Alkanes

• What is the bonding in Alkanes?

  • Single, covalent C-C bonds therefore alkanes are saturated
  • Single bonds are sigma bonds - result of overlap of orbitals between bonding atoms
  • Tetrahedral shape - bond angle 109.5
• Alkanes have general formula CnH2n+2
• Why does the boiling point of alkanes increase as C chain length increases?

  As there's greater surface points of contact for London forces, therefore greater London forces require more energy to break. However, if number of branches increases then boiling point decreases, as surface points of contact decreases, leading to weaker London forces
• What is the reactivity of alkanes like?

  Alkanes don't react with common reagents, as electronegativity of C-H bond is similar, therefore they are non-polar. Also C-H bonds and C-C bonds are strong
• Why do we combust alkanes?

  As they're valuable fuels
• What is the equation for complete combustion of alkanes?

  Alkane +O2 --> CO2 + H2O
• What is the equation for incomplete combustion of alkanes?

  • Alkane + O2 --> CO + H2O
  • CO is toxic as it binds to haemoglobin in red blood cells, preventing it form binding to O2, and this starves the bodies tissues of oxygen which can be fatal
• How to do combustion calculations:

  1. Write a balanced equation
  2. Calculate mols of alkane using mols=mass/Mr
  3. Deduce mols of CO2 using stoichiometry
  4. Calculate volume of gas using vol=mol/24(24,000)
• What are the 2 types of bond fission?

  1. Heterolytic bond fission - When both electrons go to 1 of the bonded atoms, forming a + and - ion
  2. Homolytic bond fission - When 1 electron goes to each of the bonded atoms, forming radical
• Definition of a radical:

  A species with an unpaired electron
• What are the conditions for radical substitution?

  UV light
• What is the mechanism of radical substitution?

  1. Intitiation - UV light provides enough energy to break Cl-Cl bond Cl2 --> Cl. + Cl.
  2. Propagation - Remove a hydrogen, add a halogen Cl. + CH4 --> HCl + .CH3 , .CH3 + Cl2 --> CH3Cl + Cl. The Cl radical acts as a catalyst as its regenerated
  3. Termination - Removes radicals, stopping the reaction Cl. + Cl. --> Cl2 , .CH3 + Cl. --> CH3Cl , .CH3 + .CH3 --> C2H6
• What are the limitations of radical substitution?

  Can lead to a mixture of products, as there's more than one termination step. Also during propagation, further substitution could occur