Tests for Unsaturation

Unsaturation is a property of unsaturated organic compounds that have multiple bonds (double or triple bonds). These bonds help in identifying the functional groups of a compound for qualitative analysis. There are two major tests for unsaturation in Hydrocarbons- Alkaline potassium permanganate test (Baeyer’s test) and Bromine Test. In Baeyer’s test, the disappearing of pink colour indicates that the compound is unsaturated. Bromine test can be done separately for water-soluble, water-insoluble, and gaseous compounds. However, in the bromine test disappearance of orange-red colour indicates the presence of the unsaturated compound. If both of the above tests are positive, then only it is confirmed that there is unsaturation in an organic compound. 

Key Terms: Hydrocarbons, Unsaturation, Potassium Permanganate, Baeyers Test, Bromine Tests, Potassium Hydroxide Solution, Anhydrous Sodium Carbonate, Bromine

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What is Unsaturation? 

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Unsaturation can be defined as the property of unsaturated compounds (hydrocarbons) that have multiple bonds (double or triple bonds). The hydrocarbons that contain a double bond (one sigma bond and one pi bond) between the two carbon atoms are called alkenes. The hydrocarbons that contain triple bonds (one sigma bond and 2 pi bonds) between two carbon atoms are called alkynes. Some examples of alkenes and alkynes are propene, propyne, butene, and butyne.

Also Read: Sigma and Pi Bonds

The unsaturation tests like Baeyer and Bromine tests are done for unsaturation of alkenes and alkynes. Aromatic compounds (example-benzene) are unsaturated chemical compounds that are characterized by one or more planar rings of atoms joined by covalent bonds of two different kinds. They do not come under alkenes. Therefore, no reaction would take place which means no results.

Read More: Aromaticity

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Importance of Tests of Unsaturation

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Test of unsaturation is done to find out the double and triple bond in organic compounds. These bonds help in identifying the functional groups of a compound for qualitative analysis.

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Experimental Tests For Unsaturation

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There are two major tests for testing the unsaturation of unknown compounds-

(A) Alkaline Potassium Permanganate (KMnO₄)Test (Baeyer’s Test)

(B) Bromine Test

• Bromine tests and Baeyer’s test are two major tests for testing the unsaturation of unknown compounds. 
• They detect whether an unknown compound consists of double or triple bonds. These tests will come negative for saturated organic compounds.
• Bromine test can be done separately for water-soluble, water-insoluble, and gaseous compounds. 
• In Baeyer’s test, the disappearing of pink colour indicates that the compound is unsaturated. However, in the bromine test disappearance of orange-red colour indicates the presence of the unsaturated compound. 
• If the only case if both of the above tests are positive, then only it is confirmed that there is unsaturation in an organic compound. 
• In the Bromine test, it must be noted that in place of CCl₄ any other solvent such as CHCl₃/dioxane and even water can be used to dissolve the organic compound for carrying out the reaction.

Read More: Unsaturated Hydrocarbons

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Experiment 1

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Aim: To perform the Alkaline Potassium Permanganate (KMnO₄)Test (Baeyer’s Test)

Materials Required

• Potassium hydroxide solution
• Anhydrous sodium carbonate
• Acetone
• Sample compound to be tested
• Distilled water
• Test tube
• Test tube holder

Methodology

• Preparation of Reagents

• To prepare this reagent, dissolve 1g of KMnO₄ in 100ml of distilled water in a beaker. 
• Now, add 10g of anhydrous sodium carbonate to the above solution and shake the mixture. 
• Shake to dissolve it completely.

Procedure

1. Put a small amount of the given organic compound in a test tube. 
2. Now, you have to dissolve 25-30 mg of organic compound in 2 mL of water or acetone (free of alcohol) in another test tube.
3. Add 1% alkaline potassium permanganate solution drop by drop into the test tube (step 2) and shake the test tube gently.
4. Notice the colour of the mixture.
5. If the mixture colour disappears from pink this means the given organic compound is unsaturated.

Alkaline Potassium Permanganate (KMnO4)Test (Baeyer’s Test)

Chemical Reaction:

Precautions: The test must be performed at room temperature.

Read More: Structure of Amines

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Experiment 2

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Aim: Test for the unsaturation by Bromine test

Materials Required

1. Bromine water solution/ Bromine in CCl₄ or chloroform
2. Carbon tetrachloride/chloroform
3. Sample compound to be tested
4. Distilled water
5. Test tubes
6. Test tube holder
7. Jar

Procedure

(A) Preparation of Reagents

1. Bromine in CCl₄: Pour 5ml of liquid bromine in 100ml of carbon tetrachloride and shake it gently.
2. Bromine Water: Take 5ml of bromine and then add 100 ml of distilled water. Shake well and decant off the clear liquid.

(B) Bromine Test

(a) Water-soluble Compounds

1. Take the unknown organic compound in a test tube.
2. Dissolve this compound in 2mL of distilled water.
3. Add bromine water drop by drop to the above mixture with constant shaking.
4. If the given compound is unsaturated, then the orange-red colour of bromine disappears. 

Bromine Test

(b) Water-insoluble Compounds

1. Take the unknown organic compound in a test tube.
2. Dissolve this compound in 1mL of carbon tetrachloride (CCl₄).
3. Add bromine to the above mixture with constant shaking.
4. The disappearance of the orange-red colour of Bromine indicates that the compound is unsaturated. 

(c) Gaseous Compounds

1. Take an unknown gaseous compound in a jar and add 1-2mL of bromine solution.
2. Shake the mixture well.
3. If the orange-red colour of Bromine disappears this means the compound is unsaturated. 

The chemical reaction is shown below:

Precautions

• The tests must be performed at room temperature. 
• Do not inhale the bromine vapours.
• Avoid the contact of bromine vapours with the skin.
• Wear gloves and a mask while transferring the solutions.

Read More: Saturated Hydrocarbons

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Things to Remember

• Unsaturation is a property of unsaturated compounds (hydrocarbons) that have multiple bonds (double or triple bonds). For example- propene, propyne, butene, and butyne. 
• The tests are done to find out the double and triple bond in organic compounds. These bonds help in identifying the functional groups of a compound for qualitative analysis.
• Bromine test and Baeyer’s Test are two major tests for testing the unsaturation of unknown compounds. 
• In Baeyer’s test, the disappearing of pink colour indicates that the compound is unsaturated. However, in the bromine test disappearance of orange-red colour indicates the presence of the unsaturated compound. 
• It is confirmed that there is unsaturation in an organic compound if both of the above tests are positive.
• Most of the solvents or chemicals involved in both tests are hazardous. The methodology should be performed with precautions. 

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