Module 4.1.2- Alkanes

• Sigma bonds


  a single covalent bond that is formed when an electron pair is shared by the direct overlap of bonding orbitals
• Shape of alkanes

  Tetrahedral 109.5 atoms can rotate freely around the sigma bonds which act as axes
• Effect of chain length on boiling point

  As chain length increases, molecules have a larger surface area so more surface contact is possible between molecules

  London forces are stronger so more energy is needed to overcome them, increasing boiling point

  
• Effect of branching on boiling point

  Branched alkanes have a lower boiling point

  This is because there are fewer surface points of contact between molecules, giving fewer London forces

  Branches also get in the way and prevent branched molecules from getting as close together, further decreasing intermolecular forces

  
• Reactivity of alkanes

  unreactive C-C and C-H bonds are non polar C-C and C-H bonds are strong (high bond enthalpy)
• Combustion of alkanes

  alkane + oxygen -> carbon dioxide + water

  Balance Carbon first, Hydrogen second and Oxygen third
• Incomplete complete of alkanes

  insufficient amount of oxygen to react with alkane

  either Carbon Monoxide (CO) is made or Carbon (C)
• Alkane + Halogen (X)

  substituting an alkane C-H with a C-X, requires UV or heat
• Radical substitution


  A type of substitution reaction in which a radical replaces a different atom or group of atoms
• Bromination of alkanes: Initiation


  Br2 -> 2Br•

  Bromine molecule is broken by homolytic fission using UV radiation

  
• Bromination of alkanes: Propagation 1 and 2


  Propagation step 1: CH4 + Br• -> •CH3 + HBr

  Propagation step 2: •CH3 + Br2 -> CH3Br + Br•
• Bromination of alkanes: Termination


  Br• + Br• -> Br2

  CH3• + CH3• -> C2H6

  CH3• + Br• -> CH3Br
• Bromination of alkanes summaryInitiation, Propagation 1 and 2, Termination

  Initiation: Br2 -> 2Br•

  Propagation step 1: CH4 + Br• -> •CH3 + HBr

  Propagation step 2: •CH3 + Br2 -> CH3Br + Br•

  Termination: Br• + Br• -> Br2

  CH3• + CH3• -> C2H6

  CH3• + Br• -> CH3Br
• Limitations of radical substitution

  Further substitution

  Substitution at different positions in a carbon chain