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The detection of extra elements in organic compounds is crucial in its investigation. Carbon is included in all organic molecules. Hydrogen is also found in most organic molecules (the few exceptions are the compounds such as CCl₄, CS₂, etc.). Other elements included in organic molecules include oxygen, nitrogen, sulfur, and halogens, in addition to carbon and hydrogen.
Because virtually all organic substances include carbon and hydrogen, procedures to identify them are frequently unnecessary, and their existence may be presumed without testing. The tests for detecting nitrogen, sulfur, and halogens will be discussed here.
What is the Lassaigne’s Test, and how does it work?
Lassaigne’s test can identify nitrogen, sulfur, and halogens organic molecules. A little bit of Na metal is heated with the organic compound in a fusion tube.
The idea is that Na changes all of the elements present into ionic form.
- Na + C + N → NaCN
- 2Na + S → Na₂S
- Na + X → NaX ( X= Cl, Br, or I)
By boiling the fused mixture with distilled water, the produced ionic salts are removed. Sodium fusion extract is what it’s called.
Lassaigne’s Extract Preparation
In a fusion tube, place a little piece of dry sodium. Warm the line until it melts into a gleaming globule. A pinch of the organic substance should be added. First, heat it slowly to allow the chemical to react with sodium metal. Now bring it to a high temperature. Plunge the red hot tube into distilled water in a porcelain dish. Using a glass rod, crush the ingredients and bring to a boil. Filtration is used to remove the insoluble materials. Lassaigne’s extract is the name given to the filtrate.
Lassaigne’s extract is used to detect extra elements in organic compounds like nitrogen, sulfur, and halogens in an organic molecule.
Do you know about the Halogen Test?
The newly transformed ionic salts are extracted by heating the fused (bonded) bulk and distilled WATER. Sodium fusion extract is the name for the complete process.
Sodium halide is a halide of sodium. Halogen compounds are made by fusing sodium metal with an organic substance inside the tube. Water can be used to eliminate sodium halide.
By adding silver nitrate solution after applying the acid, dilute HNO₃, it may be easily identified.
If any chlorine atom is present, a creamy curdy precipitate may develop, soluble in ammonium hydroxide solution.
Cl + Na → NaCl
AgNO₃+ NaCl → NaNO₃+ AgCl
If bromine is present in the solution, a yellowish-white precipitate that is somewhat soluble in ammonium hydroxide may occur.
Br + Na → NaBr
AgNO₃ + NaBr → NaNO₃ + AgBr
If the ammonium hydroxide solution contains iodine, a yellow precipitate may form insoluble in the ammonium hydroxide solution.
I + Na → NaI
AgNO₃ + NaI → NaNO₃ + AgI
Procedure for the Lassaigne Test
Lassaigne’s test, commonly known as the sodium fusion test, is used in elemental analysis for the subjective estimate of the presence of foreign components in an organic molecule or detection of extra elements in organic compounds, especially halogens, nitrogen, and sulphur.
- L. Lassaigne created this method. By fusing the sample with pure sodium metal, the piece is roughly boiled in this experiment.
There have been many different sorts of techniques defined. The water aids the “fused” sample’s emergence inside it, and the typical conditional trials threat detection and response for the various probable components are carried out on the resultant solution.
Discuss the Lassaigne Test.
A process exists to detect extra elements in organic compounds if a halogen, nitrogen, or sulfur exists in an organic solution. Heat is used to simulate a model in a test tube containing sodium. The boiling tube is submerged in clear water, and the wreckage has fully adhered.
The presence of a halogen can be detected by using silver nitrate solution to precipitate it. The presence of nitrogen can be seen by forming a Prussian blue precipitate on the heated section of the solution with iron. The precipitation of Lead Ethanoate or Sodium Nitroprusside indicates the presence of sulfur.
Sulfur testing
Sodium nitroprusside is used to treat the extract. The emergence of violet colour indicates the presence of sulfur.
The following reaction occurs:
S²⁻ + Fe(CN)₅NO²⁻ → [Fe(CN)5NOS]⁴⁻
Halogens testing
HNO3 is used to acidify the extract, subsequently treated with AgNO₃. The presence of Cl is indicated by a white precipitate soluble in NH₄OH. The presence of Br is shown by a yellowish deposit sparingly soluble in NH₄OH. The company of I is characterized by a yellow precipitate insoluble in NH₄OH.
NaX + AgNO₃ → NaNO₃ +AgX ↓
Nitrogen testing
The extract is acidified with concentrated H₂SO₄ and heated with FeSO₄. The appearance of Prussian blue indicates the presence of nitrogen.
The following reactions occur
- 6CN⁻ + Fe²⁺→ [Fe(CN)₆]⁴⁻
- H⁺+ Fe²⁺ → e⁻ + Fe³⁺
- 4Fe³⁺ + Fe(CN)₆⁴⁻ → Fe₄Fe(CN)₆.H₂O
Ferrous ions are oxidized to ferric ions with the assistance of the acid. The production of ferriferous cyanide indicates the presence of nitrogen.
Note that compounds with N but no C atoms do not pass this test. NH₂NH₂, despite possessing an N atom, fails this test. The CN⁻ ion requires both C and N to form.
Diazonium salts cannot be used for this test because they break down into nitrogen gas when heated.
| Experiment | Observations | Inference |
Preparation of Lassaigne’s extract In a fusion tube, fuse a little bit of sodium. Heat it with a few organic ingredients. Immerse it in distilled water in a china bowl. Filter the contents once they’ve been brought to a boil. Lassaigne’s extract is the name given to the filtrate. | ||
1. Test for Nitrogen: To 2 ml of Lassaigne’s extract, add 2 ml of freshly prepared FeSO4, boil, and add a few drops of FeCl3 solution and concentrated hydrochloric acid. | Prussian blue color | Nitrogen present |
2. Test for Sulphur : (a) To 2 ml of Lassaigne’s extract, add 2-3 drops of acetic acid and 1 ml of lead acetate solution. (b) To 2 ml of Lassaigne’s extract, add a few drops of sodium nitroprusside solution. | Black ppt. Purple color | Sulfur present
Sulfur present |
3. Test for Halogens : (a) Silver nitrate test Acidify a little of Lassaigne’s extract with concentrated HNO3 boil, cool and add silver nitrate solution. (b) Carbon disulfide test Acidify another portion of Lassaigne’s extract with dil. HCl and add 1 ml of CS2 and two drops of chlorine water and shake. |
(i) A curdy white ppt. Soluble over NH4OH. (ii) A pale yellow ppt. Sparingly soluble in NH4OH. (iii) A bright yellow ppt. Insoluble in NH4OH.
(i) Orange color in CS2 layer. (ii) Violet color in the CS2 layer. | Chlorine present Bromine present Iodine present Bromine present Iodine present |
Conclusion
Lassaigne’s test detects nitrogen, sulfur, halogens, and phosphorus. The quantities of carbon dioxide and water generated estimate carbon and hydrogen. The Dumas or Kjeldahl methods estimate nitrogen, whereas the Carius technique is used to measure halogens. By oxidizing sulfur and phosphorus to sulphuric and phosphoric acids, respectively, sulfur and phosphorus are determined. The difference between the total percentage (100) and the sum of alwl other elements present is commonly used to calculate the percentage of oxygen.
