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Organic Chemistry

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Lassaigne’s Test

a) Lassaign’s test:  The method for the detection of a foreign element like sulphur, Nitrogen, and halogens in an organic compound is called Lassign's test. It is the test carried out using sodium in the fusion tube. Sodium plays an important role in this test.

Sodium is reacted with the elements organic compound as follows:  

When both the N and S are present in the mixture, then the following reaction occurs: 

The sodium salt is soluble in water, can be predicted easily as:

1) Detection of Nitrogen

The brown colour of ferric hexacyanoferrate when it is treated with Ferrous Sulphate and sulphuric acid gives us the clue of Nitrogen present in the organic sample. 

2)Detection of sulphur: The extract contains sodium sulphide if the organic compound contains sulphur, the following test is performed.

  • Sodium nitroprusside test

  • Lead acetate test:

3)Detection of halogens: Sodium halide is formed if the sodium extract contains halide. The extract is boiled with nitric acid (dilute) to decompose NaCN or Na2S (if present), if not a white ppt. of AgCN or Ag2S will be seen even if there is an absence of halogen. Finally, the solution is cooled, and AgNO3 is added to see characteristic ppt.

  • White ppt soluble in aq. Ammonia indicates chlorine.
  • Light yellow ppt. Sparingly soluble in aq. Ammonia indicates bromine.
  • Pale yellow ppt. Insoluble in aq. Ammonia indicates Iodine

 

 

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