problem with recycling:

  • every time polymer is heated -> some of the chains will break so can only be heated a limited no. of times -> degrades plastic's properties