Question

Discuss the chemistry of Lassaigne’s test.

Answer 1

Lassaigne's test
 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 of an organic compound. These are converted into the ionic form by fusing the compound with sodium metal. 
Na+C+N \(\xrightarrow[]{\Delta}\) NaCN
2Na+S \(\xrightarrow[]{\Delta}\) Na₂S
Na+X \(\xrightarrow[]{\Delta}\) NaX
(X=Cl, Br, I)
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 extract is then tested for the presence of nitrogen, sulphur, halogens, and phosphorous.

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 (a) Test for nitrogen
 Chemistry of the test In 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
6CN^-+Fe²⁺\(\rightarrow\)[Fe(CN)₆]^4-
3[Fe(CN)₆]^4- + 4Fe3⁺ \(\xrightarrow[]{xH_2O}\)  Fe₄[Fe(CN)₆]₃.xH₂O 

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(b) Test for sulphur 
Lassaigne’s extract+Lead acetate  \(\xrightarrow[]{acetic \,acid}\,Black\,precipitate\)
Chemistry of the test 
In 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, indicates the presence of sulphur in the compound.
\(S^{2-}+Pb^{2+}\xrightarrow[]{} PbS\)
\(Lassaigne’s \,extract+sodium \,nitroprusside\xrightarrow[]{} Violet \,color\)
Chemistry of the test
The sodium fusion extract is treated with sodium nitroprusside. The 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, the formation of NaSCN takes place. 
This NaSCN (sodium thiocyanate) gives a blood-red colour. Prussian colour is not formed due to the absence of free cyanide ions. 

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(c) Test for halogens:  Chemistry of the test
In Lassaigne's test for halogens in an organic compound, the sodium fusion extract is acidified with nitric acid and then treated with silver nitrate.
X^-+Ag⁺\(\xrightarrow[]{}\)AgX (X=Cl, Br,I)
Suppose nitrogen and sulphur both are present in the organic compound. In that case, the Lassaigne's extract is boiled to expel nitrogen and sulphur, which would otherwise interfere with the test for halogens.