Tests For Alkanes And Alkenes

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Reactions of Alkanes
Reactions of Alkenes
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Chemistry Lessons

This activity compares the reaction with bromine water of several liquid alkanes and alkenes. The test for unsaturation is demonstrated.

The activity can be extended to include testing the samples with a 1% potassium manganate(vii) solution. Small samples of the liquids are also ignited and the appearance of the flames compared.

Testing Alkanes And Alkenes Using Bromine Water

How to distinguish between and alkene and alkane using bromine water.
Alkenes are unsaturated and decolourise an orange solution of bromine water. Alkanes are saturated and do not react with bromine water, so the orange colour persists.

In the following experiment, the first test tube contained cyclohexene and the second test tube containes cyclohexane.

Addition of bromine (bromine water) and potassium manganate (VII) KMnO₄ to cyclohexene

The cyclohexene decolourises the bromine water and changes the potassium manganate from purple to brown.

There will be no colour change if cyclohexane is used instead of cyclohexene.

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Combustion of cyclohexane and cyclohexene

Flammability:
Place about 5 drops of the liquid in an evaporating basin.
Set the liquid on fire by placing a lighted taper or splint near it.
Note the colour and smokiness on the flame in each case.
In addition, note whether any carbon residue is left in the dish.

Observation:
Cyclohexane produces less colour intensity and less soot given off during combustion test compared to cyclohexene.

Reason:

1. Cyclohexane is a saturated hydrocarbon and cyclohexene is an unsaturated hydrocarbon due to the presence of double bond.
2. Both hydrocarbons produce carbon dioxide and water during combustion test. When the oxygen is limited, the product will be carbon monoxide and water.
3. Cyclohexene burns and produces more soot because of the higher percentage of carbon compared to cyclohexane.

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Questions

1. What difference was observed between the flames for alkanes and alkenes?
2. What feature of the molecules gives rise to this difference?

• Show Answers
  1. Alkanes produces less colour intensity and less soot given off during combustion test compared to alkenes.
  2. The alkenes have a higher proportion of carbon in their molecules - they have a higher C:H ratio.

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