Detection of Elements: Lassaigne's Test

Objective:

Our objective is to detect the presence of nitrogen, sulphur, chlorine, bromine and iodine in organic compounds by Lassaigne's test.

The Theory:

Carbon is the main constituent element of all organic compounds. Hydrogen is also present in most of the organic compounds, but there are few exceptions, such as: CCl4, CS2 , etc. Elements other than these elements such as nitrogen, sulphur and halogens (chlorine, bromine and iodine) may also be present in organic compounds. These extra elements are usually detected by Lassaigne's Test that was developed by the French Chemist J.L Lassaigne. In this test, the organic compound is fused with metallic sodium to convert these elements into water soluble sodium salt. Usual qualitative tests are performed on this extract for the detection of respective elements.

Detection of Nitrogen

If nitrogen is present in the compound, the Lassaigne's extract would contain sodium cyanide formed during fusion. Sodium cyanide is converted to sodium ferrocyanide on treating with ferrous sulphate. On further treating it with ferric chloride, a prussian blue complex, ferricferrocyanide is formed.

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3Na_4[Fe(CN)_6] + 4FeCl_3 \rightarrow \underset{Ferric\ ferrocyanide} {\ Fe_4[Fe(CN)_6]_3} + 12NaCl

Detection of Sulphur

1. Sodium nitroprusside test

During the preparation of Lassaigne's extract, sulphur from the organic compound reacts with sodium to form sodium sulphide. It gives a purple colour with sodium nitroprusside due to the formation of sodium thionitroprusside.

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2. Lead acetate test

Sodium sulphide formed during the preparation of Lassaigne's extract reacts with lead acetate to yield lead sulphide as black precipitate.

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Detection of halogens

1. Silver nitrate test

a. Chlorine

During the preparation of Lassaigne's extract, chlorine form the organic compound reacts with sodium to form sodium chloride. Sodium chloride gives a white precipitate of silver chloride with silver nitrate solution. The precipitate is soluble in ammonium hydroxide.

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b. Bromine

Sodium bromide formed during the preparation of Lassaigne's extract reacts with silver nitrate to form pale yellow precipitate of silver bromide, which is sparingly soluble in ammonium hydroxide.

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c. Iodine

Sodium iodide formed during the preparation of sodium fusion extract reacts with silver nitrate solution to form yellow precipitate of silver iodide, which is insoluble in ammonium hydroxide.

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2. Carbon disulphide test

When sodium bromide and sodium iodide in the Lassaigne's extract is treated with chlorine water, the bromide and iodide oxidise to the corresponding halogens. This halogen dissolves in carbon disulphide. Bromine imparts orange colour and iodine imparts violet colour in carbon disulphide layer.

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Learning Outcomes:

  • Students acquire the skill to prepare Lassaigne's extract.
  • Students understand various tests to detect the presence of nitrogen, sulphur and halogens in an organic compound.
  • Students understand the chemical reactions and their balanced equations that takes place during each test.
  • Students acquire the skill to perform the experiment in the real lab once they understand different steps in the procedure.

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