Alkanes

• Alkane basics

  • general formula: CnH2n+2
  • alphatic = carbons in a straight/branched chain
  • alicyclic = carbons in a circle
  • aromatic = carbons with a ring of delocalised electrons
  • boiling point: increased chain length -> more van der Waals forces -> more energy needed
  • melting point: longer molecules -> more surface contact between molecules -> more energy needed to vapourise substance
  • branched alkane have lower MP & BP than straight alkane -> less surface contact
  • alkanes aren't soluble in water but are in organic solvents
  • non-polar molecules
• Fraction = molecules with similar chain length & BP range

  • seperated by fractional distillation
  • differently sourced crude oil have different compositions with other dissolved compounds

  1. Crude oil vapourised by heating & enters column
  2. fractionating column hotter at bottom than top
  3. negative temp gradient bottom to top
  4. Vapour travels up the fractions condense at point cool enough for them to do so
  5. longer chain -> higher BP
  6. short alkane fractions leave at top of column & long alkane fractions leave at bottom of column
• Fractions

  A) refinery oil

  B) C1-C4

  C) fuel - furnaces, blowtorches

  D) petrol

  E) C5-C12

  F) vehicle fuel, lubricants, photochemical industry

  G) naptha

  H) C2-C14

  I) hydrocarbon cracking, soaps, solvents, fuels

  J) paraffin (kerosine)

  K) C11-C15

  L) heaters, aviation, as a wax

  M) diesel

  N) C15-C19

  O) vehicle fuel

  P) lubricating/fuel oil

  Q) C20-C30

  R) ship fuel

  S) bitumen

  T) C30+

  U) roads (tarmac)

  
• Cracking = process of breaking down larger hydrocarbons into shorter, more useful hydrocarbons (more in demand)

  • cracking involves homolytic fission of bonds via a free radical mechanism

  thermal cracking:

  • conditions
  • very high temp: 400 - 1000°C
  • very high pressure: up to 70 atm
  • conditions applied for a very short time
  • 1s -> to prevent further decomposition
  • produces high % of alkenes & short chain alkanes
  • uses: manufacture of chemicals & polymers
• Catalytic cracking

  • conditions:
  • temp: 450°C
  • pressure: between 1-2 atm
  • time taken: 2s-4s
  • catalyst: zeolite or aluminosilicates
  • honeycomb structure -> increase SA -> increase rate of reaction
  • produces aromatic hydrocarbons, branched alkanes & cycloalkanes
  • uses: fuels for cars
• Combustion of alkanes

  • complete combustion: excess/sufficient/plentiful O2
  • always produces H2O & CO2
  • incomplete combustion: insufficient/limited O2
  • products: C/CO/CO2 & H2O
  • gaseous product = CO or CO2
  • toxic gas = CO
  • solid product = C
• Pollution caused by combustion

  1. CO2 - greenhouse gas
  2. CO - toxic/poisonous gas
  3. C particulates - exacerbates asthma & can cause cancer
  4. H2O water vapour - greenhouse gas
  5. NOx nitrogen oxides - form nitric acid or photochemical smog (ground-level ozone)
  6. petrol engines hot enough to produce nitric acid(acid rain)
  7. smog made if reacted with unburnt hydrocarbons
  8. unburnt hydrocarbons - some greenhouse gases & can form photochemical smog
  9. sulfur dioxide - forms acid rain
  10. some fossil fuels have traces of sulfur but can be removed by reacting with base
• Removal of polluting gases

  • flue gas desulfurisation = process of removing sulfur dioxide from flue gases
  • flue gases = gases released by power stations
  • CaO & CaCO3 can be used to produce gypsum using SO2
  • gypsum = builder's plaster/plasterboard
• Removing polluting gases

  • most gases formed from combustion in engines
  • engines built with catalytic converters to reduce output of CO, NOx & unburnt hydrocarbons
  • honeycomb structure made of ceramic coated in platinum & rhodium metals (catalysts)
  • honeycomb increases SA massively
  • polluting gases pass over catalysts
  • catalyses reaction between polluting gases to form less harmful products
  • SO2 can't be removed by a catalytic converter
• Photochemical reactions/Free radical substitution mechanism

  • free radical = chemical species with an unpaired electron
  • extremely reactive & can start a chain reaction
  • substitution reaction = 1 or more (hydrogen) atoms replaced in a molecule
  • caused by homolytic fission = covalent bond breaks evenly & each bonded atom takes 1 of the shared pair of electrons
  • free radical = single electron
  • UV energy/energy greater than H-halogen bond needed
  • only 1 quantum of UV energy absorbed
• Free radical substitution (forms halogenoalkanes)


  Stage 1: initiation - homolytic fission occurs

  • Cl-Cl bond breaks not C-H which is too strong

  Cl:Cl -> 2Cl• reagent: Cl2 (can be other halogens)

  Stage 2: Propagation (2 stages)

  • stage 1: Cl• takes H from CH4 & leaves •CH3 & HCl

  Cl• + CH4 -> HCl + •CH3

  • stage 2: •CH3 reacts with Cl2 & leave Cl• & CH3Cl

  Stage 3: Termination

  termination = joining of 2 free radicals to form no free radicals

  2Cl• -> Cl2

  Cl• + •CH3 -> CH3Cl

  •CH3 + •CH3 -> C2H6
• CFCs = chlorofluorocarbons

  • used to be used for refrigerants, aerosols, foam fire extinguishers & air conditioning
  • good properties: low reactivity & volatility & non-toxic
  • bad properties: catalyses break down of ozone
  • ozone (O3) prevents UV light reaching Earth's surface
  • ground-level ozone can cause lung irritation & respiratory problems
  • Chlorine free radicals from CFCs can attack ozone molecules

  initiation: CCl3F -> •CCl2F + •Cl

  propagation: Cl• + O3 -> ClO• + O2 & ClO• + O3 -> 2O2 + Cl•

  overall: 2O3 -> 3O2