Question

Discuss the chemistry of Lassaigne’s test.

Answer 1

This test is employed to detect the presence of nitrogen, sulphur, halogens, and phosphorous in an organic compound. These elements are present in the covalent form in an organic compound. These are converted into the ionic form by fusing the compound with sodium metal.



The cyanide, sulphide, and halide of sodium formed are extracted from the fused mass by boiling it in distilled water. The extract so obtained is called Lassaigne’s extract. This Lassaigne’s extract is then tested for the presence of nitrogen, sulphur, halogens, and phosphorous.

(a) Test for nitrogen



Chemistry of the test

In the Lassaigne’s test for nitrogen in an organic compound, the sodium fusion extract is boiled with iron (II) sulphate and then acidified with sulphuric acid. In the process, sodium cyanide first reacts with iron (II) sulphate and forms sodium hexacyanoferrate (II). Then, on heating with sulphuric acid, some iron (II) gets oxidised to form iron (III) hexacyanoferrate (II), which is Prussian blue in colour. The chemical equations involved in the reaction can be represented as



(b) Test for sulphur



Chemistry of the test

In the Lassaigne’s test for sulphur in an organic compound, the sodium fusion extract is acidified with acetic acid and then lead acetate is added to it. The precipitation of lead sulphide, which is black in colour, indicates the presence of sulphur in the compound.





Chemistry of the test

The sodium fusion extract is treated with sodium nitroprusside. Appearance of violet colour also indicates the presence of sulphur in the compound.



If in an organic compound, both nitrogen and sulphur are present, then instead of NaCN, formation of NaSCN takes place.

Na + C + N + S → NaSCN

This NaSCN (sodium thiocyanate) gives a blood red colour. Prussian colour is not formed due to the absence of free cyanide ions.



(c) Test for halogens



Chemistry of the test

In the Lassaigne’s test for halogens in an organic compound, the sodium fusion extract is acidified with nitric acid and then treated with silver nitrate.



If nitrogen and sulphur both are present in the organic compound, then the Lassaigne’s extract is boiled to expel nitrogen and sulphur, which would otherwise interfere in the test for halogens.

Answer 2

Hey !!

In this test, a fleshly cut pea-size sodium metal is placed in a fusion tube and strongly heated till it forms a shining globule. The fusion tube is withdrawn from the flame and a pinch of organic halide added. The fusion tube is again heated in the flame to red-heat and then plunged into distilled water and filtered. The filtrate thus obtained is called ''Lassaigne's extract''

The Lassaigne's extract is then boiled with the dil. HNO₃ to expel gases, if evolved. The solution is then cooled and AgNO₃ solution added to it.

(a) A white precipitate soluble in NH₄OH indicates the presence of chlorine.

(b) A pale yellow precipitate partially soluble in NH₄OH indicates the presence of bromine.

(c) A bright yellow precipitate insoluble in NH₄OH indicates the presence of halogens.

Good luck !!