Give the chemistry of the Lassianghe's test for the detection of nitrogen in Organic Compounds.

R-CONH₂+3Na+H₂OHeat (fusion) → Extract with water → Add FeSO₄ + H₂SO₄ → HeaNaCN+Na₂CO₃+NaOH+Na₂S

Chemistry of Lassaigne’s Test for Nitrogen in Organic Compounds

Lassaigne’s test is a qualitative method used to detect the presence of nitrogen in organic compounds, particularly those containing nitrogen in the form of amines, amides, or nitro groups. The test involves fusion with sodium metal followed by chemical reactions to liberate nitrogen gas, which is then identified.

Step 1: Fusion with Sodium Metal

When an organic compound containing nitrogen is heated with sodium metal in a fusion tube, the nitrogen is converted into sodium cyanide (NaCN): $$\mathrm{R}-\mathrm{NH}{}_2+2\mathrm{Na}\to \mathrm{NaCN}+\mathrm{NaNH}{}_2\text{(for amines)}$$ $$\mathrm{R}-\mathrm{CONH}{}_2+4\mathrm{Na}\to \mathrm{NaCN}+\mathrm{Na}{}_2\mathrm{CO}{}_3+2\mathrm{NaOH}\text{(for amides)}$$ The sodium cyanide formed is then extracted with water.

Step 2: Conversion of Cyanide to Carbonate

The sodium cyanide is treated with ferric sulfate (Fe₂(SO₄)₃) to convert it into sodium ferrocyanide (Na₄[Fe(CN)₆]): $$6\mathrm{NaCN}+\mathrm{Fe}{}_2(\mathrm{SO}{}_4){}_3\to \mathrm{Na}{}_4[\mathrm{Fe}(\mathrm{CN}){}_6]+2\mathrm{Fe}(\mathrm{CN}){}_3+3\mathrm{Na}{}_2\mathrm{SO}{}_4$$ However, the key step involves oxidation of cyanide to carbonate using bromine water (NaOBr) in an alkaline medium (NaOH).

NaCN+FeSO₄+H₂SO₄HeatFeCO₃+Na₂SO₄+NH₄HSO₄

Step 3: Liberation of Nitrogen Gas

The sodium cyanide is treated with bromine water in the presence of sodium hydroxide: $$\mathrm{NaCN}+\mathrm{NaOBr}+\mathrm{NaOH}\to \mathrm{Na}{}_2\mathrm{CO}{}_3+\mathrm{N}{}_2+\mathrm{NaBr}+\mathrm{H}{}_2\mathrm{O}$$ Here, NaOBr (sodium hypobromite) acts as an oxidizing agent, converting cyanide into nitrogen gas (N₂). The evolution of colorless effervescence (N₂ gas) confirms the presence of nitrogen in the organic compound.

NaCN+NaOH+Br₂HeatNa₂CO₃+N₂+3NaBr

Observation

• A colorless gas (N₂) is evolved when the extract is treated with bromine water and sodium hydroxide.
• The gas can be tested using a glowing splint, which remains unlit (since N₂ does not support combustion).

Chemical Explanation

The reaction proceeds via the formation of sodium cyanide (NaCN), which is then oxidized to sodium carbonate (Na₂CO₃) while releasing nitrogen gas (N₂). The overall reaction can be summarized as: $$\mathrm{NaCN}+3\mathrm{NaOBr}+2\mathrm{NaOH}\to \mathrm{Na}{}_2\mathrm{CO}{}_3+\mathrm{N}{}_2+3\mathrm{NaBr}+2\mathrm{H}{}_2\mathrm{O}$$

This test is particularly useful for detecting nitrogen in amides, amines, and nitro compounds, but not in azo compounds (–N=N–) or hydrazines, which require different methods.