Chemistry · General Principles Of Organic Chemistry · NEET
It is a test to find out if an organic compound contains nitrogen, sulphur, or halogens. You heat (fuse) the compound with a small piece of sodium metal. The sodium breaks the covalent bonds and joins with N, S, or X to make salts: sodium cyanide (NaCN), sodium sulphide (Na2S), and sodium halide (NaX). You boil this fused mass in water to get the 'sodium fusion extract', then test that extract with colour reactions.
In organic compounds, N, S and halogens are held by covalent bonds, so they are 'hidden' and cannot be detected by normal salt tests. Sodium is very reactive. When you fuse the compound with sodium, these atoms are converted from covalent form into ionic form (CN-, S2-, X-). Ions are easy to detect with simple inorganic tests. This 'covalent to ionic' idea is a favourite NEET question.
The extract contains cyanide ions (CN-) from nitrogen. When boiled with iron(II) sulphate, CN- forms sodium hexacyanoferrate(II), Na4[Fe(CN)6]. On heating with conc. sulphuric acid, some Fe2+ is oxidised to Fe3+. The Fe3+ then reacts to form iron(III) hexacyanoferrate(II), Fe4[Fe(CN)6]3.xH2O, which is Prussian blue. So Prussian blue confirms nitrogen.
When nitrogen AND sulphur are both in the compound, sodium fusion makes sodium thiocyanate (NaSCN) instead of separate NaCN and Na2S. The SCN- ion reacts with Fe3+ to give a blood-red complex [Fe(SCN)]2+. Because there are no free cyanide ions left, you do NOT get Prussian blue. This exact point was asked in NEET 2023 (answer: [Fe(SCN)]2+).
Sulphur becomes sulphide ion (S2-) in the extract. Two tests confirm it: (1) acidify with acetic acid and add lead acetate, a black precipitate of lead sulphide (PbS) means sulphur is present; (2) add sodium nitroprusside, a violet/purple colour means sulphur is present.
If nitrogen or sulphur is also present, the extract contains CN- and S2- ions. These ions would also form precipitates with silver nitrate and give a wrong result. Boiling with concentrated nitric acid decomposes the cyanide and sulphide first, so only the halide ions remain to react with AgNO3. Then a white precipitate = chlorine, pale yellow = bromine, yellow = iodine.
In Lassaigne's extract of an organic compound, both nitrogen and sulphur are present, which gives a blood-red colour with Fe3+ due to the formation of:
During Lassaigne's test, the elements present in an organic compound are converted from:
Which one of the following reactions does NOT belong to Lassaigne's test?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is qualitative. It only tells you WHICH elements (N, S, halogens) are present, not how much. To measure the amount, you use estimation methods like Dumas or Kjeldahl for nitrogen.
No. Lassaigne's test detects nitrogen, sulphur, halogens and phosphorus. Oxygen is not detected by this method; there is no reliable direct test for oxygen, so it is usually found by difference.
After fusing the compound with sodium, you boil the fused mass with distilled water. This dissolves the sodium salts (NaCN, Na2S, NaX) and gives a clear liquid called the sodium fusion extract, which is used for all the detection tests.
After acidifying the extract with nitric acid and adding silver nitrate: a white precipitate (soluble in ammonia) means chlorine, a pale yellow precipitate means bromine, and a yellow precipitate means iodine.
Sodium is highly reactive and easily breaks the covalent bonds in the organic compound, combining with N, S and halogens to form simple ionic sodium salts that dissolve in water and are easy to test.
