Question
How is the Lassaigne test used to detect nitrogen, sulphur, and halogens in an organic compound? What happens when both N and S are present together? Write the reactions involved.
(JEE Main 2023 asked the test for detecting nitrogen; CBSE 11 boards ask the complete procedure)
Solution — Step by Step
Heat the organic compound with a piece of sodium metal in an ignition tube. Sodium converts covalently bonded elements into ionic form that dissolves in water.
Plunge the hot tube into distilled water and filter. This filtrate is the sodium extract — the starting point for all tests.
Add FeSO₄ solution to the sodium extract, boil, then add dilute H₂SO₄ and FeCl₃.
A Prussian blue colour or precipitate confirms nitrogen.
Add freshly prepared sodium nitroprusside solution to the sodium extract.
A violet colour confirms sulphur. Alternatively, add lead acetate solution — a black precipitate of PbS also confirms S.
Acidify the sodium extract with dilute HNO₃, then add AgNO₃ solution.
- White precipitate (soluble in NH₃) → Chlorine (AgCl)
- Pale yellow precipitate (partially soluble in NH₃) → Bromine (AgBr)
- Yellow precipitate (insoluble in NH₃) → Iodine (AgI)
flowchart TD
A[Organic compound + Na → heat] --> B[Sodium Extract in water]
B --> C{Test for N}
B --> D{Test for S}
B --> E{Test for Halogens}
C -->|FeSO₄ + H₂SO₄ + FeCl₃| F{Prussian blue?}
F -->|Yes| G[N present]
D -->|Na₂Fe CN₅NO| H{Violet colour?}
H -->|Yes| I[S present]
E -->|dil. HNO₃ + AgNO₃| J{Precipitate colour?}
J -->|White| K[Cl present]
J -->|Pale yellow| L[Br present]
J -->|Yellow| M[I present]Why This Works
The whole logic of the Lassaigne test is conversion. Organic compounds have elements locked in covalent bonds — you cannot test for them directly with ionic reagents. Sodium fusion breaks these covalent bonds and creates ionic compounds (NaCN, Na₂S, NaX) that dissolve in water and respond to standard qualitative tests.
The Prussian blue test works because cyanide ions form the hexacyanoferrate complex, which then reacts with Fe³⁺ to give the characteristic deep blue colour.
Alternative Method
When both N and S are present, sodium thiocyanate (NaSCN) forms instead of NaCN and Na₂S separately. The Prussian blue test may fail because CN⁻ is tied up as SCN⁻. In this case, add excess Na during fusion to ensure both NaCN and Na₂S form. Alternatively, test for N using the blood-red colour with FeCl₃ (which detects SCN⁻ directly): (blood red).
Common Mistake
When testing for halogens, students forget to boil off any HCN or H₂S by adding excess dilute HNO₃ before adding AgNO₃. If CN⁻ or S²⁻ are still present, they form AgCN (white) or Ag₂S (black) precipitates that give false positives for halogen tests. Always acidify thoroughly with HNO₃ first.