CIE AS Chemistry (9701) 2019-2021

Revision Notes

3.2.6 Reactions of Alkenes

Reactions of Alkenes

  • Alkenes are very useful compounds as they can undergo many types of reactions
  • They can therefore be used as starting molecules when making new compounds

Electrophilic addition

  • Electrophilic addition is the addition of an electrophile to a double bond
  • The C-C double bond is broken, and a new single bond is formed from each of the two carbon atoms
  • Electrophilic addition reactions include the addition of:
    • Hydrogen (also known as hydrogenation reaction)
    • Steam (H2O (g))
    • Hydrogen halide (HX)
    • Halogen

Hydrocarbons Electrophilic Addition Reactions, downloadable AS & A Level Chemistry revision notes

The diagram shows an overview of the different electrophilic addition reactions alkenes can undergo

Oxidation

  • Alkenes can also be oxidised by acidified potassium manganate(VII) (KMnO4) which is a very powerful oxidising agent
  • Alkenes can be oxidised by both hot and cold KMnO4 which will result in different products being formed
    • When shaken with cold dilute KMnO4 the pale purple solution turns colourless and the product is a diol
    • When alkenes are reacted with hot concentrated KMnO4 the conditions are harsher causing the C-C double bond to completely break
    • The O-H groups in the diol formed are further oxidised to ketones, aldehydes, carboxylic acids or carbon dioxide gas
    • The actual products formed depend on what is bonded to the carbon atoms in the alkene

 

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

Alkenes can be oxidised by cold dilute and hot concentrated KMnO4 to give different products

  • The reactions of alkenes with hot concentrated KMnO4 can be used to determine the position of the double bond in larger alkenes

 

Hydrocarbons Determining Position of Double Bond, downloadable AS & A Level Chemistry revision notes

The above reactions can be used to predict where the double bond in a larger molecule is

Worked example: Oxidation of alkenes

Hydrocarbons Worked example - Oxidation of alkenes, downloadable AS & A Level Chemistry revision notes

Answer

The products are propanone (a ketone), carbon dioxide and water.

 

Hydrocarbons Oxidation of 2-methylprop-1-ene, downloadable AS & A Level Chemistry revision notes

Worked example: Identifying alkenes from oxidation reactions

Hydrocarbons Worked example - Identifying alkenes from oxidation reactions, downloadable AS & A Level Chemistry revision notes

Answer

The alkene is 1-butene.

 

Hydrocarbons Oxidation of 1-butene, downloadable AS & A Level Chemistry revision notes

Addition polymerisation

  • Addition polymerisation is the reaction of many monomers containing at least one double C-C bond to form the long-chain polymers as the only product
    • Monomers are small, reactive molecules that react together to make the polymer
    • A polymer is a long-chain molecule made up of many repeating units (monomers)
  • In addition polymerisation reaction, the C-C double bond is broken to link together the monomers and form a polymer
  • This is a common method to make plastics

 

Hydrocarbons Addition Polymerisation Ethene, downloadable AS & A Level Chemistry revision notes

The diagram shows a polymerisation reaction of ethene to poly(ethene)

 

Hydrocarbons Addition Polymerisation Propene, downloadable AS & A Level Chemistry revision notes

The diagram shows a polymerisation reaction of propene to poly(propene)

  • Other alkenes and substituted alkenes can also polymerise to make polymers with different properties
    • Eg. poly(chloroethene), also known as PVC is the most versatile plastic used

 

Hydrocarbons PVC, downloadable AS & A Level Chemistry revision notes

Poly(chloroethene) is used as plastic

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