Qualitative Analysis Of Organic Compounds

This topic explains about the qualitative analysis of organic compounds. Organic compounds primarily consist of carbon and hydrogen atoms. Other than this, some compounds also contain Oxygen, Nitrogen, Sulphur, Halogens, and Phosphorus. In order to qualitatively determine the presence of different elements in an organic compound, scientists have devised various methods. Following are some of the most commonly used methods:

Detection of carbon and hydrogen:

On heating, the compounds in the presence of copper (II) oxide, the presence of carbon and hydrogen can be detected. Carbon is oxidized to carbon dioxide (tested by lime water, it turns the solution turbid) and hydrogen to water (tested by anhydrous copper sulphate, turns it blue).

C + 2CuO  → 2Cu + CO2

2H +CuO → Cu + H2O

CO2  +  Ca(OH)→ CaCO3   +  H2O

5H2O  +CuSO4    →  CuSO4.5H2O

Detection of other elements:

Lassaigne’s test is used to detect the presence of other elements in an organic compound. Conversion of covalent bonds into sodium metals takes place in this method by fusing the compound with sodium metal. The extract obtained after the fusion of elements with sodium metal is known as sodium fusion extract.

  • Test for nitrogen:

    • Sodium fusion extract is reacted with iron (II) sulphate and it is then mixed with concentrated sulphuric acid. Formation of Prussian blue colour confirms the presence of nitrogen.

6CN  +  Fe2+   → [Fe(CN)6]4-

3[Fe(CN)6]4-   +   Fe3+  xH2OFe4[Fe(CN)6]3.xH2O

Prussian blue

  • Test For sulphur:

    • The reaction of sodium fusion extract with acetic acid followed by the addition of lead acetate yields a black precipitate of lead sulphide which confirms the presence of sulphur.

S2-   +   Pb2+→ PbS

  • Test for halogens:

    • Formation of a white precipitate which is soluble in ammonium hydroxide on reacting sodium fusion extract with nitric acid and then with silver nitrate confirms the presence of halogens in the compound.

X +  Ag+→ AgX
X represents a halogen I, Br or Cl

  • Test for phosphorus:

    • Formation of a yellow coloured precipitate on treating the organic compound with nitric acid and ammonium molybdate shows the presence of phosphorus.

Na3PO4  +  3HNO3à   H3PO4  +  3NaNO3

H3PO4  +  12(NH4)2MoO4  +  21HNO→ (NH4)3PO4.MoO3  +  21NH4NO3  +   12H2O

          Ammonium molybdate               Ammonium phosphomolybdate

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Practise This Question

Rearrangement involves the migration of atoms on group of atoms to another position within the molecule under suitable conditions.