Alkanes

• CO2 absorb IR radiation. C=O bonds are polar.
• Carbon particulates and carbon monoxide are released due to incomplete combustion
• Carbon dioxide, and water are released due to complete combustion
• Nitrogen oxide is released due to complete combustion of N2 in the air
• Unburnt hydrocarbons are a product of unburnt fuel
• Sulphur dioxide is released as a product of combustion of sulphur impurities.
• Thermal cracking

  1. Strongly heating the molecules to 700 to 1200K and a high pressure of 7000kPa (7MPa) or 70 times atmospheric pressure

  2. Produces a high percentage of alkenes
• Catalytic cracking

  1. Uses catalysts to break the C-C and C-H bonds

  2. Can be carried out under less extreme conditions

  3. High temperature; about 720 K

  4. Pressure slightly above atmospheric

  5. Uses zeolite catalysts (crystalline aluminosilicates)

  6. Produces mainly branched alkanes, cycloalkanes and aromatic (benzene) hydrocarbons
• Zeolite catalysts

  • Crystalline aluminosilicates
• Branched-chain alkanes


  Burn more smoothly than straight-chain alkanes
• Branched-chain alkanes


  • Used as motor fuels
• Haloalkanes can be made from alkanes through free radical substitution, or from alkenes through electrophilic addition.
• Free radical substitution occurs in three steps:

  1. initiation
  2. propagation
  3. termination
• Initiation phase:


  Must take place with UV light as it provides energy to break Cl-Cl bonds.
• Initiation Phase

  The Cl-Cl bonds break to form two chlorine radicals that both contain one free unpaired electron in their outer shells.
• Initiation Phase Equation


  Cl2 --> 2Cl•

  UV
• Propagation Phase Part 1


  A chlorine radical reacts with a methane, and bonds with a hydrogen, forming hydrochloric acid and a radical methyl that has an unpaired free electron on the carbon.
• Propagation Phase Equations


  i) Cl• + CH4 --> HCl + •CH3

  ii) •CH3 + Cl2 --> CH3Cl + Cl•
• Propagation Phase Part 2

  The radical methyl bonds with a Cl from a CL2, forming chloromethane and a radical chlorine.
• Termination


  Any two radicals react to form a stable, neutral molecule.
• Propagation Stage Overall Equation

  CH4 + Cl2 --> HCl + CH3Cl