Lesson 8, Topic 1
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Alkanes from crude oil notes

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  • Crude oil (petroleum) is a mixture of hydrocarbons, mainly alkanes
  • These alkanes range from small ones like pentane to larger alkanes of 50 carbons or more
  • Crude oil can be separated into more useful parts called fractions by fractional distillation, cracking and reforming

Crude oil is vapourised at 350oC


The vapourised crude oil rises up the fractionating column. The larger hydrocarbons that do not vapourise sink to the bottom forming a thick residue


As the vapour rises up the column it gets cooler and hydrocarbon of different chain lengths condense at different temperatures and run off at different parts of the column


The hydrocarbons with the lowest chain lengths (lowesr boiling points) to not condense and are drawn of as gases at the top of the column

  • Shorter chain fractions such as petrol are more valuable than longer chain ones like bitumen
  • Cracking involves breaking long-chain akanes at the C-C bonds into smaller chain hydrocarbons (which can include alkenes

There are two types of cracking: thermal and catalytic

  • Thermal crack takes place at high temperatures (1000C) and high pressures (70 atm)
  • It produces a lot of alkenes
  • These alkenes are used to make lots of valuable products like polymers
  • Catalytic cracking uses a zeolite catalyst (hydrated aluminosilicate) at a slight pressure and high temperature (about 4500C)
  • It mostly produces aromatic hydrocarbons and motor fuels
  • Using a catalyst cuts costs because the reaction can be done at a low pressure and a lower temperature.
  • The catalyst also speeds up the reaction saving time and money
  • Straight chain alkanes can be converted into branched chain alkanes and cyclic hydrocarbons by reforming 
  • This uses a catalyst (e.g platinum stuck on aluminium oxide)
  • Hexane can be reformed into cyclohexane and hydrogen gas, which can be reformed into benzene (C6H6) and hydrogen gas
  • Octane can be reformed into 2,5 dimethylhexane