Question
How do we detect nitrogen, sulphur, and halogens in organic compounds using the Lassaigne (sodium fusion) test? What reactions occur at each step?
Solution — Step by Step
Organic compounds contain elements in covalent bonding. To detect them using ionic reagents, we first convert them to ionic form by fusing the compound with sodium metal.
The fusion extract (dissolved in water and filtered) is called the Lassaigne extract or sodium fusion extract. All subsequent tests are performed on this extract.
If nitrogen is present, sodium fusion produces NaCN. We add solution, boil, then add dilute and .
A Prussian blue colour/precipitate confirms nitrogen.
If sulphur is present, sodium fusion produces .
Test 1: Add sodium nitroprusside — a violet colour confirms sulphur.
Test 2: Add lead acetate solution — a black precipitate of PbS confirms sulphur.
If halogen (Cl, Br, I) is present, sodium fusion produces NaCl, NaBr, or NaI. Acidify the extract with dilute , then add .
- White precipitate soluble in → Chlorine (AgCl)
- Pale yellow precipitate sparingly soluble in → Bromine (AgBr)
- Yellow precipitate insoluble in → Iodine (AgI)
graph TD
A[Organic compound + Na fusion] --> B[Lassaigne extract]
B --> C{Test for N}
B --> D{Test for S}
B --> E{Test for X}
C -->|FeSO4 + FeCl3| F[Prussian blue = N present]
D -->|Na nitroprusside| G[Violet colour = S present]
E -->|dil HNO3 + AgNO3| H{Precipitate colour?}
H --> I[White, sol. in NH4OH = Cl]
H --> J[Pale yellow = Br]
H --> K[Yellow, insol. = I]Why This Works
The entire logic rests on converting covalent organic bonds to ionic compounds that we can test with standard inorganic reagents. Sodium is used because it is highly reactive and converts C-N to NaCN, C-S to Na2S, and C-X to NaX at high temperature.
The key to remembering the halogen test is the solubility trend: AgCl is most soluble in ammonia (smallest, most ionic), AgI is least soluble (largest, most covalent). This follows Fajans’ rules — as the anion size increases, covalent character increases, and solubility in ammonia decreases.
Alternative Method
If both N and S are present in the compound, the fusion produces NaSCN (sodium thiocyanate) instead of separate NaCN and Na2S. In this case:
- The Prussian blue test for N will fail (no free CN)
- We must add excess sodium during fusion to ensure both NaCN and Na2S form separately
For the halogen test, if N or S is also present, we must boil the extract with dilute HNO3 first to decompose NaCN and Na2S before adding AgNO3. Otherwise, AgCN or Ag2S precipitates will give false positives.
Common Mistake
The most frequent error: students add directly to the Lassaigne extract without boiling with dilute first. If nitrogen or sulphur is present, (white) or (black) precipitates form and are mistaken for silver halides. Always acidify and boil first to destroy CN and S ions. This is tested in JEE Main as a “which step is incorrect” type question.