Tests For Unsaturation: Procedure And Results Interpretation

Saturated And Unsaturated Hydrocarbons

Hydrocarbons, or molecules which contain hydrogen atoms and carbon atoms that are bonded to each other and may or may not contain other atoms, are classified into two distinct categories: saturated and unsaturated. 

Saturated Hydrocarbons

A Saturated hydrocarbon is a hydrocarbon in which all the carbon-carbon bonds are single bonds. A hydrocarbon is an organic compound whose only constituents are carbon and hydrogen. As the name suggests, saturated hydrocarbons are hydrocarbons in which all the carbon atoms are bonded to four other atoms and are ‘saturated’, implying that no carbon-carbon multiple bonds exist in these organic compounds.Generally, the term ‘saturated hydrocarbon’ is used to refer to alkanes – acyclic hydrocarbons containing only sp³ hybridized carbon atoms.

A few other examples of saturated hydrocarbons include:

• Butane (C₄H₁₀)
• Octane (C₈H₁₈)
• Cyclohexane (C₆H₁₂)
• Cyclopropane (C₃H₆)

Unsaturated Hydrocarbons

Unsaturated hydrocarbons are organic compounds that are entirely made up of carbon and hydrogen atoms and consist of a double or a triple bond between two adjacent carbon atoms. The hydrocarbons having at least one double bond between two adjacent carbon atoms are called alkenes, whereas the hydrocarbons which contain a carbon-carbon triple bond are referred to as alkynes.

Examples Of Unsaturated Hydrocarbons

• Ethene, C₂H₄
• Benzene, C₆H₆
•  Acetic acid, C₂H₄O₂

Difference Between Saturated and Unsaturated Hydrocarbon

Unsaturated hydrocarbons feature at least one double or triple bond between two adjacent carbon atoms. The key differences between saturated and unsaturated hydrocarbons are tabulated below.

Tests for Unsaturation

There are two methods for detecting the unsaturation in an organic compound.

1. Alkaline potassium permanganate test (Baeyer’s test)
2. Bromine test

Materials Required

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

Preparation of Reagents

1. Alkaline KMnO₄ (1%): Dissolve about 1g of potassium permanganate in 100ml of distilled water. To that solution add 10g of anhydrous sodium carbonate. Shake to dissolve it completely.
2. Bromine in CCl₄: Take 5ml of liquid bromine in 100ml of carbon tetrachloride and shake it to dissolve properly.
3. Bromine Water: Take 5ml of bromine add 100ml of distilled water and shake well. Decant off the clear liquid.

1. Bromine Test

Bromine solution is brown. In this test when bromine solution is added to the unsaturated hydrocarbon the brown colour disappears if the hydrocarbon is unsaturated. Bromine forms an addition product with the unsaturated hydrocarbon.

Procedure

When the compound is soluble in water.

• Dissolve a part of the given compound (solid or liquid) in about 2 ml of distilled water in a test-tube and add a drop of bromine water and shake.
• Disappearance of orange-red colour of bromine indicates unsaturation. Continue the addition of bromine water drop wise with constant shaking. The disappearance of orange- red colour continues so long as there are unsaturated bonds. When all the pi-bonds are broken, the orange-red colour shall persist. Bring a rod dipped in NH₄OH near the mouth of the test-tube.
• Absence of white fumes confirms unsaturation.

When the compound is insoluble in water.

• Dissolve a small amount of the given compound in 1 ml of CCl₄ in a test tube and add 2% solution of bromine in CCl₄. Shake the mixture.
• Disappearance of orange-red colour indicates unsaturation.
• Continue the drop wise addition of Br2 with constant shaking until the brown colour persists. Bring a rod dipped in NH₄OH near the mouth of the test-tube. Absence of white fumes confirms unsaturation.

When the compound is a gas.

• Add 1-2 ml of bromine solution into a gaseous compound taken in a jar and shake. Disappearance of orange-red colour of bromine indicates unsaturation.
• Note. No HBr is evolved in bromine test for unsaturation. However, some compounds such as aniline form substituted products with bromine and evolve HBr.
• Caution: Handle bromine solution carefully. Avoid its context with skin.

2. Alkaline KMnO, Test (Baeyer’s Test)

In this test the pink colour potassium permanganate disappears when an alkaline potassium permanganate is added to an unsaturated hydrocarbon. The disappearance of pink colour may take place with or without the formation of brown precipitate of manganese oxide.

Procedure

1. Take a little of the given organic compound to be tested in a test tube.
2. Dissolve the organic compound in 2ml of water or acetone in the test tube.
3. Add 1% alkaline potassium permanganate solution dropwise and shake the mixture.
4. Observe the solution, if pink colour persists then it is saturated compound. If the pink colour disappears then the given organic compound is unsaturated.