Question

Discribe the chemistry of Lassaigne method for detection of nitrogen in an organic compound.

Answer 1

Theory:

 

It is a general test for the detection of halogens, nitrogen and sulphur in an organic compound. These elements are covalently bonded to the organic compounds. In order to detect them, these have to be converted into their ionic forms. This is done by fusing the organic compound with sodium metal. The ionic compounds formed during the fusion are extracted in aqueous solution and can be detected by simple chemical tests. The extract is called sodium fusion extract or Lassaigne's extract.

 

Test for Halogen:

 

Halogens present in an organic compound forms sodium halide on fusion with sodium metal. Sodium halide extracted with water can be easily identified by adding silver nitrate solution after acidifying with dil. HNO3.

 

If chlorine is present, a white curdy precipitate soluble in ammonium hydroxide solution is formed.

 

Na + Cl → NaCl

 

NaCl + AgNO3 → AgCl + NaNO3

 

If bromine is present, an yellowish white precipitate sparingly soluble in ammonium hydroxide solution is formed.

 

Na + Br → NaBr

 

NaBr + AgNO3 → AgBr + NaNO3

 

If iodine is present, an yellow precipitate insoluble in ammonium hydroxide solution is formed.

 

Na + I → NaI

 

NaI + AgNO3 → AgI + NaNO3

 

 

Test for Nitrogen:

 

The carbon and nitrogen present in the organic compound on fusion with sodium metal gives sodium cyanide (NaCN) soluble in water. This is converted in to sodium ferrocyanide by the addition of sufficient quantity of ferrous sulphate. Ferric ions generated during the process react with ferrocyanide to form prussian blue precipitate of ferric ferrocyanide.

 

Na + C + N → NaCN

 

6NaCN + FeSO4 → Na4[Fe(CN)6] + Na2SO4

                                                                                                     Sodium ferrocyanide

 

 Na4[Fe(CN)6] + Fe3+ → Fe4[Fe(CN)6]3

                                                                                                                       Ferric ferrocyanide

                                                                                

 

Test for Sulphur:

 

If sulphur is present in the organic compound, sodium fusion will convert it into sodium sulphide. Sulphide ions are readily identified using sodium nitroprusside.

 

 Na + S → Na2S

 

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

                                                                              Sodium nitroprusside                 violet colour

 

Test for both Nitrogen and Sulphur:

 

If both nitrogen and sulphur are present in an organic compound, sodium fusion will convert it into sodium thiocyanate which then react with Fe3+ to form blood colour complex [Fe(SCN)]2+

  

 

Na + C + N + S → NaSCN

 

 

    Fe3+ + 2Na → [Fe(SCN)]2+

 

Note: If the Lassaigne's extract containing excess of sodium metal, sodium cyanide and sulphides are formed instead of sodium thiocyanate.

 

NaSCN + 2Na → NaCN + Na2S

 

Here in this type of cases, both sulphur and nitrogen are to be identified in separate tests