Reactions

• Halogenation


  The reaction of the carbon-carbon double bond in alkenes such as ethene with halogens such as chlorine, bromine and iodine
• Reaction of ethene and fluorine

  Ethene reacts explosively with fluorine to give carbon and hydrogen fluoride gas
• Reaction of ethene and chlorine or bromine or iodine

  Addition reaction, e.g. bromine adds to give 1,2-dibromoethane
• Reaction of alkenes with bromine

  1. Bubbling gaseous alkene through pure liquid bromine or bromine solution, or shaking liquid alkene with bromine or bromine solution

  2. Bromine is decolourised as it reacts with the alkene
• Chlorine reacts faster than bromine, but the chemistry is similar. Iodine reacts much, much more slowly, but again the chemistry is similar
• Using bromine water as a test for alkenes


  1. Shaking an alkene with bromine water (or bubbling a gaseous alkene through bromine water)

  2. The solution becomes colourless, as alkenes decolourise bromine water
• The major product of the reaction of alkenes with bromine water is not 1,2-dibromoethane, as the water also gets involved in the reaction, and most of the product is 2-bromoethanol
• Addition of sulphuric acid to alkenes

  1. Alkenes react with concentrated sulphuric acid in the cold to produce alkyl hydrogensulphates

  2. Ethene reacts to give ethyl hydrogensulphate
• Alkyl hydrogensulphate


  The product of the reaction of an alkene with concentrated sulphuric acid, with the structure CH3CH2OSO3H
• Reaction of propene with sulphuric acid

  Sulphuric acid adds to the unsymmetrical alkene propene, with the hydrogen attaching to the carbon with the most hydrogens already attached, in line with Markovnikov's Rule
• Making ethanol


  Ethene is passed into concentrated sulphuric acid to make ethyl hydrogensulphate, which is then diluted with water and distilled to produce ethanol
• Making propan-2-ol


  More complicated alkyl hydrogensulphates react with water in the same way as ethyl hydrogensulphate to produce alcohols
• Oxidation of alkenes with cold dilute potassium manganate(VII) solution

  1. Under acidic conditions, the manganate(VII) ions are reduced to manganese(II) ions

  2. Under alkaline conditions, the manganate(VII) ions are first reduced to green manganate(VI) ions, then to dark brown solid manganese(IV) oxide
• Carbonyl compounds


  Compounds containing the carbonyl group, C=O, formed by the oxidation of alkenes with potassium manganate(VII)
• Oxidation of alkenes with hot concentrated acidified potassium manganate(VII) solution

  1. The diols formed in the cold dilute reaction are themselves oxidised, so the final products depend on the arrangement of groups around the original carbon-carbon double bond

  2. If both attached groups are alkyl groups, a ketone is formed

  3. If one group is a hydrocarbon and the other is hydrogen, an aldehyde is formed which is further oxidised to a carboxylic acid

  4. If both groups are hydrogen, carbon dioxide and water are formed
• Ketones are not easily oxidised further by potassium manganate(VII)
• Aldehydes are readily oxidised to carboxylic acids by potassium manganate(VII)
• Alkene + Cl2 -> chloroalkane
• Alkene + H2O -> alcohol
• Alkene + HCl -> haloalkanes
• Alkene + O2 -> peroxides
• Alkene + O3 -> aldehyde/ketone
• Alkene + HBr -> bromoalkanes
• Alkene + KMnO4 -> carboxylic acid
• Alkene + NaHCO3 -> alkanol
• Manufacturing ethanol


  1. Reacting ethene with steam

  2. Reaction is reversible
• Only 5% of the ethene is converted into ethanol at each pass through the reactor
• Achieving 95% overall conversion


  1. Removing the ethanol from the equilibrium mixture

  2. Recycling the ethene
• Manufacturing other alcohols


  1. Adding water across the carbon-carbon double bond of an unsymmetrical alkene

  2. Applying Markovnikov's Rule
• Markovnikov's Rule


  When you add a molecule HX across a carbon-carbon double bond, the hydrogen joins to the carbon atom which already has the more hydrogen atoms attached to it
• Poly(ethene) (polythene or polyethylene)

  • An example of addition polymerisation
  • Ethene is the monomer
  • Poly(ethene) is the polymer
• Manufacture of poly(ethene)


  1. Ethene molecules join together

  2. Number of molecules joining up is 2000 to 20000

  3. Temperature: about 200°C

  4. Pressure: about 2000 atmospheres

  5. Initiator: a small amount of oxygen as an impurity
• Isotactic poly(propene)

  • Regular arrangement of the CH3 groups
  • Allows close packing and maximizes van der Waals bonding
  • Stronger material
• Atactic poly(propene)

  CH3 groups are orientated randomly along the chain
• Syndiotactic poly(propene)

  • Every alternate CH3 group is orientated in the same way
  • Allows close packing but not as strong as isotactic
• Poly(chloroethene) (polyvinyl chloride): PVC

  • Made by polymerising chloroethene, CH2=CHCl
  • Mainly atactic polymer molecules with chlorines orientated randomly
• Poly(tetrafluoroethene): PTFE

  Just like poly(ethene) but each hydrogen is replaced by a fluorine atom