Alkenes

• Alkenes are unsaturated hydrocarbons meaning they contain at least one carbon-carbon double bond.

  • This is an area of high electron density making it susceptible to attack from electrophiles (species attracted to d- areas). It consists of a normal covalent bond and a pi bond.
• Bromine water is used to identify the double bond in alkenes and other unsaturated compounds.

  • It turns the solution from orange-brown to colourless if a double bond is present in the substance.
• Alkenes undergo electrophilic addition about the double bond.

  • Electrophiles are electron acceptors and are attracted to areas of high electron density.
  • Examples of common electrophoresis include: HBr, Br2, H2SO4, etc.
  • Electrophiles can be used to form alkyl hydrogensulphates or halogenoalkanes from alkenes.
• Electrophilic addition is the reaction that shows how electrophiles attack the double bond in alkenes.

  • When the double bond is broken a carbocation forms. This is a carbon atom with only three bonds, meaning it has a positive charge.
• Carbocations can have varying stability, with tertiary being the most stable and primary being the least stable.

  • The more stable the carbocation, the more likely it is to form.
  • In an addition reaction, multiple products can form but the major product will always be the most stable possible.
• Example of Electrophilic Addition:

  
• Example of Electrophilic Addition:

  
• Addition polymers are produced from alkenes where the double bond is broken to form a repeating unit.

  
• The reaction conditions used in the production of these polymer chains can be altered to give the plastics produced different properties.

  • High pressures and temperatures produce branched chain polymers with weak intermolecular forces.
  • Lower pressure and temperatures produce straight chain polymers with strong intermolecular forces.
• Uses of polymers:

  • Polymers are I reactive hydrocarbon chains with multiple strong,non-polar covalent bonds. This makes them useful for manufacturing many everyday plastic products such as shopping bags (poly(ethene)).
  • However, the unreactive nature of the bonds in addition polymers means they are not biodegradable and cannot be broken down by species in nature.
• Poly(chloroethene), more commonly known as PVC, is an addition polymer with waterproof properties. It gains these properties by the addition of plasticisers during the reaction.