Discuss the chemistry of Lassaigne's test.

Exercises · 8.21

Q. Discuss the chemistry of Lassaigne's test.

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Concept understanding — Lassaigne Test Chemistry

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.

Note

The Lassaigne test is also called the sodium fusion test. It was developed by the French chemist J.L. Lassaigne in the 19th century.

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.

Watch out

Sodium metal is extremely reactive with water and moisture. It must be handled with dry apparatus and stored under kerosene. Never let it come in contact with water directly — the fusion tube is heated, then dropped into water after cooling.

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 $\text{Fe}_4[\text{Fe(CN)}_6]_3$ .

6\text{NaCN} + \text{FeSO}_4 \rightarrow \text{Na}_4[\text{Fe(CN)}_6] + \text{Na}_2\text{SO}_4

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.

\text{Na}_2\text{S} + \text{Na}_2[\text{Fe(CN)}_5\text{NO}] \rightarrow \text{Na}_4[\text{Fe(CN)}_5\text{NOS}] \text{ (violet)}

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.

Halogen	Precipitate	Colour	Solubility in $\text{NH}_3$
Chlorine	$\text{AgCl}$	White	Soluble
Bromine	$\text{AgBr}$	Pale yellow	Partially soluble
Iodine	$\text{AgI}$	Yellow	Insoluble

Watch out

If nitrogen or sulfur is present, you must remove them before testing for halogens. Why? Because $\text{NaCN}$ and $\text{Na}_2\text{S}$ also react with $\text{AgNO}_3$ to form precipitates ( $\text{AgCN}$ and $\text{Ag}_2\text{S}$ ), giving false positives. To remove them, boil the extract with dilute $\text{HNO}_3$ — this converts $\text{CN}^-$ to $\text{HCN}$ gas and $\text{S}^{2-}$ to $\text{H}_2\text{S}$ gas, both of which escape.

Common Mistakes Students Make …

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Lassaigne’s test converts covalently bonded nitrogen, sulphur, or halogens in an organic compound into water-soluble ionic salts by fusing the sample with sodium metal. The resulting sodium fusion extract is then tested with specific reagents to detect each element. The key is that sodium reduces the organic molecule, forming NaCN, Na₂S, or NaX, which then give characteristic precipitates or colour changes.

The Core Idea: Why Sodium Fusion?

Organic compounds are built from covalent bonds — carbon, hydrogen, and heteroatoms like nitrogen, sulphur, or halogens are all tightly held in molecules. You cannot simply dissolve the compound in water and test for these elements because they aren’t present as free ions. Lassaigne’s test solves this by destroying the organic framework and converting the heteroatoms into their inorganic sodium salts.

When you heat the organic compound with a piece of sodium metal, the sodium acts as a powerful reducing agent. It breaks the C–N, C–S, and C–X bonds, and the heteroatom ends up as a negatively charged ion (cyanide, sulphide, or halide), paired with Na⁺. These salts are water-soluble, so you can extract them and run standard qualitative tests.

Watch out

A common mistake is to think the sodium simply “reacts” with the heteroatom directly. In reality, the sodium first reduces the carbon skeleton, and the heteroatom is captured during the breakdown. If the compound contains both nitrogen and sulphur, you may get sodium thiocyanate (NaSCN) instead of separate NaCN and Na₂S — this gives a blood-red colour with Fe³⁺, which can be confused with the Prussian blue test for nitrogen alone.

Step-by-Step Procedure

Fusion: Take a small piece of dry sodium metal (about the size of a pea) in a clean, dry fusion tube. Add a pinch of the organic compound (about 50–100 mg). Heat the tube gently at first, then strongly until the bottom of the tube is red hot. The sodium melts and reacts vigorously with the organic matter.
Quenching: Plunge the red-hot tube into a porcelain dish containing about 10–15 mL of distilled water. The tube cracks, and the sodium fusion extract dissolves. Boil the solution for a few minutes to ensure complete extraction, then filter it. The clear filtrate is your sodium fusion extract (SFE).
Testing for Nitrogen: Take 2 mL of the SFE. Add a few drops of freshly prepared ferrous sulphate solution ( $\ce{FeSO4}$ ). Boil for a minute — this converts any cyanide into ferrocyanide. Then add dilute sulphuric acid until acidic, followed by a drop of ferric chloride ( $\ce{FeCl3}$ ). A Prussian blue precipitate or deep blue colouration confirms nitrogen.

The chemistry:

$\ce{NaCN + FeSO4 -> Fe(CN)2 + Na2SO4}$

$\ce{Fe(CN)2 + 4NaCN -> Na4[Fe(CN)6]}$ (sodium ferrocyanide)

$\ce{3Na4[Fe(CN)6] + 4FeCl3 -> Fe4[Fe(CN)6]3 (Prussian blue) + 12NaCl}$
Testing for Sulphur: Take 2 mL of the SFE. Add a few drops of sodium nitroprusside solution ( $\ce{Na2[Fe(CN)5NO]}$ ). A violet to purple colour indicates sulphide ions ( $\ce{S^{2-}}$ ). Alternatively, acidify with acetic acid and add lead acetate — a black precipitate of $\ce{PbS}$ confirms sulphur.

The nitroprusside test is more sensitive:

$\ce{S^{2-} + [Fe(CN)5NO]^{2-} -> [Fe(CN)5NOS]^{4-}}$ (violet complex)
Testing for Halogens: Take 2 mL of the SFE. Acidify with dilute nitric acid (to destroy any cyanide or sulphide that would interfere). Boil to expel $\ce{HCN}$ and $\ce{H2S}$ . Then add silver nitrate solution ( $\ce{AgNO3}$ ).
- A white curdy precipitate (soluble in $\ce{NH4OH}$ ) = chlorine
- A pale yellow precipitate (partially soluble in $\ce{NH4OH}$ ) = bromine
- A yellow precipitate (insoluble in $\ce{NH4OH}$ ) = iodine
If nitrogen or sulphur is present, they must be removed first because $\ce{AgCN}$ (white) and $\ce{Ag2S}$ (black) would mask the halide precipitate.

Tip

To distinguish between bromine and iodine more cleanly, after acidifying the SFE with $\ce{HNO3}$ , add a little carbon tetrachloride ( $\ce{CCl4}$ ) and then chlorine water. Shake: iodine gives a violet colour in the organic layer, bromine gives a reddish-brown colour.

Special Cases and Pitfalls …

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Q.Discuss the chemistry of Lassaigne's test.

The Core Idea: Why Sodium Fusion?

Step-by-Step Procedure

Special Cases and Pitfalls …

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