Organic Chemistry – Some Basic Principles and Techniques

Chemistry · Class 11

Simulation · Chemistry · Class 11

Lassaigne's test for N, S and halogens

From the lesson Qualitative analysis: detecting elements in Organic Chemistry – Some Basic Principles and Techniques. Change the values and watch what happens.

Lassaigne's test for N, S and halogensChemistry · Class 11

The idea behind it

NCERT § "Qualitative Analysis of Organic Compounds"

  • To detect carbon and hydrogen, heat the compound with copper(II) oxide: its carbon ends up as CO₂ (lime water turns milky) and its hydrogen as water (white anhydrous copper sulphate turns blue).
  • In Lassaigne's test the compound is fused with sodium metal, turning N, S and halogens into NaCN, Na₂S and NaX, which are then extracted with water as the sodium fusion extract.
  • Nitrogen: iron(II) sulphate is added to the extract, which is boiled and then acidified with sulphuric acid; a Prussian blue colour, from iron(III) hexacyanoferrate(II), Fe₄[Fe(CN)₆]₃·xH₂O, confirms nitrogen.
  • Sulphur: acetic acid and lead acetate give a black precipitate of PbS; sodium nitroprusside gives a violet colour.
  • When both N and S are present, NaSCN forms and gives a blood-red colour with Fe³⁺ instead of Prussian blue.
  • Halogens: acidify the extract with nitric acid, boil it to destroy any cyanide or sulphide, then add silver nitrate; chlorine gives a white precipitate that dissolves in ammonia, bromine a pale yellow one sparingly soluble in ammonia, and iodine a yellow one insoluble in ammonia.
  • Phosphorus: heating the compound with sodium peroxide, an oxidising agent, turns its phosphorus into phosphate; after boiling with nitric acid, this gives a yellow precipitate with ammonium molybdate.