Elements Present in Organic Compounds
The essential elements present in organic compounds are carbon and hydrogen. In addition to these, organic compounds may also contain oxygen, nitrogen, sulphur, halogens and phosphorus.
1. Detection of Carbon and Hydrogen
Carbon and hydrogen are detected by heating the organic compound with copper(II) oxide (CuO).
- Carbon present in the organic compound is oxidised to carbon dioxide (CO₂).
- Hydrogen present in the organic compound is oxidised to water (H₂O).
Chemical Reactions
Reaction 1: Oxidation of Carbon
C + 2CuO → 2Cu + CO₂
The carbon dioxide produced is passed through limewater, which turns milky due to the formation of calcium carbonate.
CO₂ + Ca(OH)₂ → CaCO₃ + H₂O
Observation: Limewater turns milky.
Reaction 2: Oxidation of Hydrogen
H₂ + CuO → Cu + H₂O
The water produced is tested using anhydrous copper sulphate, which changes from white to blue.
CuSO₄ + 5H₂O → CuSO₄·5H₂O
Observation: Anhydrous copper sulphate changes from white to blue.
Summary Table
| Element Detected | Reagent Used | Product Formed | Observation |
|---|---|---|---|
| Carbon | Copper(II) oxide (CuO) | Carbon dioxide (CO₂) | Limewater turns milky |
| Hydrogen | Copper(II) oxide (CuO) | Water (H₂O) | Anhydrous copper sulphate changes from white to blue |
Important Points for JEE/NEET
- Carbon is detected through the formation of carbon dioxide, which turns limewater milky.
- Hydrogen is detected through the formation of water, which turns anhydrous copper sulphate from white to blue.
- Copper(II) oxide acts as an oxidising agent in this process.
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2. Detection of Other Elements (Lassaigne’s Test)
Nitrogen, sulphur, halogens and phosphorus present in an organic compound are detected by Lassaigne’s test.
The elements present in an organic compound are converted from their covalent form into ionic form by fusing the compound with sodium metal.
Chemical Reactions
1. Detection of Nitrogen
Na + C + N → NaCN
Sodium cyanide is formed when the organic compound containing carbon and nitrogen is fused with sodium.
2. Detection of Sulphur
2Na + S → Na₂S
Sodium sulphide is formed when the organic compound containing sulphur is fused with sodium.
3. Detection of Halogens
Na + X → NaX
Here, X represents chlorine (Cl), bromine (Br) or iodine (I).
Sodium halide is formed when the organic compound containing a halogen is fused with sodium.
Preparation of Lassaigne’s Extract
The sodium salts formed during sodium fusion are extracted from the fused mass by boiling it with distilled water. The solution obtained is called Lassaigne’s extract (sodium fusion extract).
The extract contains the elements originally present in the organic compound in ionic form, making their detection easier.
Important: In Lassaigne’s test, covalently bonded elements are converted into water-soluble ionic sodium salts for qualitative analysis.
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(A) Test for Nitrogen
Nitrogen present in an organic compound is detected by Lassaigne’s test.
Procedure
The sodium fusion extract is boiled with freshly prepared iron(II) sulphate (FeSO₄) solution and then acidified with concentrated sulphuric acid (H₂SO₄).
Chemical Reactions
Step 1: Formation of Sodium Hexacyanidoferrate(II)
Sodium cyanide reacts with iron(II) sulphate to form sodium hexacyanidoferrate(II).
6NaCN + FeSO₄ → Na₄[Fe(CN)₆] + Na₂SO₄
Step 2: Formation of Prussian Blue
On heating with concentrated sulphuric acid, some iron(II) ions are oxidised to iron(III) ions. These react with sodium hexacyanidoferrate(II) to form ferric ferrocyanide, which is known as Prussian blue.
Observation
The formation of a Prussian blue colour confirms the presence of nitrogen in the organic compound.
Important Points for JEE/NEET
- Reagents used: Iron(II) sulphate (FeSO₄) and concentrated sulphuric acid (H₂SO₄).
- The characteristic observation is the formation of a Prussian blue colour.
- Prussian blue is ferric ferrocyanide.
- Nitrogen is converted into sodium cyanide (NaCN) during sodium fusion.
Correct and balance the reaction equations
(B) Test for Sulphur
Sulphur present in an organic compound is detected by using Lassaigne’s extract. Two important tests are used for the detection of sulphur.
1. Test with Lead Acetate
Procedure:
The sodium fusion extract is acidified with acetic acid (CH₃COOH), and lead acetate solution is added to it.
Chemical Reaction:
S²⁻ + Pb²⁺ → PbS↓
Observation:
A black precipitate of lead sulphide (PbS) confirms the presence of sulphur in the organic compound.
2. Test with Sodium Nitroprusside
Procedure:
The sodium fusion extract is treated with sodium nitroprusside solution.
Chemical Reaction:
S²⁻ + [Fe(CN)₅NO]²⁻ → [Fe(CN)₅NOS]⁴⁻
Observation:
The appearance of a violet colour confirms the presence of sulphur.
When Both Nitrogen and Sulphur Are Present
If both nitrogen and sulphur are present in an organic compound, sodium thiocyanate (NaSCN) is formed during sodium fusion.
Chemical Reaction:
Na + C + N + S → NaSCN
Sodium thiocyanate reacts with iron(III) ions to produce a blood-red complex.
Fe³⁺ + SCN⁻ → [Fe(SCN)]²⁺
Observation:
A blood-red colour confirms the presence of both nitrogen and sulphur.
In this case, the Prussian blue test for nitrogen is not obtained because free cyanide ions are absent.
Effect of Excess Sodium
If sodium fusion is carried out with excess sodium, sodium thiocyanate decomposes to form sodium cyanide and sodium sulphide.
Chemical Reaction:
NaSCN + 2Na → NaCN + Na₂S
The cyanide and sulphide ions formed give their respective characteristic tests.
Summary Table
| Test | Reagent | Positive Observation |
|---|---|---|
| Lead acetate test | Acetic acid and lead acetate | Black precipitate of PbS |
| Sodium nitroprusside test | Sodium nitroprusside | Violet colour |
| Nitrogen and sulphur together | Iron(III) ions | Blood-red colour due to the formation of an iron(III) thiocyanate complex |
Important Points for JEE/NEET
- Lead acetate test: Black precipitate indicates sulphur.
- Sodium nitroprusside test: Violet colour indicates sulphur.
- Nitrogen and sulphur together: Blood-red colour is obtained with iron(III) ions.
- Excess sodium decomposes sodium thiocyanate into sodium cyanide and sodium sulphide.
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Test for Halogens
The extract is acidified with HNO3 and then treated with AgNO3. A white precipitate soluble in NH4OH indicates the presence of Cl, a yellowish precipitate sparingly soluble in NH4OH indicates the presence of Br, and a yellow precipitate insoluble in NH4OH indicates the presence of I.
AgNO3 + NaX → AgX ↓ + NaNO3
