• 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?KMnO_4?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 KMnO_4?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