Question

How do we use bromine to distinguish among saturated hydrocarbons, unsaturated hydrocarbons and phenols?

Answer 1

Bromine decolorization is used as a simple qualitative test for unsaturation. Bromine is a dark red-brown liquid, but alkenes and dibromo alkanes are colorless. Thus, a dilute solution of bromine in an inert colorless solvent, such as dichloromethane, is rapidly decolorized when it is added to an alkene.

Complete answer:
The bromine test is a qualitative test for the presence of alkenes, alkynes, and phenols. A solution of bromine has a deep reddish-brown color. If we add the solution dropwise to an alkene or alkyne, the color disappears almost immediately, because the product is colorless. These are additional reactions. Bromine is used in the form of bromine water. A solution of bromine in water is called bromine water. Bromine water has a red-brown color due to the presence of bromine in it.
When bromine water is added to an unsaturated compound, then bromine gets added to the unsaturated compound and the red-brown color of bromine water is discharged. So, if an organic compound decolorizes bromine water, then it will be an unsaturated hydrocarbon, but saturated hydrocarbons do not decolorize bromine water. If a white precipitate forms along with the discoloration, a phenol is present. Phenols are so reactive that they react instantaneously with bromine to form a white precipitate.

Note:
The rapid disappearance of the bromine color is a positive test for unsaturation. The test is not unequivocal, however, because some alkenes do not react with bromine, and some react very slowly. In the case of a negative test, therefore, the potassium permanganate test should be performed.