Detection of Nitrogen, Sulphur, Phosphorus and Halogens

The Lassaigne’s Test is a general method for detecting halogens, nitrogen, and sulphur in organic compounds. The organic substances are covalently bound to these elements. These must be transformed into their ionic forms to be detected. The organic molecule is fused with sodium metal to accomplish this. The ionic chemicals generated during fusion are extracted in an aqueous solution and recognised using simple chemical tests. Sodium fusion extract, also known as Lassaigne’s extract.

In a fusion tube with the organic compound, a little piece of Na metal is heated. The idea is that Na changes all of the elements present into ionic form in this way. By boiling the fused mixture with distilled water, the produced ionic salts are removed. Sodium fusion extract is what it’s called. Covalent bonding is the sort of bonding between halogens, nitrogen, sulphur and organic molecules. The components must be converted into their ionic forms to be identified. The method is made possible by the organic component fusing in the presence of sodium metal.

The sodium fusion extract is heated with iron (II) sulphate and then acidified with sulphuric acid in Lassaigne’s test for nitrogen in an organic molecule. Sodium cyanide interacts with iron (II) sulphate to generate sodium hexacyanoferrate in this procedure (II). After that, some iron (II) is oxidised by sulphuric acid to generate iron (III) hexacyanoferrate (II), which is Prussian blue.

Lassaigne’s Extract Preparation

Lassaigne’s extract is used to detect the presence of nitrogen, sulphur, and halogens in an organic molecule. Lassaigne’s extract is prepared by taking a little piece of dry solution in a fusion tube. Warm the tube until it melts into a gleaming globule. A pinch of the organic compound should be added. To begin, 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 china dish. Using a glass rod, crush the contents and bring to a boil. Filtration is used to remove the insoluble materials. 

Detection of nitrogen in organic compounds by lassaigne method

All organic substances have carbon as the primary component ingredient. Most organic molecules contain hydrogen, although there are a few outliers, such as CCl4, CS2, and others. Organic compounds may also contain components other than these, such as nitrogen, sulphur, and halogens (chlorine, bromine, and iodine). Lassaigne’s Test, created by French chemist J.L Lassaigne, is commonly used to detect these additional elements. The organic chemical is fused with metallic sodium in this assay, resulting in water-soluble sodium salt. This extract is subjected to standard qualitative testing to discover specific components.

The extracted salt is heated with FeSO4 and then sprayed with concentrated acid called H2SO4. The presence of nitrogen can be proven if a Prussian blue colour is obtained. The reactions that occur throughout this procedure are as follows:

1.Fe2+ + 6CN– → [Fe(CN)6]4-

2.Fe2+ + H+→ Fe3+ + e–

3.[Fe(CN)6]4- + 4Fe3+ → Fe4[Fe(CN)6].H2O

The oxidation of ferrous ions is carried out by acids, which are beneficial in the formation of ferric ions. The presence of nitrogen in a solution can be established by the presence of ferriferous cyanide in that solution. The crucial aspect of this test is that it is unaffected by compounds containing N but not C atoms. For example, although having an N atom, NH2NH2 does not respond to this test. This is why both C and N must be present to form CN– ions. The diazonium salts we’re using don’t pass the test because when they boil, they disintegrate into nitrogen gas.

Sulphur Determination

Lassaigne’s Test: A black precipitate of lead sulphide can indicate the presence of sulphur. This can be accomplished by mixing the clean fusion solution with diluted Acetic acid to produce a lead acetate solution. Sulphur is designated by a purple pattern when 2-3 drops of a freshly made dilute solution of sodium pentacyanonitrosyl ferrate Na2[Fe(CN)5NO] are added to the solution.

S2- + [Fe(CN)5NO]2- → [Fe(CN)5NOS]4-

Sodium Nitroprusside Test: A little amount of sodium fusion extract treated with a few drops of sodium nitroprusside turns violet, indicating the presence of sulphur. The violet tint fades slowly while your standstill.

Na2S+Na2[Fe(CN)5(NO)]→Na4[Fe(CN)5(NOS)] violet colour

Lead Acetate Test: To acidify the second half of the portion, dilute acetic acid is added, followed by a few drops of lead acetate. If a black precipitate indicating the formation of lead sulphide is noticed, the presence of sulphur in the organic component has been established.

Na2S+(OAc)2Pb→PbS+2AcONa

Halogens Detection

When halogens in an organic substance fuse with sodium metal, sodium halide is formed. After acidifying with dilute HNO3, sodium halide extracted with water can be easily detected by adding silver nitrate solution.

If there is chlorine present, a white curdy precipitate is generated that is soluble in ammonium hydroxide solution.

Na + Cl→NaCl

NaCl + AgNO3→AgCl + NaNO3

A yellowish-white precipitate was sparingly soluble in ammonium hydroxide solution forms when bromine is present.

Na + Br→NaBr

NaBr + AgNO3→AgBr + NaNO3

When iodine is present, a yellow precipitate is obtained that is insoluble in ammonium hydroxide solution.

Na + I→NaI

NaI +AgNO3→AgI + NaNO3

Detection of Phosphorus

The presence of phosphorus in an organic substance can be detected by fusing it with sodium peroxide, an oxidising agent. The phosphorus in the organic molecule is oxidised to sodium phosphate.

5Na2O2 + 2PΔ → 2Na3PO4 + 2Na2O

The fused material is removed with water, and the resulting aqueous solution is heated with strong nitric acid. This is supplemented with ammonium molybdate. A yellow precipitate or colour indicates the presence of phosphorus (due to the production of ammonium phosphomolybdate).

CONCLUSION

A process exists for determining whether a halogen, nitrogen, or sulphur 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 thoroughly adhered. The presence of a halogen can be detected by using silver nitrate solution to precipitate it. The presence of nitrogen can be detected 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 sulphur. Nitrogen, sulphur, and halogens are covalently bound in organic molecules. If they are in the ionic state, they can be detected using “Lassaigne’s test.” By fusing the organic chemical with sodium metal, this can be accomplished.