Tests for Unsaturation

Table of Content

Aim Theory Apparatus Procedure Result FAQs

Functional group plays a vital role in determining the nature of the organic compounds and their properties. Identification of functional groups is a very important part of qualitative analysis. Tests for unsaturation can be used to identify the double and triple bonds present in the organic compound.

Aim:

To identify the presence of functional group like test for saturation and unsaturation in a given organic compound.

Theory:

Organic compounds containing carbon and hydrogen are called hydrocarbons. Most of the organic compounds are derived from hydrocarbons by the displacement of one or more hydrogen atoms by single atom or a group of atoms such as acid, aldehyde, Hydroxide and halogen functional groups etc. Hydrocarbons are classified as saturated and unsaturated hydrocarbons.

1. Saturated Hydrocarbons

Saturated hydrocarbons molecules which have no double bonds in them are called saturated or commonly called alkanes. This simply means that there are as many hydrogen atoms as possible in the molecule, and no more can be added. They are also called paraffins and least reactive. Saturated hydrocarbon has long chains as well as branched ones and further classified as aliphatic hydrocarbons and cycloalkanes. Some examples are methane, ethane, propane and butane.

2. Unsaturated Hydrocarbons

In organic compounds unsaturation is due to the presence of double and triple bonds. The aliphatic hydrocarbons that contain a double bond between the two carbon atoms are called alkenes and the hydrocarbons that contain triple bonds between two carbon atoms are called alkynes. In other words an aliphatic hydrocarbon containing one sigma bond and one pi bond constitute a double bond called alkenes. The hydrocarbon containing one sigma bond and 2 pi bond constitute a triple bond called alkynes. Some examples for alkenes and alkynes are propene, propyne, butene and butyne.

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

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

1. Alkaline potassium permanganate 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.

Note: Decolourization of pink colour of KMnO₄ indicates unsaturation.

The chemical reaction is given below.

2KMnO₄ + H₂O → 2KOH + 2MnO₂ + 3[O]

-C=C- + H₂O → -C(OH)-C(OH)- + KOH

(Colourless)

2. 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.

Note: Decolourization of orange-red colour of bromine solution indicates unsaturation.

The chemical reaction is given below.

-C=C- + Br₂ → -C(Br)-C(Br)-

(colourless)

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

Apparatus Setup:

Procedure:

(a) 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.

(b) Alkaline potassium permanganate test (Baeyer’s test):

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.

(c) Bromine test

(i)Water-soluble Compounds:

1. The organic compound to be tested is taken in a test tube.
2. Dissolve it in 2ml of distilled water.
3. Add bromine water drop wise with constant shaking.
4. If the orange red colour of bromine disappears then the given organic compound is unsaturated. When all the pi bonds are broken then the colour persists.
5. If the colour of bromine persists then the given organic compound is saturated.

(ii) Water insoluble Compounds:

1. Dissolve the given organic compound in 1ml of carbon tetrachloride in a test tube.
2. Add bromine in CCl₄ drop wise in the test tube with constant shaking.
3. Disappearance of orange red colour indicates unsaturation.
4. Continue the drop wise addition of bromine with constant shaking until the brown colour persists.
5. Blow across the mouth of the test tube or bring a rod dipped in ammonium hydroxide. Absence of white fumes confirms unsaturation.

(iii) Gaseous Compounds:

1. Add 1-2ml of bromine solution into a gaseous compound taken in a jar.
2. Shake the mixture well.
3. Disappearance of orange red colour of bromine indicates unsaturation.

Observations:

Results and Discussions:

1. Baeyer’s test:

The given organic compound is _________ compound.(saturated/unsaturated)

2. Bromine test:

The given organic compound is _________ compound.(saturated/unsaturated)

Precautions:

1. The bromine solution should be handled carefully.
2. Do not inhale the vapours of bromine and chloroform.
3. The test should be carried out at room temperature.
4. Avoid bromine solution in contact with skin.

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