chapter 12 - alkanes

• bonding in alkanes

  • bonds are sigma bonds
  • each carbon atom has 4 sigma bonds
• sigma bonds is the result of the overlap of two orbitals, and is in line with the bonding atoms
• alkanes are all tetrahedral shape, with 109.5 bond angles
• fractional distillation is the separation of crude oil into fractions based on boiling points
• alkanes boiling points

  • increases as chain length increases - greater surface area for contact - stronger london forces
  • derceases as branching increases - less surface area for contact - weaker london forces
• combustion of methane

  • CH4 + 2O2 -> CO2 + 2H2O

  incomplete combustion of methane (2 reactions)

  • CH4 + 3/2O2 -> CO + 2H2O
  • CH4 + 2O2 -> C + 2H2O
• reactions of alkanes with halogens

  alkane + halogen -UV radiation-> haloalkane + hydrogenhalide

  this is radical substitution
• mechanism for bromination of alkanes

  step 1 - initiation

  • Br-Br -> Br. +.Br

  step 2 - propogation

  • CH4 + Br. -> CH3 + HBr
  • .CH3 + Br2 -> CH3Br + Br.

  step 3 - termination

  • Br. + Br. -> Br2
  • .CH3 + .CH3 -> C2H6
  • .CH3 + .Br -> CH3Br
• problem with mechanism for bromination of alkanes

  further substitution can occur, leading to a mix of different products, where sometimes all the hydrogen atoms have been substituted