QUALITATIVE ORGANIC ANALYSIS

We will discuss the analysis of elements (C,N,O,S,X,P) present in an organic compound.

1. Detection of carbon and Hydrogen:

2. Detection of N,S and Halogen:

Lassaigne test

: Fuse the organic compound with Na , which converts covalently bonded N,S and X present in organic compound to water soluble NaCN, Na2S and NaX. If organic compound contain both ''N'' and "S'', then converts to NaSCN, which reduced by excess Na to give NaCN and Na2S.

(a) Detection of Nitrogen:

• Take the sodium fusion mixture and add equal volume of FeSO4 and then boil the mixture. Cool the mixture and add few drops of H2SO4. Formation of Prussian blue conforms the presence of Nitrogen.

Note: 1) Diazo compounds donot give Lassaigne tese because they loose N2 gas even at moderate temperature.

2) Hydrazine (NH2NH2) also donot give the test since it does not contain "C''.

3) Some nitrogen compounds like amides on heating with sodalime gives NH3 [ NH3 tests: adding Nesslers reagent]

(b) Detection of sulphur:

• Take sodium fusion extract and add sodium nitroprusside, violet colour indicates the presence of "sulphur''.

• Take the sodium fusion extract and acidify with CH3COOH and add lead acetate, black ppt. indicates the presence of Sulphur.

• When both Nitrogen and sulphur oresent, but not used excess of Na during fusion extract, then NaSCN does not convert to NaCN and Na2S. Hence the following test should be used.

       

• Oxidation test for sulphur detection: Fuse organic compound with Na2CO3 and KNO3, if sulphur is present in organic compound it changes to Na2SO4. Extract the fused mass with water and then filter and add BaCl2 solution, white precipitate ( BaSO4) which is insoluble in mineral acids-indicates the presence of sulphur

                          

  (c) Detection of halogens:

•  If organic compound contain N/S/ and halogens, then sodium fusion extract contain NaCN/Na2S/ and NaX. So, before testing for halogens first we have to treat fusion extract with Conc. HNO3 which removes NaCN/Na2S ( Otherwise these interfere with tests of NaX). Conc. HNO3 decomposes NaCn and Na2S to HCN(g) and H2S(g). 

Now cool the extract and then add few drops of AgNO3 solution, formation of white ppt soluble in dil.NH4OH indicates the presence of Cl, Pale yellow ppt soluble in conc.NH4OH indicates the presence of Br and if yellow ppt. insoluble in NH4OH indiactes the presence of Iodine.

(d) Detection of Phosphorous:

• If we strongly heat the mixture of organic compound with sodium peroxide  and Na2CO3 then all the phosphorous present in the organic compound oxidises to phosphate. Extract the fused mass with water and filter the solution. Boil the filtrate with conc. HNO3 and add ammonium molybdate- yellow ppt. indicates the presence of ''P''.