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Lassaigne's Test


Lassaigne's Test for Nitrogen
Lassaigne's solution (sodium extract) contains sodium cyanide if the organic compound contains nitrogen.
Na    +   C    +    N    NaCN
              free organic
              compound
The sodium extract is treated with ferrous sulphate and the solution is heated. Ferrous cyanide thus formed reacts with more of sodium cyanide to form sodium ferrocyanide, Na4[Fe(CN)6].
FeSO4 + 2NaCN   Fe(CN)2 + Na2SO4
                                ferrous
                                cyanide
Fe(CN)2 + 4NaCN    Na4[Fe(CN)6]
                                   sodium ferrocyanide
A drop of ferric chloride is added to this solution. A prussian-blue precipitate of ferric ferrocyanide, Fe4[Fe(CN)6]3 is formed. The formation of the prussian blue precipitate shows that nitrogen is present in the organic compound.
3Na4 [Fe(CN)6]     +   4 FeCl3       Fe4[Fe(CN)6]3   +    12NaCl
sodium ferrocyanide                            ferric ferrocyanide
                                                        (prussian blue)

Lassaigne's Test for Halogens


The organic compound is fused with sodium to prepare Lassaigne's solution (sodium extract).
  1. Lassaigne's Test for Chlorine
    Lassaigne's extract is acidified with dilute nitric acid and silver nitrate solution is added. In case chlorine is present, a white precipitate of AgCl is formed. This dissolves on adding excess of ammonium hydroxide (NH4OH), indicating the presence of chlorine in the organic compound.
    Na   +   Cl         NaCl
    NaCl   +   AgNO3       AgCl     +    NaNO3
                                       white

    AgCl +   2NH4OH       [Ag(NH3)2]Cl   +   2H2O

    Insoluble                  soluble complex
  2. Lassaigne's Test for Bromine
    Lassaigne's extract is acidified with dilute nitric acid and silver nitrate solution is added. A light yellow precipitate of AgBr indicates the presence of bromine in the organic compound. The precipitate dissolves with difficulty on adding ammonium hydroxide.
    Na   +   Br        NaBr

         from organic
          compound
    AgNO3 + NaBr     AgBr   +   NaNO3

                                    light yellow
  3. Organic Layer Test for Bromine
    The sodium extract is acidified with dilute nitric acid and chlorine water is added to it. The sodium bromide present in the sodium extract liberates bromine due to the reaction with chlorine water. Shaking with an organic solvent, such as chloroform, carbon tetrachloride or carbon disulphide, can easily identify the bromine thus liberated.

    Bromine being more soluble in these organic solvents passes into the organic layer. Therefore, the layer of the organic solvent becomes reddish brown if bromine is present in the organic compound (Fig. 18.9).
    2NaBr + Cl2      2NaCl   +   Br2

                                  reddish brown

 Organic Layer Test for Bromine              Organic Layer Test for Iodine
  1. Lassaigne's Test for Iodine
    The sodium extract is acidified with dilute nitric acid and silver nitrate solution is a added. A yellow precipitate of AgI indicates the presence of iodine in the organic compound. This precipitate of silver iodide does not dissolve on adding ammonium hydroxide.
    Na    +    I        NaI
         from organic
          compound
    NaI + AgNO3   AgI   +   NaNO3
                                 yellow
  2. Organic Layer Test for Iodine
    Sodium extract is acidified with dilute nitric acid and chlorine water is added. The iodine, which is liberated, is identified by adding an organic solvent, such as carbon tetrachloride, chloroform, carbon disulphide, etc. (Fig. 18.10). As iodine is more soluble in these organic solvents, the organic layer becomes violet if iodine is present in the organic compound.
    2NaI + Cl2    2NaCl   +   I2
                                       violet colour   




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