CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.2.7 Production of Alkenes

Production of Alkenes: Elimination, & Dehydration Reactions & Cracking

  • Alkenes can be made by a series of reactions including elimination, dehydration reactions and cracking

Elimination reaction

  • Alkenes can be produced from the elimination reaction of a halogenoalkane
  • An elimination reaction is one in which a small molecule is lost
    • In the case of halogenoalkanes, the small molecule that is eliminated is a hydrogen halide, HX, where X is the halogen
  • The halogenoalkane is heated with ethanolic sodium hydroxide

 

Hydrocarbons Elimination Reaction, downloadable AS & A Level Chemistry revision notes

Production of an alkene from a halogenoalkane by reacting it with ethanolic sodium hydroxide and heating it

  • The eliminated H+ in HBr reacts with the ethanolic OH to form water
  • The eliminated Br in HBr reacts with Na+ to form NaBr

 

Hydrocarbons Elimination Reaction 2, downloadable AS & A Level Chemistry revision notes

The eliminated HBr reacts with ethanolic OH and Na+ to form H2O and NaBr

  • Note that the reaction conditions should be stated correctly as different reaction conditions will result in different types of organic reactions
  • NaOH (ethanol): an elimination reaction occurs to form an alkene
  • NaOH (aq): a nucleophilic substitution reaction occurs, and an alcohol is one of the products

 

Hydrocarbons Reaction Conditions, downloadable AS & A Level Chemistry revision notes

Different reaction conditions will give different products

Dehydration reaction

  • Alkenes can also be produced from the elimination reaction of alcohols in which a water molecule is lost
    • This is also called a dehydration reaction
  • Alcohol vapour is passed over a hot catalyst of aluminium oxide powder (Al2O3)
    • Concentrated acid, pieces of porous pot or pumice can also be used as catalysts

 

Hydrocarbons Elimination Reaction Alcohol, downloadable AS & A Level Chemistry revision notes

Production of an alkene from an alcohol by using a hot aluminium oxide powder catalyst

 

Hydrocarbons Elimination Reaction Alcohols 2, downloadable AS & A Level Chemistry revision notes

The formation of ethene from ethanol is an example of a dehydration reaction of alcohols

 

  • The smaller alkenes (such as ethene, propene and butene) are all gases at room temperature and can be collected over water

 

Hydrocarbons Apparatus, downloadable AS & A Level Chemistry revision notes

The smaller alkenes are gases at room temperature and collected over water

Cracking

  • Alkenes can also be produced from the cracking of long hydrocarbon molecules in crude oil
  • An aluminium oxide (Al2O3) catalyst and high temperatures are used to speed up this reaction.
  • It is important to ensure that the crude oil doesn’t come into contact with oxygen as this can cause combustion of the hydrocarbons to produce water and carbon dioxide
  • The cracking of crude oil produces a smaller alkane and alkene molecules

 

Hydrocarbons Cracking of Long Alkanes, downloadable AS & A Level Chemistry revision notes

Long hydrocarbon fraction is cracked into two smaller ones

  • The low-molecular mass alkenes are more reactive than alkanes as they have an electron-rich double bond
  • They can therefore be used as feedstock for making new products

 

Hydrocarbons Reactions of Alkenes, downloadable AS & A Level Chemistry revision notes

Alkenes are reactive molecules and can undergo many different types of reactions making them useful as starting compounds

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