3.1 alkanes

• carbon-carbon bond is relatively strong and non-polar
• carbon-hydrogen bond is strong and relatively non-polar
• fractional distillation

  1. oil is preheated
  2. the oil is passed into a column
  3. the temperature of the column decreases as you go up
  4. the fractions condense at different heights
• vacuum distillation

  • heavier residue is distilled in a vacuum at lowered pressure to reduce the boiling point
• fractional distillation in a laboratory equipment:

  • flask
  • bunsen burner
  • fractionating column
  • thermometer
  • condenser
• cracking is the formation of smaller hydrocarbon chains by breaking c-c bonds in larger hydrocarbon chains
• larger mr alkanes (cracked) - smaller mr alkanes + alkenes + (hydrogen)
• what are the two ways cracking can occur?

  thermal and catalytic
• economic reasons for cracking

  1. demand
  2. using excess hydrocarbons
  3. products are more valuable
• thermal cracking

  • high pressures
  • high temperature
  • produces alkenes and sometimes hydrogen
• catalytic cracking

  • moderate pressure
  • high temperature
  • zeolite catalyst
  • produces: fuels and aromatic hydrocarbons
• alkanes readily burn in the presence of oxygen
• complete combustion occurs when there is excess oxygen
• incomplete combustion occurs when there is a limited amount of oxygen producing carbon monoxide which is toxic and carbon/soot which causes global dimming
• sulfur containing impurities can be found which produces sulfurdioxide when burnt which then dissolves in the atmosphere causing acid rain
• coal is high in sulfur content
• flue gas desulfurisation

  • SO2 can be removed if passed through a scrubber containing basic calciumoxide which reacts with acidic SO2 in a neutralisation reaction producing calciumsulfite which can be used for plasterboard
• nitrogenoxides

  • form when there N2 reacts with O2 in car engines at high temperatures and sparks that break the strong N2 bond
• polllutants from combustion of internal engines

  • NO - toxic and produces acidic gases
  • NO2 - toxic and produces acidic rain
  • CO - toxic
  • CO2 - global warming
  • unburnt hydrocarbons - smog
  • soot/C - global dimming and respiratory problems
• catalytic converters

  • remove carbon monoxide , nitrogen oxides and unburnt hydrocarbons from the exaust gas into harmless carbon dioxide , water and nitrogen
  • 2CO + 2NO = 2CO2 + N2
  • CH + NO = CO2 + N2 + H2O
• catalytic converters

  • are ceramic honeycomb coated with a thin layer of a metal catalyst such as platinum , rhodium and palladium
• name three green house gases?

  water vapor (H2O), carbondioxide (CO2) and methane (CH4)
• mechanism of greenhouse effect:

  1. uv radiation passes through the atmoshphere and warms up the earths surface
  2. infared long wavelength radiation is radiated back by the eart
  3. the carbon to oxygen bond in carbon dioxide absorbs the infared radiation so it doesnt escape
  4. the energy is transferred to other molecules in the atmosphere by collision
• synthesis of haloalkanes

  • in the presence of uv light, alkanes react with halogens to form a mixture of products by substitution of Hydrogen atoms
  • the mechanism is called free radical substitution
• step 1 ( initiation) : free radical substitution

  process- homolytic fission

  A) cl2

  B) 2cl

  
• homolytic fission in initation is when a bond is broken by homolytic fission it produces free radicals as each atom gets 1 electron from the covalent bond
• a free radical is a reactive species which possess an unpaired electron and does not have a charge
• step 2 (propogation): free radical substitution

  • the chlorine radical is very reactive and removes a hydrogen from methane
  • the methyl free radical reacts with chlorine to form your haloalkane and another cl radical
  • all propogation has a free radical in the products and reactant

  A) CH4

  B) cl

  C) HCl

  D) CH3

  E) CH3

  F) cl2

  G) CH3Cl

  H) cl

  
• step 3 (termination) : free radical substitution

  • collision of two free radicals does not produce more free radicals so the chain is terminated
• petroleum is a mixture of mainly alkane hydrocarbons which can be seperated by fractional distillation
• alkanes are saturated hydrocarbons
• alkanes don't react with many reagents because the C-C and C-H bonds are relatively strong
• whats the name of the mechanism when alkanes react with free radicals?


  free radical substitution
• what are the three steps of free radical substitution?


  1. initiation
  2. propogation
  3. termination
• what happens in initiation?


  • bond had been broken in a process called homolytic fission
  • each atom gets one electron from the covalent bond
• what happens when a bond breaks by homolytic fission?


  free radicals form
• do free radicals have a charge?


  no
• what are free radicals?


  a reactive species which posses and unpaired electron
• how do we write the reactants and products in the termination step?


  in structural formula