Producing Alkenes (Cambridge (CIE) A Level Chemistry): Revision Note

Production of Alkenes: Elimination, Dehydration & 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

Making alkenes from halogenoalkanes

• The eliminated H⁺ in HBr reacts with the ethanolic OH^- to form water

• The eliminated Br^- in HBr reacts with Na⁺ to form NaBr

Overview of halogenoalkane elimination

• 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

 Comparing reaction conditions

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

 Making alkenes from alcohols

Overview of alcohol elimination

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

Practical set-up to form alkenes from alcohol

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 the combustion of the hydrocarbons to produce water and carbon dioxide

• The cracking of crude oil produces smaller alkane and alkene molecules

 Cracking hydrocarbons

• 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

Possible compounds formed from alkenes