Test for Unsaturation (Unsaturation Test) in the Identification of Organic Compounds (Assignment / PPT)

What is Unsaturated Compounds and Unsaturation test?

• Organic compounds containing  > C = C <   or carbon-carbon triple bonds are called unsaturated compounds.
• An unsaturation test is a test that determines does the compound have any double bonds, triple bonds( multiple bond).
• Generally addition and oxidative reactions are used to detect unsaturation linkages.
• There are following tests are given below-

1. Bromine in carbon tetrachloride test,

2. Potassium permanganate test( Bayer’s test)

3. Tetranitromethane test

• However, no single reaction gives sufficient information that can be used to identify the presence of double/triple bond. Therefore, both the test (qualitative ) given above should be performed before reaching to any conclusion.

BROMINE IN CARBON TETRACHLORIDE TEST: 

• Bromine undergoes addition reaction with nearly all active unsaturated compounds like alkenes & alkynes.
• The decolourisation of solution takes place during the course of reaction.
• The discharge of bromine colour  accompanied by the liberation of  HBr  gas indicates substitution and not addition.
• Such reactions is given by phenoles , aromatic  amines, aldehydes , ketones & some compounds containing active methylene groups.

Mechanism for Bromine Test:

Procedure (Bromin test for Unsaturation in the lab): 

In a test tube dissolve 100 mg of compounds in 2 ml of carbon tetrachloride and add few drops with shaking 5% solution of bromine in carbon tetrachloride. If decolourisation is observed , the compound may be unsaturated.

Some other examples

 

Precautions for Bromine Test:

(a)The tests should be performed at room temperature.

(b) Handle bromine solution  carefully. Do not inhale the vapours and also avoid its contact with the skin.

Potassium Permanganate test(Bayer’s Test):

1% Cold, dilute alkaline solution of Potassium Permanganate is known as Bayer's Reagent.

• Potassium Permanganate is the strong oxidizing agent.
• Unsaturated compounds disappearance of the purple colour of potassium permanganate and formation of brown coloured ppt of manganese hydroxide .
• With water  soluble substances, reaction is carried out in water  whereas with those insoluble in water ,benzene or acetone solvent is used.
• The test is based on the fact that permanganate ion oxidise  alkenes  and alkynes type of compounds to glycols( diols) and also oxidises Phenols and Aldehydes.
• Alkenes decolourise the neutral/alkaline KMnO4 solution and vicinal glycols are formed (Bayer’s test). Reaction takes place as follows.

 Mechanism for Bayer's Test:

Procedure for Bayer's Unsaturation Test in the Lab:  In a test tube dissolve 100 mg of compound in 5 ml of water or acetone .Add 2% aqueous permanganate solution drop by drop and shake the test tube vigorously . Disappearance of purple colour with the formation of brown solution indicates that the compound may be unsaturated.

Some other examples:

Precaution for Bayer's test: Add the reagent drop wise. 

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