Nuggets: Alkenes

last updated 3rd Oct

  1. Physical Chemical of Alkenes
  2. (i) Insoluble in water
  3. (ii) Low boiling point
  4. General formula for alkanes: CnH2n
  5. Name Formula State
    Ethene C2H4 gas
    Propene C3H6 gas
    Butene C4H8 gas
    Pentene C5H10 liquid
  6. Chemical Properties of Alkenes
  7. The functional group of alkenes is the carbon-carbon double bond. (C = C). There is only one pair of double bonds in alkenes.
  8. (i) Alkenes are more reactive than alkanes because of the presence of the double bond.
  9. (ii) Complete combustion of alkenes produces carbon dioxide and water.
  10. Alkenes produce more soot than alkanes during combustion because of the higher percentage of carbon compared with alkanes.
  11. (iii) Polymerisation. Alkenes can undergo addition polymerization at high pressure (1000 atm) and high temperature (200oC). Thousands of alkene molecules combine to form polymers (e.g. polyethene)
  12. Polyethene is flexible and difficult to break. It is used in production of cling film, plastic bags.
  13. (iv) Addition Reactions. The double bond in alkenes can be broken and other molecules can add on to the carbon atoms to form new products. This is called addition reaction. The product formed is saturated/polyunsaturated (e.g. no more double bonds).
  14. Examples of addition reactions are hydrogenationbrominationhydration.
  15. Hydrogenation is the addition of hydrogen to alkenes. (reaction takes place at 200oC using nickel as catalyst) e.g. C2H4 + H2 –> C2H6 : ethene to ethane.
  16. Hydrogenation is used to convert vegetable oils (unsaturated) into margarine.
  17. Vegetable oil turned into margarine, from a liquid to a solid state, because of the addition of hydrogen which increases the molecular mass. Margarine has larger melting point than vegetable oil.
  18. Unsaturated oil is healthier for consumption. Saturated (trans fat) fat are unhealthy for consumption.
  19. Bromination is the addition of bromine to alkenes. When ethene is passed through liquid bromine, decolorization occurs. (C2H4 + Br2 –> C2H4Br2) The product is 1,2-dibromoethane, a saturated compound.
  20. An organic compound is said to be saturated if it only contains single bonds or when the double bond is broken to form single bond.
  21. Hydration is the addition of steam to alkenes at temperature 300oC, pressure 60atm, with phosphoric acid as catalyst.
  22. Manufacturing of Alkenes by Cracking
  23. Alkenes can be obtained by cracking (break down) of petroleum fractions. 
  24. Cracking is the process of breaking larger hydrocarbons into smaller molecules. 
  25. The products of cracking are smaller alkane and alkene molecules.
  26. Alkenes can also be prepared by thermal cracking of alcohols. Ethane is produced from ethanol: C2H5OH –> C2H4 + H2O
  27. Preparation of Ethene by dehydration of alcohols
  28. Ethanol is heated to 180oC with excess concentrated sulphuric acid. Sulphur dioxide is removed from ethene by passing it through sodium hydroxide solution and ethene is collected over water. 
  29. Manufacturing of Alkenes by Cracking
  30. Cracking is the process in which larger hydrocarbon molecules are broken down into smaller molecules.
  31. When a large alkane molecule is cracked, a mixture of smaller alkanes, alkenes and hydrogen gas could be formed. 
  32. When a large alkane molecule is cracked, sometimes we get a mixture of alkenes and alkane without the hydrogen gas.
  33. E.g. C13H28 –> C2H4 + C3H6 + C8H18 
  34. E.g. C13H28 –> 2C2H4 + C9H20
  35. E.g C13H28 –> 2C2H4 + C3H6 + C6H12 + H
  36. Cracking happens at a temperature of 600oc with catalyst Silicon Dioxide or Aluminium Oxide
  37. The cracking process obtains petrol, alkenes, hydrogen

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