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.
Detection of foreign elements
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 sodium extract or Lassaigne’s extract test. This is done because organic compounds are covalent compounds. And 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.
Preparation of Sodium extract (Lassaigne’s extract)
Sodium metal is generally stored in kerosene.
- A small piece of sodium metal is taken and dried in the fold of blotting paper.
- It is kept in a fusion tube or ignition tube and heated gently so that the sodium metal just melts into a shiny globule.
- 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.
- It is heated, first gently, then strongly, until red-hot and plunged into a porcelain basin containing water.
- The fusion tube and content 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.
1. Detection of Nitrogen
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.
- 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 following reactions are involved.
Test for both Sulphur and Nitrogen
During this test, the formation of blood red colouration indicates the presence of both nitrogen and sulphur in the organic compound.
Detection of Sulphur
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.
b) Sodium nitroprusside test
To a small portion of the sodium extract solution, nitroprusside solution is added.
Detection of Halogen
To detect halogens, the first 1 ml of the sodium extract is boiled with dilute nitric acid to remove cyanide or sulphide if present.
Because 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.
Chlorine
Bromine
Iodine