What is Lassaigne Test?
Lassaigne Test is a qualitative analysis tool to detect the presence of nitrogen, sulphur, and halogens in an organic compound using sodium metal as a reagent. The organic compound for which the test is to be carried out, is heated in a fusion tube in the presence of sodium metal. The sodium metal converts the elements present in the compound into an ionic form called as sodium fusion extract. This extract is then further treated with specific reagents to check for some observable changes that infer the presence of a particular element in the compound.
Principle of the Lassaigne Test
The principle of Lassaigne Test is that when sodium metal is heated with an organic compound, it converts all the elements present in the compound into an ionic form. The extract obtained in the ionic form can further be treated with specific reagents to get observable changes which infer the presence of specific elements in the compound. Let us have a look on how different elements in organic compounds get converted when heated in presence of sodium in a fusion tube.
Conversion of Organic Compounds to Ionic Forms
- Na + C + N → NaCN
- 2Na + S → Na2S
- Na + X → NaX, where X = Cl, Br or I
These ionic salts can be extracted by boiling the fused mass with distilled water. The extract is therefore called the sodium fusion extract.
Detection of Elements via Formation of Coloured Compounds
The ionic salts are further treated with some specific reagents to form different coloured compounds which indicates presence or absence of certain elements in the organic compound under consideration. This is discussed in detail as follows.
Lassaigne Test
Lassaigne Test is a set of procedures used to detect the presence of nitrogen, sulphur and halogens in an organic compound. In the theoretical approach, we use different symbols in the molecular formula of a compound to show the presence of a particular element in it but in the practical approach, we use a variety of tests to identify the presence of specific elements through observable reactions or characteristic behaviours when brought in contact with a specific reagent. For instance, when heated, certain metals exhibit certain characteristic flame colours, indicating their presence.
In this article, we will learn about the Lassaigne Test, its principle, the preparation of the Lassaigne Extract, the procedure for the Lassaigne Test, its applications, and some problems based on it.
Table of Content
- What is Lassaigne Test?
- Procedure for the Lassaigne Test
- Examples of Lassaigne Test
- Applications of Lassaigne Test