Reactions of Alkenes (Cambridge (CIE) A Level Chemistry): Revision Note

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

Electrophilic addition reactions of alkenes

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

 Oxidising alkenes using KMnO₄

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

Predicting the position of double bonds formed

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

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 an addition polymerisation reaction, the C-C double bond is broken to link together the monomers and form a polymer

• This is a common method of making plastics

 The polymerisation of ethene

The polymer backbone consists of a carbon chain with monomers that contain 2 carbon atoms

The polymerisation of propene

The polymer backbone consists of a carbon chain with monomers that contain 2 carbon atoms with the methyl group, from the propane monomer, as a side chain

• Other alkenes and substituted alkenes can also polymerise to make polymers with different properties

  • E.g. poly(chloroethene), also known as PVC is the most versatile plastic used

The polymerisation of chloroethene

Poly(chloroethene) is used as plastic