The alkenes are a homologous series of compounds. They have a double carbon bond, meaning they are unsaturated. The alkenes have a formula CnH2n They are made when large alkanes from the fractional distillation of crude oil are cracked. Alkenes are much more reactive than alkanes as they have a double bond which reacts easily with other substances.

The first few[edit | edit source]

  • Ethene: CH2--CH2
  • Propene: CH2--CH-CH3
  • Butene: CH2--CH-CH2-CH3
  • Pentane:
  • Hextane:
  • Heptane:
  • Octane:

Structure[edit | edit source]

Ethene Ethene.jpg

Propene Propene.jpg

Butene Butene.jpg

Reactions of Alkenes[edit | edit source]

Combustion[edit | edit source]

The flame is usually yellow and sooty. There is either complete combustion, where the products are only carbon dioxide and water or incomplete combustion, where carbon monoxide and water are produced.

Reaction with hydrogen[edit | edit source]

This is an addition reaction, where the unsaturated alkene is changed into a saturated alkane. The catalyst is nickel.

Example: C2H4 + H2 = C2H6

Ethene + Hydrogen = Ethane

Reaction with bromine[edit | edit source]

This is an addition reaction also, forming dibromoalkanes. This is also the test to distinguish between alkanes and alkenes (alkanes do not react quickly, and the orange bromine stays orange, while alkenes turn it colourless).

Reaction with acidified potassium manganate(VIII)[edit | edit source]

This is another addition reaction, where the purple KMnO4 loses its colour. A 'diol' is formed.

Reaction with water vapour[edit | edit source]

This addition reaction needs a catalyst and extreme conditions: 300oC, 60atm; catalyst: phosphoric acid. An alcohol is produced.

Addition polymerisation[edit | edit source]

See main article Polymerisation

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