alkenes

• Fractional distillation

  -crude oil is worlds main source of organic chemicals

  -as a mixture its not useful but different length hydrocarbon chains are

  -mixture of mostly alkanes, unbranched and branched

  -size and length of hydrocarbon determines which fraction it will be spserated
• fractional distillation process

  -carried out in fractionating column, hot at bottom, cool at top

  -crude oil enters column and heated so vapours rise

  -hydroc. with high boiling points will immediately condense at high temps lower down

  -hydroc. with low boiling points will rise up column and condense at different heights acc. to bp

  -tapped off as liquids

  -smaller hydroc. are collected at the top
• crude oil: cracking

  -smaller hydroc. chains are more useful and higher in demand

  -excess heavier fractions are broken down into smaller, more useful compounds

  -process is cracking
• production of alkanes from cracking

  -large, less useful hydroc. chains broken into small, ,ore useful ones

  -large hydroc. fed into steel chamber and heated at high temp

  -then passed over Al2O3 catalyst

  -chamber doesn't contain oxygen to prevent combustion of hydroc. inot water and co2

  -smaller alkane and one (or more) alkene formed
• complete combustion of alkanes

  -when alkanes are burn in excess oxygen

  -all carbon and hydrogen will be oxidised to co2 and h2o

  -alkane + oxygen ----> carbon dioxide + water
• incomplete combustion of alkanes

  -alkanes burnt in a limited supply of oxygen

  -not all carbon is fully oxidised

  -some carbon only partially oxidised to form carbon monoxide

  -alkane + oxygen --incomp--> carbon monoxide + water

  -takes place inside car engine

  -with reduced oxygen supply, carbon will be reduced as soot

  -alkane + oxygen --incomp--> carbon + water
• carbon monoxide

  -toxic and odourless gas which can cause dizziness, loss of consciousness and eventually death

  -CO binds to haemoglobin which therefore cannot bind to oxygen and co2
• oxides of nitrogen

  -normally nitrogen is too unreactive to react with oxygen in air

  -in car engine, high temps and press. are reached causing oxidation of nitrogen to take place

  N2(g) + O2(g) ---> 2NO(g)

  N2(g) + 2O2(g) ---> 2NO2(g)

  -oxides are released in the exhaust - goes to atmosphere

  -car exhaust fumes also contain unburnt hydrocarbons (VOCs)

  -nitrogen oxides can react with VCOs to form peroxyacetyl nitrate (PAN) main pollutant in photochemical smog

  -nitric oxides can also dissolve and react in water with oxygen to form nitric acid which causes acid rain

  -acid rain corrodes building and endangers aquatic life as rivers or lakes can become too acidic
• catalytic removal

  -to reduce pollutants from car exhausts, cars are fitted with catalytic converters

  -precious metals such as platinum are coated on a honeycomb to provide a large surface area

  -reactions that take place include:

  oxidation of CO to CO2

  reduction of NO to N2

  oxidation of unburnt hydrocarbons
• free radical substitution

  -alkanes undergo frs in which hydrogen atom gets substituted b a halogen (cl or br)

  -alkanes are unreactive so UV light is needed

  -initiation - halogen bond broken to form 2 radicals

  -propagation - creates further radicals

  -termination - 2 radical collide
• initiation step


  -Cl-Cl or Br-Br bond broken by energy from UV light

  -produces 2 radicals in a homolytic fission

  Cl2 ---UV---> 2Cl*
• propagation step

  CH4 + Cl (dot) ----> CH3 (dot) + HCl

  CH3 (dot) +Cl2 ----> CH3Cl + Cl (dot)
• termination step

  2Cl* ----> Cl2

  CH3 (dot) + Cl* ----> CH3Cl

  CH3 (dot) + CH3* ----> CH3CH3