Chapter 12 - Alkanes

• Alkanes are saturated hydrocarbons, meaning they do not have a double carbon bond.
• all carbon atoms are joined by Sigma bonds.
• Bonding in Alkanes:

  • Sigma bonds are formed from the overlap of orbitals
  • An atomic orbital with a hydrogen atom overlaps with an atomic orbital from a Carbon atom
  • The carbon atoms also join together from the overlap of their atomic orbitals
• Straight Chain:

  • As chain length increases, so does boiling point
  • Greater surface area of contact
  • Stronger London forces
  • More energy required to overcome forces
• Branched Alkanes:

  • As branching increases, boiling point decreases
  • Because there are fewer surface points of contact between molecules
  • Weaker London forces so less energy required to overcome forces
• Alkanes are unreactive:

  • C-C and C-H sigma bonds are strong
  • C-C bonds are non-polar
  • C-H bonds are considered non-polar
• A Haloalkane is an organic compound containing at least one halogen.
• Free Radical Substitution:

  • Methane .+ Bromine —> Bromomethane + Hydrogen Bromide
  • Conditions: UV light
• Substitution: The swapping of atoms/ groups of atoms
• Radical: A species with an unpaired electron
• Initiation: The UV light breaks the Br-Br sigma bond homolytically to produce two bromine radicals

  
• Propagation:

  • The bromine radical reacts with methane
  • The bromine radical forms a sigma bond with a hydrogen atom
  • A methyl radical is formed

  
• Propagation (2):

  • The methyl radical reacts with Bromine
  • The methyl forms a sigma bond with a bromine atom
  • A bromine radical is formed

  
• Termination:

  • Two bromine radicals collide and form a sigma bond
  • Exposure to UV light will just break this sigma bond

  
• Issues with free radical substitution:

  • Further substitution can occur
  • Substitution can occur at different positions in carbon chain- the hydrogen removed is random