Alkenes are unsaturated hydrocarbons whose molecules contain a carbon-carbon (C=C) double bond in the chain.

Alkenes have the general formula: CNH2N

The presence of a carbon-carbon double bond (C=C) in an alkene makes them more reactive than alkanes.

Chemical reactions of alkenes

Combustion of alkenes

When an alkene is combusted, it forms carbon dioxide gas and water.

Hydrogenation of alkenes

The addition of hydrogen across a double bond is known as hydrogenation.

Conditions needed for hydrogenation:

  1. 150-300oC temperature
  2. Nickel catalyst

As hydrogen is added, this reaction is also known as addition reaction. The addition of an hydrogen molecule breaks up the Carbon-Carbon double bond and hence an alkane forms.

Many vegetable oils contain alkenes (which are unsaturated). they are processed by hydrogenation which make them saturated.

Saturated hydrocarbons contain saturated fat which is linked with coronary heart disease. hence their consumption is recommended in small amounts.

Food containing saturated hydrocarbons include: butter, hydrogenated vegetable fat, cheese, etc.

Hydration of alkenes

Alkenes are also involved in a reaction where steam is made to react with them. This is known hydration.

hydration results in the formation of a compound called an alcohol.

Conditions needed for hydration:

  1. 300oC temperature
  2. Phosphoric acid catalyst
  3. 60 atm pressure

Halogenation: the test for unsaturation

Addition of an halogen to an unknown hydrocarbon determines whether it is an unsaturated hydrocarbon or a saturated hydrocarbon.

Bromine is used in this reaction as reactions with fluorine and chlorine can be too explosive or highly exothermic.

Hence the test for unsaturation is also known as bromine water test.

  • Test: Bromine water test.
  • Result for alkene: In the presence of an alkene, the colour of the solution changes from orange-brown to colourless.
  • Result for alkane: In the presence of an alkane, the colour of the solution remains the same.

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