chemistry of lassaignes test
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Answer:
Lassaignes 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.
Na + C + N + Δ → NaCN
2Na + S + Δ → Na2S
Na + X + Δ → 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 Lassaignes extract. This Lassaigne’s extract is then tested for the presence of nitrogen, sulphur, halogens, and phosphorous.
(a) Nitrogen 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.
Lassaignes extract + Ferrous sulphate + Conc. H2SO4 → Ferriferro cyanide (Prussian blue colour)
6CN- + Fe2+ → [Fe(CN)6]4-
3[Fe(CN)6]4- + 4Fe3+ → Fe4[Fe(CN)6]3. H2O (Prussian blue)
(b) Sulpher test
In the Lassaignes 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.
Lassaignes extract + Lead acetate + Acetic acid → Black precipitate
S2- + Pb2+ → PbS (Black)
The sodium fusion extract is treated with sodium nitroprusside. Appearance of violet colour also indicates the presence of sulphur in the compound.
S2- + [Fe(CN)5NO]2- + 4Fe3+ → [Fe(CN)5NOS]4-
If in an organic compound, both nitrogen and sulphur are present, then instead of NaCN, formation of sodium thiocyanate (NaSCN) takes place.
Na + C + N + S → NaSCN
This NaSCN gives a blood red colour. Prussian colour is not formed due to the absence of free cyanide ions.
Fe3+ + SCN- → [Fe(SCN)]2- (Red)
(c) Halogen test
In Lassaignes test for halogens in an organic compound, the sodium fusion extract is acidified with nitric acid and then treated with silver nitrate.
X- + Ag+ → AgX (X = Cl. Br, I)
If nitrogen and sulphur both are present in the organic compound, then the Lassaignes extract is boiled to expel nitrogen and sulphur, which would otherwise interfere in the test for halogens.
The sodium fusion test, or Lassaigne's test, is used in elemental analysis for the qualitative determination of the presence of foreign elements, namely halogens, nitrogen, and sulphur, in an organic compound. It was developed by J. L. Lassaigne......