Tests For Alkanes And Alkenes

Alkanes (saturated hydrocarbons) and alkenes (unsaturated hydrocarbons with C=C bonds) can be identified using specific chemical tests.

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.

The following table gives a summary of the tests for alkane and alkene: bromine test, Baeyer’s test, and combustion test. Scroll down the page for more examples and videos.

 

1. Bromine Water Test

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.

1. Add 1-2 drops of bromine water (Br₂ in H₂O) to the hydrocarbon.
   
2. Shake and observe:
   Negative (Alkane): Stays orange.
   Positive (Alkene): Orange → Colorless.
   

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2. Baeyer’s Test (Potassium Permanganate Test)

Alkenes react with KMnO₄ (purple) turning the solution brown (MnO₂ precipitate).
Alkanes do not react (purple remains).

1. Add KMnO₄ to the hydrocarbon.
   
2. Observe:
   Negative (Alkane): Purple persists.
   Positive (Alkene): Purple → Brown precipitate.
   

Addition of bromine (bromine water) and potassium manganate (VII) KMnO₄ to cyclohexene
In the following experiment, 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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3. Combustion Test (Flame Characteristics)

Alkanes burn with a cleaner/blue flame (complete combustion).
Alkenes burn with a sooty/yellow flame (higher C:H ratio, incomplete combustion).

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.

Key Notes

• Bromine Water is the most reliable test for unsaturation.
• Baeyer’s Test is sensitive to other reducing agents (false positives possible).
• Combustion is qualitative; use with other tests for confirmation.

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