CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.2.8 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

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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