Q.Explain how to test for the presence of sulphur in an organic compound by Lassaigne's method. OR Explain the homologous series.

The Lassaigne Test: Why We Burn the Sample First

Imagine you have an organic compound — say, a drug, a pesticide, or a dye — and you need to know if it contains nitrogen, sulfur, or a halogen (chlorine, bromine, iodine). You can't just test the compound directly because these atoms are covalently bonded inside the molecule. They won't simply fall off and react with a reagent.

The core problem: covalent bonds are stubborn. You need to break the molecule apart and convert those atoms into simple, water-soluble ions that you can detect with standard inorganic tests. That's exactly what the Lassaigne test does.

The Intuition: Fusion with Sodium

The trick is to heat the organic compound with a piece of metallic sodium. Sodium is a powerful reducing agent. When you fuse them together (heat strongly in a fusion tube), the sodium rips the molecule apart. Here's what happens to the key elements:

• Nitrogen → gets converted to sodium cyanide ($\text{NaCN}$)
• Sulfur → gets converted to sodium sulfide ($\text{Na}_2\text{S}$)
• Halogens (Cl, Br, I) → get converted to sodium halides ($\text{NaX}$, where X = Cl, Br, I)

The product of this fusion is a dark, charred mass. You then extract it with distilled water, boil, and filter. The clear filtrate is called the Lassaigne extract (or sodium fusion extract). This extract now contains the ions you can test for.

The Precise Statement

Lassaigne test: A qualitative analysis method in which an organic compound is fused with metallic sodium to convert covalently bonded nitrogen, sulfur, and halogens into their respective water-soluble inorganic sodium salts (NaCN, Na₂S, NaX). These ions are then detected in the aqueous extract using specific chemical tests.

How to Detect Each Element in the Extract

1. Detection of Nitrogen

Test: Add a few drops of freshly prepared ferrous sulfate ($\text{FeSO}_4$) solution to the extract. Boil, then cool. Add dilute sulfuric acid and a drop of ferric chloride ($\text{FeCl}_3$).

What happens: The cyanide ion ($\text{CN}^-$) reacts with ferrous ions to form ferrous cyanide, which then reacts with ferric ions to form Prussian blue — a deep blue precipitate of $\text{Fe}_4[\text{Fe(CN)}_6]_3$.

Result: A blue colour or precipitate confirms nitrogen.

2. Detection of Sulfur

Test: Add a few drops of sodium nitroprusside ($\text{Na}_2[\text{Fe(CN)}_5\text{NO}]$) solution to the extract.

What happens: Sulfide ions ($\text{S}^{2-}$) react with sodium nitroprusside to form a violet colour complex.

Result: A violet colour confirms sulfur.

3. Detection of Halogens

Test: Acidify the extract with dilute nitric acid ($\text{HNO}_3$), then add silver nitrate ($\text{AgNO}_3$) solution.

What happens: Halide ions ($\text{Cl}^-$, $\text{Br}^-$, $\text{I}^-$) form precipitates with silver ions.

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