Question

Discuss the chemistry of Lassaigne's test.
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Answer 1

HI BRAINLY QUEEN01,

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)

☺️

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.

Hope it helps you ❤️

Answer 2

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.

• In organic compounds, nitrogen, sulphur and halogens are covalently bonded. Their detection in 'Lassaigne's test' is possible if they are in the ionic form. This can be achieved by fusing the organic compound with sodium metal.

Chemistry for test for nitrogen:

Sodium fusion extract is boiled with ferrous sulphate and acidified with sulphuric acid. Sodium cyanide reacts with ferrous sulphate and forms sodium hexacyanoferrate(II). On heating with sulphuric acid, some ferrous is oxidized to ferric hexacyanoferate (II) Fe

4

[Fe(CN)

6

]

3

which is prussian blue in colour.

Chemistry of the test for sulphur:

Acetic acid is added to sodium fusion extract. complete precipitation of sulphur in the form of lead sulphate occurs which is black in colour. Also, the sodium fusion extract is treated with sodium nitroprusside to obtain violet color. However if N and S are present, then instead of NaCN, NaSCN is obtained. This gives blood red colour on reaction with Fe

3+

ions.

Chemistry of test for halogens:

Sodium fusion extract is acidified with nitric acid and then treated with silver nitrate. However if nitrogen and sulphur both are present, then Lassaigne's extract is boiled to expel nitrogen and sulphur which would otherwise interfere in the test for halogens.

Explanation:

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