Lassaigne's test class 11 - organic compounds - detection nitrogen sulfur chlorine bromine iodine

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Lassaigne’s test is used for the detection of elements: Nitrogen (N), Sulfur (S), Chlorine (Cl), Bromine (Br) and Iodine (I). This test involves following two steps.

i) preparation of sodium fusion extract (SFE).

ii) Detection of elements using SFE.

i) Preparation of SFE for sodium fusion test & principle - reactions

A small amount of organic substance is fused with a small quantity of sodium metal in a fusion tube. The red hot fusion tube is then plunged into distilled water and the contents are boiled for a few minutes, then cooled and filtered. then cooled and filtered.

The filtrate obtained is called sodium fusion extract (SFE) or Lassaigne’s extract. It is usually alkaline. If it is not alkaline, a few drops of NaOH solution may be added to make it alkaline.

) Detection of elements using SFE

Thus obtained SFE is used to detect the presence of elements like N,S,Cl, Br & I.

The elements in the organic compound react with sodium during fusion reaction as follows:


Where

X = Cl/Br/I

Hence SFE may contain any of or all of ionic forms of respective elements

SUMMARY SHEET OF LASSAIGNE'S TEST
Test Observation Inference
SFE + FeSO4 + FeCl3 + HCl i) A prussian blue color is formed.

ii) Blood red coloration is observed. i) Nitrogen is confirmed.

ii) Both nitrogen and sulfur are confirmed.
i) SFE + Sodium nitroprusside

ii) SFE + CH3COOH + Pb(CH3COO)2 i) A violet coloration is observed.

ii) A black precipitate is formed.
Sulfur is confirmed.

SFE + HNO3 + AgNO3 i) A white ppt. soluble in NH4OH is formed.

ii) A pale yellow ppt. partially soluble in NH4OH is formed.

iii) A yellow ppt. insoluble in NH4OH is formed. i) Chlorine is confirmed.

ii) Bromine is confirmed.

iii) Iodine is confirmed..