Explain the reason for the fusion of an organic compound with metallic sodium for testing nitrogen, sulphur an

Exercises · 8.26

Q. Explain the reason for the fusion of an organic compound with metallic sodium for testing nitrogen, sulphur and halogens.

Tripura TbseTextbookSubjective· 2mImportance★★★★★est

38% · 50/130 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

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 …

Start your 14-day free trial to unlock the full solution →

The Lassaigne test works because fusing an organic compound with metallic sodium converts covalently bonded elements (N, S, X) into water-soluble ionic salts (NaCN, Na₂S, NaX), which can then be detected in aqueous solution by simple precipitation or colour reactions.

Why sodium fusion? The core idea

Organic compounds are built from covalent bonds — carbon, hydrogen, nitrogen, sulphur, and halogens are all held together by shared electron pairs. If you simply dissolve the compound in water and add a reagent, nothing happens: the atoms are locked inside molecules and cannot react as free ions.

The Lassaigne test solves this by breaking the covalent framework and converting the elements we want to detect into their inorganic sodium salts. Sodium metal is a powerful reducing agent. When heated with the organic compound, it rips apart the molecule, reducing nitrogen to cyanide ( $\ce{CN-}$ ), sulphur to sulphide ( $\ce{S^{2-}}$ ), and halogens to halide ions ( $\ce{X-}$ ). These ions are water-soluble and can be detected by standard qualitative tests.

Important

The fusion must be done with freshly cut, bright metallic sodium — if the sodium has oxidised to a dull grey crust (NaOH/Na₂O), it won't reduce the organic compound effectively, and the test fails.

Step-by-step reasoning

1. The fusion reaction — what actually happens

A small piece of sodium is heated in a fusion tube until it melts (sodium melts at 97.8°C, well below red heat). The organic compound is added in small portions. The tube is heated strongly until it glows red. At this temperature, sodium vapour reacts violently with the organic matter.

For a compound containing carbon, nitrogen, and a halogen, the overall reaction can be written as:

\ce{Na + C + N -> NaCN}

\ce{2Na + S -> Na2S}

\ce{Na + X -> NaX} \quad (\text{X = Cl, Br, I})

But this is a simplification. In reality, the sodium first reduces the organic molecule to elemental carbon, nitrogen, etc., and then immediately reacts with them. The key point: every atom of nitrogen ends up as cyanide ion, every sulphur atom as sulphide, and every halogen as halide.

Watch out

If the compound contains both nitrogen and sulphur, a side reaction can form sodium thiocyanate ( $\ce{NaSCN}$ ), which gives a blood-red colour with ferric ions — this can be mistaken for the phenol test. To avoid confusion, the extract is always boiled with dilute acid before testing for sulphur.

2. Why not just dissolve the compound in water?

Consider aniline ( $\ce{C6H5NH2}$ ). If you add water and then silver nitrate, you get no precipitate — the nitrogen is covalently bonded, not free $\ce{NH2-}$ ions. Similarly, chloroform ( $\ce{CHCl3}$ ) does not give a white precipitate with $\ce{AgNO3}$ because the chlorine is covalently attached to carbon.

Only after sodium fusion does the chlorine become $\ce{NaCl}$ , which dissociates in water to give $\ce{Cl-}$ ions that can be detected.

3. The extraction step — why water?

After fusion, the tube contains a black mass (excess carbon, sodium salts, and unreacted sodium). This is boiled with distilled water and filtered. The filtrate, called the Lassaigne extract, contains the soluble sodium salts. The carbon residue is left behind on the filter paper.

Tip

Always boil the extract for a minute or two — this destroys any leftover sodium metal (which would react violently with acids later) and ensures complete dissolution of the salts.

4. Testing for nitrogen

To the extract, add freshly prepared ferrous sulphate solution, then boil, cool, and add ferric chloride followed by dilute HCl. A Prussian blue colour or precipitate confirms nitrogen.

The chemistry:

Ferrous sulphate reacts with cyanide to form ferrous cyanide: $\ce{FeSO4 + 6NaCN -> Na4[Fe(CN)6] + Na2SO4}$
On boiling, some ferrous ions oxidise to ferric. Adding ferric chloride gives: $\ce{3Na4[Fe(CN)6] + 4FeCl3 -> Fe4[Fe(CN)6]3 (Prussian blue) + 12NaCl}$

The acidification with HCl dissolves any ferrous/ferric hydroxides that might mask the blue colour.

5. Testing for sulphur

To the extract, add sodium nitroprusside solution. A violet colour appears due to the formation of $\ce{Na4[Fe(CN)5NOS]}$ .

Alternatively, acidify the extract with acetic acid and add lead acetate — a black precipitate of $\ce{PbS}$ confirms sulphur.

6. Testing for halogens …

Start your 14-day free trial to unlock the full solution →

←Previous question · exercises8.25 — Why is nitric acid added to sodium extract before adding silver nitrat…→Next question · exercises8.27 — Name a suitable technique for the separation of the components from a …

Q.Explain the reason for the fusion of an organic compound with metallic sodium for testing nitrogen, sulphur and halogens.

Why sodium fusion? The core idea

Step-by-step reasoning

🎓Unlock everything free for 14 days

Unlock everything free for 14 days