chapter 13

• alkenes are unsaturated hydrocarbons with at least one double carbon bond
• double bond formation:

  a sigma bond is formed between the two carbons that each have two hydrogens. the electrons left on each carbon is in a p orbital and the sideways overlap of these two p orbitals cause a pi bond. The pi bond prevents the rotation of the carbons
• the shape around each of there carbon atoms in the double bond is triagonal planar, as there are three regions of electron density that repel each other as far as possible in the same plane with a bond angle of 120.
• stereoisomers have the same molecular and structural formula but a different arrangement of atoms in space. this causes E/Z isomers
• a molecule will have e/z isomerism if it had a c=c bond and different groups attached the each carbon atom on the double bond
• e isomers have groups that are diagonally opposite, whereas z isomers have groups on the same size
• cis trans isomerism is when molecules have the exact same groups

  cis is the z isomer whereas trans is the e isomer
• the group with priority on each carbon will be the group with the atom with the highest atomic number as you go across the chain
• alkenes are very reactive due to the pi bond as its weaker than the sigma bond
• alkenes go through addition reactions :

  • hydrogenation = h2 with a nickel catalyst while heated
  • halogenation = x2 at room temperature (bromine water is decolourised when an alkene is present)
  • hydrogen halides= gas at room temperature, gives two possible products
  • hydration= steam with a phosphoric acid catalyst
• the mechanism for addition reactions on unsaturated compounds is called electrophilic addition. The more electropositive atom is added first (or an induced dipole happens)
• markownikoffs rule:

  two carbocations are possible in electrophillic addition, primary or secondary

  the major product is formed via the most stable carbocation, which would be the secondary carbocation because it has more alkyl groups that spreads out the charge on the carbon atom
• unsaturated alkene molecules go through addition polymerisation to produce long saturated chains
• some common polymers include:

  • polyethene = straws, bags, childrens toys
  • polychloroethene or pvc = pipes, sheeting, insulation
  • polypropene= fibres, childrens toys, packing crates
  • polystyrene
  • teflon= non stick pans
• disposing of waste polymers can fill up landfills as they are non biodegradable, or recycling them can reduce environmental impact and help make new products.

  • pvc recycling= pvc is hazardous due to high chlorine content so its grinded and recovered by precipitation
  • using waste polymers as fuel
  • feedstock recycling
• biodegradable means its broken down the microorganisms naturally whereas photodegradable means it broken down by absorbing light