9. Qualitative analysis of organic compounds

QUALITATIVE ANALYSIS OF ORGANIC COMPOUNDS

An organic compound mainly contains carbon and hydrogen. Some compounds may also contain oxygen, nitrogen, sulphur, halogens and phosphorus.

Detection of Carbon and Hydrogen

Organic compound is heated with copper (II) oxide [CuO]. Carbon present in the compound is oxidised to carbon dioxide and hydrogen to water. CO₂ can be tested by passing through lime-water, which turns milky and water can be tested with anhydrous copper sulphate, which turns blue.

C + 2CuO ⎯⎯Δ → 2Cu + CO₂

2H + CuO ⎯⎯Δ → Cu + H₂O

CO₂ + Ca(OH)₂ ⎯→ CaCO₃↓ + H₂O

Detection of Nitrogen, Sulphur and Halogens

Nitrogen, sulphur and halogens present in an organic compound are detected by “Lassaigne’s test”. Here the organic compound is fused with metallic sodium in a fusion tube. It is then plunged into distilled water taken in a china dish. The solution is boiled and filtered. The filtrate is known as sodium fusion extract.

Principle: In an organic compound, nitrogen, sulphur and halogen atoms are present in covalent form. By heating with metallic sodium, these elements are converted to ionic form as follows:

Na + C + N ⎯⎯Δ → NaCN

2Na + S ⎯⎯Δ → Na₂S

Na + X ⎯⎯Δ → Na X (X = Cl, Br or I)

For the detection of the elements, the following tests are done:

Test for Phosphorus

The organic compound is heated with an oxidising agent like sodium peroxide. The phosphorus present in the compound is oxidised to phosphate. The solution is boiled with nitric acid and then treated with ammonium molybdate. A yellow colouration or precipitate indicates the presence of phosphorus.