• 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

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

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

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

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

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

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

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

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