Tests for Unsaturation

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 of the examples are methane, ethane, propane and butane.

Alkanes

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.

Alkenes and Alkynes

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 colour. 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:

Baeyer's test

Bromine Test

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 dropwise 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 dropwise in the test tube with constant shaking.
  3. Disappearance of orange red colour indicates unsaturation.
  4. Continue the dropwise 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:

Alkaline potassium permanganate test

Either disappearance of pink colour or pink colour persists.

Bromine test

Either disappearance of orange -red colour or orange -red colour persists.

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.

Viva Questions:

1. If the given organic compound is aldehyde, then what test is performed to find the unsaturation?

Ans: Aldehydes decolourise potassium permanganate solution due to its oxidation to carboxylic acid. Bromine water test can be carried out in this case to find out unsaturation.

2. What is Baeyer’s reagent?

Ans: Baeyer’s reagent is a cold potassium permanganate alkaline solution, which is a powerful oxidant that makes it a redox reaction.

3. Why does phenol decolourises bromine water?

Ans: Phenol is not an unsaturated compound also decolourises bromine water. This is due to the substitution reaction forming bromine substituted phenol.

4. What type of reaction takes place between alkenes and bromine water?

Ans: The double bond breaks when alkenes react with bromine, and each carbon is attached to a bromine atom. It gives a colorless liquid, the bromine loses its original red-brown color.

5. If the compound is insoluble in water, what reagent is used to find saturation or unsaturation?

Ans: If the compound is insoluble in water bromine in 100ml of carbon tetrachloride is used to test its unsaturation.

 

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