You are handed a test tube containing a colorless liquid. You are also told the liquid is either an alkane or an alkene. What simple test can be used to determine whether this liquid is an alkane or an alkene? Discuss the results of this test and be specific.
The Correct Answer and Explanation is:
To determine whether a colorless liquid is an alkane or an alkene, you can perform the bromine water test.
Test: Bromine Water Test
- Add a few drops of bromine water (a reddish-brown solution) to the unknown liquid in the test tube.
- Shake or mix the contents gently.
- Observe the color change.
Results:
- If the reddish-brown color disappears (decolorizes): The liquid is an alkene.
- If the reddish-brown color remains unchanged: The liquid is an alkane.
Explanation
The bromine water test is a standard qualitative test used in organic chemistry to detect the presence of carbon-carbon double bonds, which are found in alkenes. Bromine (Br₂) reacts readily with the double bond in alkenes via an electrophilic addition reaction, resulting in the formation of a dibromoalkane. During this reaction, the bromine atoms add across the double bond, and the brown color of bromine fades as it becomes part of the colorless organic product.
The reaction is as follows:
Alkene + Br₂ → Dibromoalkane
For example:
CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br
This reaction takes place without the need for light or heat, making it a quick and reliable test.
On the other hand, alkanes are saturated hydrocarbons and do not contain carbon-carbon double bonds. They do not react with bromine water under normal conditions. Therefore, when bromine water is added to an alkane, the reddish-brown color remains unchanged, indicating no reaction has occurred.
This test is specific and useful because it visually distinguishes between alkanes and alkenes based on chemical reactivity. However, if the test is conducted in bright sunlight or in the presence of UV light, even alkanes may react with bromine through a free radical substitution mechanism, though much more slowly. For accurate results, the test should be done away from strong light sources to avoid false positives.
This makes the bromine water test a simple yet powerful tool for identifying unsaturation in organic compounds.
