Alkanes

• each carbon atom in an alkane is surrounded by 4 bonding pairs of electrons, shape is tetrahedral and bond angle is 109.5 degrees
• polarity of alkanes

  c and h have similar electronegativities, so the bonds are non-polar, all alkanes are therefore non-polar. Alkanes have a simple molecular lattice structure with van der waals' between molecules which are weak intermolecular forces
• solubility

  the forces of attraction between water molecules are H bonds between molecules which are much stronger than vdw in alkanes, so alkanes are not soluble in water
• boiling points of straight chain alkanes

  as length of the carbon chain increases, boiling point increases because there are more electrons so the vdw are stronger and require more energy to break
• boiling point of branched chain alkanes

  as number of branches increases, boiling point decreases. there are fewer points of contact between the molecules so the vdw between molecules become weaker and require less energy to break
• crude oil or petroleum

  mixture consisting mainly of alkane hydrocarbons, main source of fuels and petrochemicals
• fractional distillation


  separation of the components of a liquid intro fractions which differ in boiling point
• define a fraction

  group of compounds that have similar boiling points and are removed at the same level in a fractionating column
• process of fractional distillation

  • crude oil is vaporised and the vapour is introduced near the bottom of the column
  • the vapour rises up the column and creates a temperature gradient
  • because the alkanes have different boiling points, they condense at different levels and the fractions are collected
  • the hydrocarbons with the lowest boiling points do not condense and are drawn off as gases at the top of the tower
  • the largest hydrocarbons don't vaporise and are collected at the base of the tower as a thick residue.
• longer, less useful alkanes are converted to more useful molecules through a process called cracking in which c-c bonds are broken
• thermal cracking

  very high temperature

  very high pressure

  produces alkanes and high percentage of alkenes

  the c-c bonds can break at different positions in the chain to give a mixture of products

  products are used to make polymers
• thermal cracking of 12 C atom

  C12H26 -> C2H6 + 2C5H10
• catalytic cracking


  high temp and slight pressure

  in presence of zeolite catalyst

  produces branched and cyclic alkanes and aromatic hydrocarbons, benzene

  used as motor fuels
• complete combustion

  products formed are CO2 + H2O

  combustion of methane: CH4 + O2 -> CO2 + 2H2O

  when oxygen is in excess
• incomplete combustion

  limited supply of oxygen

  forms CO + H2O and then solid C, soot

  CH4 + 1/2O2 ->CO +2H2O
• Unburnt hydrocarbons

  low level ozone causes respiratory problems

  react with NOx to produce ozone (O3)
• carbon dioxide


  greenhouse gas , leads to global warming.

  From complete combustion of alkanes
• carbon monoxide

  toxic gas

  incomplete combustion
• carbon

  respiratory problems, worsens asthma

  further incomplete combustion
• nitrogen oxides, NOx

  dissolve in rain water, HNO3

  N2 + O2 -> 2NO

  nitrogen from air reacts with oxygen in engine due to high temperatures
• sulfur dioxide, SO2

  dissolve in rain water to make acid rain, H2SO3, H2SO4

  sulfur impurities in fossil fuels, react with O2 in air
• the catalytic converter purpose

  to remove CO, NO and unburnt hydrocarbons
• catalytic converters contain a honeycombed structure coated with a thin layer of Pd/Pt/Rh metals
• why is a thin layer used on catalytic converters?

  to save money
• what does the honeycomb structure provide?

  a large surface area
• removal of NO and CO

  NO + CO -> 1/2 N2 + CO2
• Removal of unburnt hydrocarbons

  by reaction with oxygen

  C8H18 + 12 1/2 O2 -> 8CO2 + 9H2O

  by reaction with NO

  C8H18 + 25NO -> 12 1/2 N2 + 8CO2 + 9H2O
• Flue gas desulfurisation

  power stations burn coal or natural gas to produce electricity. Sulfur dioxide is also produced. Chimneys, or flues, are coated with calcium oxide or calcium carbonate which absorb and react with SO2 produced

  CaO + SO2 -> CaSO3 (calcium sulfate iv)

  CaCO3 + SO2 -> CaSO3 + CO2
• alkanes are generally unreactive because?

  They have strong covalent C-C and C-H bonds and they are non polar
• halogenation of alkanes

  reagent: halogen X2

  conditions: uv light

  it's a substitution reaction because a hydrogen atom is substituted for a halogen
• methane and chlorine

  CH4 + CL2 -> CH3CL + HCl
• free radical


  a species with an unpaired electron. reactive
• free radical substitution mechanism

  initiation- formation of radicals x-x bond broken to make 2X*, requires UV light

  propagation- formation of products (halogenoalkane and HX)

  termination- removal of radicals

  unpaired electron on a radical is shown by a dot
• the reaction between methane and chlorine

  initiation: Cl2 -> 2Cl*

  propagation: a) Cl* + CH4 -> C*H3 + HCl b) C*H3 + Cl2 -> CH3Cl + Cl*

  termination: Cl* + Cl* -> Cl2

  C*H3 + Cl* -> CH3CL

  C*H3 + C*H3 -> C2H6