Alkanes

• Alkanes


  Saturated hydrocarbons with the general formula CnH2n+2
• Alkanes


  • Methane (CH4)
  • Ethane (C2H6)
  • Propane (C3H8)
• Cycloalkanes


  Saturated cyclic hydrocarbons with the general formula CnH2n
• Fractional distillation of crude oil

  1. Crude oil vaporized

  2. Vapour enters column

  3. Longer hydrocarbons condense at lower trays

  4. Shorter hydrocarbons rise and condense at upper trays
• Fractions obtained from fractional distillation

  • Gases (1-4 carbons)
  • Petrol (5-10 carbons)
  • Kerosene (11-13 carbons)
  • Diesel oil (14-20 carbons)
  • Lubricating oil (21-50 carbons)
  • Paraffin wax (50+ carbons)
  • Asphalt
• Uses of fractions


  • Gases used as LPG
  • Petrol used in cars
  • Kerosene used in jet fuel and heating
  • Diesel oil used to fuel trains
  • Fuel oil used in ships and power stations
  • Bitumen/asphalt used in roofing and tarmac
• Cracking of alkanes


  1. Thermal cracking (high temp, high pressure)

  2. Catalytic cracking (high temp, low pressure, zeolite catalyst)
• Thermal cracking


  • Produces mainly alkenes
  • Used to make polymers/plastics
• Catalytic cracking


  • Produces mainly aromatic hydrocarbons
  • Useful in vehicle fuels
• Complete combustion of alkanes

  Alkane + Oxygen → Carbon dioxide + Water
• Incomplete combustion of alkanes

  Alkane + Limited oxygen → Carbon monoxide + Soot + Water
• Burning fossil fuels


  Produces carbon dioxide, a greenhouse gas
• Carbon dioxide absorbs infrared radiation from the sun, causing the greenhouse effect and global warming
• Particulates from burning fuels make engines and buildings dirty, clog engines because the particulates get stuck
• Filters and catalytic converters in cars help try to remove or stop particulates from causing too many problems
• Burning fuels produces complete combustion, but it is rare to get 100% complete combustion
• Burning fossil fuels produces carbon dioxide, a greenhouse gas
• Carbon dioxide absorbs infrared radiation from the Sun


  This causes a greenhouse effect, trapping heat and leading to global warming
• Evidence of global warming

  • Retreating glaciers, snow melt
• Photochemical smog is caused by the mixing of hydrocarbons and nitrogen dioxide from burning fuels, in the presence of sunlight
• Photochemical smog is harmful to the respiratory system of animals and humans
• Ozone is toxic at lower levels of the atmosphere, but we want it in the upper atmosphere
• Oxides of nitrogen are caused by the reaction of nitrogen and oxygen under high pressure and temperature in engines, and are acidic
• Catalytic converters can help remove or reduce unburned hydrocarbons and oxides of nitrogen from vehicle emissions
• Burning fossil fuels that contain sulfur impurities produces sulfur dioxide, which can form sulfuric acid and cause acid rain
• Wet scrubbing


  1. Dissolve calcium carbonate in water

  2. Spray onto acidic gases leaving chimneys

  3. Neutralizes sulfur dioxide
• Free radical reactions


  Involve initiation, propagation, and termination steps
• Initiation


  Radicals are produced using visible light or UV
• Propagation


  Radicals react with non-radicals to form new radicals
• Termination


  Two radicals collide to form a stable non-radical molecule
• Free radical reactions


  • Making chloromethane from methane and chlorine
• Multiple substitution can occur if there is an excess of chlorine radicals, forming di-, tri-, and tetrachloromethane