3.2.8 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

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

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

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

Worked example: Oxidation of alkenes

Answer

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

Worked example: Identifying alkenes from oxidation reactions

Answer

The alkene is 1-butene.

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

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

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

Poly(chloroethene) is used as plastic