Hydrocarbons

• organic chemistry is the study of the structure , properties , and reactions of organic compounds.
• all organic compounds contain the element carbon . Most contain hydrogen .
• Not all carbon-containing compounds are organic.
• A homologous series is a family of organic compounds with the same general formula , and similar chemical properties because they have the same functional group .
• alkenes are unsaturated hydrocarbons containing at least one C=C bond . their general formula is CnH2n
• characteristics

  1. Same functional group
  2. similar chemical properties
  3. each member of the series differs from the next by additional CH2 unit
  4. has a gradual change in their physical properties as the number of carbon atoms increase
• A functional group is an atom or group of atoms that gives a molecule its characteristic properties. The functional group determines the chemical properties of the homologous series .
• Alkanes are saturated hydrocarbons containing only single bonds between carbon atoms . Their general formula is CnH2n+2 .
• Hydrocarbons are organic compounds that contain only hydrogen and carbon atoms .
• remember this ! M E P B P H

  
• H

  |

  H— C — H

  |

  H

  Need to have 4 “legs” (4 valance electrons )
• physical properties

  Low melting and boiling point .

  • they are covalent molecules with a simple molecular structure. Hence , a little heat energy is needed to overcome the weak intermolecular forces of attraction.
• Alkanes show a gradual change in the following properties as the number of carbon atoms increases

  molecular size, boiling point/melting point

  molecular size increases viscosity
• When molecular size increases , intermolecular forces of attraction become more extensive and stronger hence requires more energy to overcome .
• Poor conductor of electricity

  They have a simple molecular structure and are uncharged . There are no mobile ions or electrons to act as charge carriers.
• when alkane molecules become bigger, the intermolecular forces are stronger . This makes it difficult for the liquid to flow .
• Insoluble in water , but soluble in organic solvents
• Combustion can be incomplete or complete
• Complete combustion

  • Products carbon dioxide + water (g)
  • Releases more energy
• Incomplete combustion

  • Carbon monoxide/soot + water.
  • Energy (release less energy)
• Chlorination of methane produces chloromethane (methyl chloride).

  
• Addition !!
• Test for unsaturation.

  • Add aqueous bromine to unknown sample .

  Alkane --> bromine remains reddish brown.

  Alkene --> reddish-browm aqueous bromine decolourises immediately
• Conditions for addition.

  1. 200°c , nickel catalyst
  2. Br2
  3. Steam (H2O) / polymerisation
• Cracking is the breaking down of larger hydrocarbon molecules into smaller hydrocarbon molecules.

  
• High temperature eg 600°c aluminum oxide and silicone dioxide catalyst
• why is cracking important

  1. Produce petrol
  2. Produce short chain alkenes
  3. Produce hydrogen
• Similarities of alkene and alkane

  1. Both Hydrocarbons
  2. Flammable