12. Alkanes

• Alkanes:

  • CnH2n+2
  • saturated
  • tetrahedral carbon
  • strong covalent bonds within molecules
  • London forces between molecules
• σ bond = single covalent bond

  • bonds are formed by the overlap of two atomic orbitals
  • the overlap forms a molecular orbital which is where the pair of electrons are found
  • the molecular orbital is known as the σ orbital
• Fractional distillation:

  • alkanes come from crude oil which is a mixture of hydrocarbons
  • different fractions contain alkanes with different numbers of carbon atoms
  • chain length impacts the boiling point of the alkane
• Fractional Distillation

  
• Alkane reactivity:

  • alkanes are unreactive
  • this is because of strong C-C bonds, a high bond enthalpy and non-polar bonds
• Complete combustion of alkanes:

  • alkanes undergo complete combustion to form CO2 and H2O
  • complete combustion forms a blue flame
  • combustion is an example of a redox reaction
• incomplete combustion of alkanes:

  • supply of oxygen is not plentiful
  • orange flame
  • mixture of products: CO, C, CO2, H2O
• Carbon monoxide:

  • reduces the ability of blood to transport oxygen around the body
• Alkane substitution reactions:

  • alkanes react with chlorine and bromine in the presence of UV light
  • 1 hydrogen is swapped with one halogen
• Free Radical Substitution:

  1. Initiation
  2. radical is generated using UV light to cause homolytic fission
  3. Cl2 -> 2Cl.
  4. Propagation
  5. radical reacts with non-radical to form new radical
  6. Cl. + CH4 -> HCl + CH3.
  7. CH3. + Cl2 -> CH3Cl + Cl.
  8. Termination
  9. two radicals meet and unpaired electrons become paired
  10. Cl. + Cl. -> Cl2
  11. CH3. + CH3. -> C2H6
  12. CH3. + Cl. -> CH3Cl
• Issues with FRS:

  • hard to swap one hydrogen for one chlorine (polysubstitution)
  • mixture of products due to termination
  • substitution can occur on different parts of the chain