13.2 Qualitative Analysis of Organic Compounds
🎯 Learning outcomes
By the end of this chapter, Students should be able to:
- Detect N, S, and halogens in organic compounds by Lassaigne’s test.
As we know, hydrocarbons and their derivatives are called organic compounds. It means carbon and hydrogen are the essential elements in organic compounds. Many organic compounds include oxygen as well. Besides, a few elements like nitrogen, sulphur, halogens, phosphorus, etc., may be present in organic compounds. These elements are called foreign elements.
Generally, foreign elements are detected via the formation of sodium extract, also called Lassaigne’s extract. The whole process is called the sodium extract or Lassaigne’s extract test. This is done because organic compounds are covalent compounds. Reactions involving covalent bonds are slow and complex. It is difficult to test for the presence of foreign elements as such. So, these are first changed into ionic form by strongly heating them with reactive sodium metal, whose presence could be easily tested.
Sodium metal is generally stored in kerosene. A small piece of sodium metal is taken and dried in the fold of blotting paper. Then it is kept in a fusion tube or ignition tube and heated gently so that the sodium metal just melts into a shiny globule. Then, after cooling it slightly, the sample of the organic compound, in which the presence of foreign elements is to be detected, is added in an amount nearly equal to the size of sodium. Then it is heated, first gently, then strongly, until red-hot and plunged into a porcelain basin containing water. The fusion tube and contents are crushed and warmed, then filtered. The filtrate is called sodium extract or Lassaigne’s extract, and it is used to detect foreign elements.
The sodium extract is generally alkaline due to the presence of NaOH formed by the reaction of water with excess sodium.
The following reactions occur during the preparation of Sodium extract if N, S, both N and S, or halogens are present in the organic compound, respectively.
For the detection of nitrogen, the sodium extract must be alkaline. If it is not, it should be made alkaline by adding a small amount of NaOH solution. About 1 ml of the sodium extract is taken in a test tube, and a few drops of freshly prepared FeSO₄ solution are added. Then it is boiled, where dirty green ppt. is generally formed. After cooling in tap water, a few drops of FeCl₃ are added and acidified with dilute HCl. The formation of Prussian blue or green colouration indicates the presence of nitrogen in the given organic compound. The following reactions are involved.
During this test, the formation of blood red colouration indicates the presence of both nitrogen and sulphur in the organic compound. (Test for both sulphur and nitrogen)
The presence of sulphur is detected in two ways.
a) Lead acetate test
The sodium extract is acidified with acetic acid and then treated with lead acetate. The formation of black ppt. confirms the presence of sulphur.
b) Sodium nitroprusside test
To a small portion of the sodium extract solution, nitroprusside solution is added. The appearance of violet colouration confirms the presence of sulphur in the organic compound.
To detect halogens, the first 1 ml of the sodium extract is boiled with dilute nitric acid to remove cyanide or sulphide if present.
If not removed, these ions interfere with the result of the silver nitrate test as
After boiling, it is cooled, and then a few drops of AgNO₃ solution are added. The formation of white ppt. soluble in ammonia solution (ammonium hydroxide) and reappearing in dilute HNO₃ indicates the presence of chlorine.
The formation of pale-yellow ppt. slightly soluble in ammonium hydroxide or aq. NH₃ indicates the presence of bromine.
The formation of yellow ppt. insoluble in ammonium hydroxide or NH₃ solution indicates the presence of iodine.
Test your understanding
Take a short quiz on qualitative analysis of organic compounds.