Lassaigne Test
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The Lassaigne’s Test is a chemical test used to differentiate between organic compounds containing different elements, such as carbon, nitrogen, sulfur, and halogens.
Na metal is heated in a fusion tube with an organic compound to detect Nitrogen, Sulphur, and halogens present in the compound using Lassaigne’s test. The principle behind this is that the heat converts all the elements into their ionic form.
Na + C + N → NaCN
2Na + S → Na2S
Na + Cl → NaCl
Na + Br → NaBr
Na + I → NaI
The formed ionic salts are extracted from the fused mass by boiling it with distilled water. This is called sodium fusion extract.
Also Read
Introduction to Organic Chemistry
Purification of Organic Compounds
Qualitative Analysis of Organic Compounds
Test for Nitrogen
The extract is boiled with FeSO4 and acidified with concentrated H2SO4. The presence of nitrogen is indicated by the appearance of Prussian blue colour.
The following reactions occur:
- [Fe(CN)6]4- → Fe2+ + 6CN-
2. Fe2+ + H+ → Fe3+ + e-
3Fe3+ + [Fe(CN)6]4- → Fe4[Fe(CN)6]·H2O
The oxidation of ferrous ions to ferric ions is aided by acid. The presence of nitrogen is indicated by the formation of ferriferous cyanide.
Note: This test is only applicable to compounds containing both C and N atoms. For example, NH2NH2 does not answer this test since it does not contain a C atom. This is because both C and N are necessary to form the CN– ion.
This test is not given by diazonium salts as they decompose to produce nitrogen gas upon heating.
Test for Sulphur
The extract is treated with sodium nitroprusside. If a violet colour appears, it indicates the presence of sulphur.
The following reaction occurs:
‘[Fe(CN)5NO]2- → [Fe(CN)5NOS]4-’
Testing for Halogens
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 after the extract is acidified with HNO3 and then treated with AgNO3.
AgX + NaNO3 → AgNO3 + NaX ↓
