How would you detect the presence of nitrogen in organic compound by Lassiagne test method. [5]

The Lassaigne test is a qualitative analytical method used to detect the presence of nitrogen, sulfur, and halogens in organic compounds. To specifically detect nitrogen, the following steps are performed:

Procedure for Detecting Nitrogen by Lassaigne Test

1. Fusion with Sodium Metal

   • A small amount of the organic compound is fused with a piece of sodium metal in a fusion tube.
   • The reaction converts any nitrogen present in the compound into sodium cyanide (NaCN).
   • The chemical reaction can be represented as: $$\mathrm{Na}+\mathrm{N}(\mathrm{from}\mathrm{organic}\mathrm{compound})\to \mathrm{NaCN}$$
   • If sulfur is also present, sodium thiocyanate (NaSCN) is formed instead of NaCN.

2. Extraction with Water

   • The fused mass is dissolved in distilled water and filtered to obtain a clear solution.
   • This aqueous extract contains sodium cyanide (or thiocyanate) if nitrogen (or sulfur) was present.

3. Preparation of Prussian Blue

   • A small amount of ferrous sulfate (FeSO₄) solution is added to the extract, followed by ferric chloride (FeCl₃) solution.
   • The reaction between NaCN and Fe³⁺ ions produces a deep blue or greenish-blue precipitate of ferric ferrocyanide (Prussian blue).
   • The chemical reaction is: $$\mathrm{NaCN}+\mathrm{Fe}{}^{3+}\to [\mathrm{Fe}(\mathrm{CN}){}_6]{}^{3-}(\mathrm{intermediate})$$ $$3\mathrm{Fe}{}^{3+}+2[\mathrm{Fe}(\mathrm{CN}){}_6]{}^{4-}\to \mathrm{Fe}{}_4[\mathrm{Fe}(\mathrm{CN}){}_6]{}_3(\mathrm{Prussian}\mathrm{blue})$$
   • If sulfur is present, no Prussian blue is formed (since NaSCN does not react similarly), but a blood-red color may appear due to the formation of ferric thiocyanate.

4. Observation of Prussian Blue

   • The formation of a deep blue or greenish-blue precipitate confirms the presence of nitrogen in the original organic compound.
   • If no such precipitate appears, nitrogen is absent.

Key Observations

• Positive Test: Formation of Prussian blue indicates nitrogen.
• Negative Test: No precipitate means nitrogen is absent.
• Interference: If sulfur is present, it may interfere by forming NaSCN instead of NaCN, but the test can still be modified to confirm nitrogen separately.

Chemical Explanation

• Sodium cyanide (NaCN) reacts with ferric ions (Fe³⁺) to form a complex ion, which then reacts with ferrous ions (Fe²⁺) to give the characteristic Prussian blue color.
• The test is highly sensitive and can detect even small amounts of nitrogen in organic compounds.

[Fe(CN)₆]^4-+Fe³⁺Formation of insoluble complexFe₄[Fe(CN)₆]₃

NaCN+Fe³⁺Add FeSO₄ → [Fe(CN)₆]^4- + Fe³⁺ → Prussian blue (Fe₄[Fe(CN)[Fe(CN)₆]^4-+Na⁺