Qualitative analysis : (Detection of Elements )
The qualitative analysis of an organic compound involves the detection of all the elements present in it.
Carbon is an essential constituent of an organic compound whereas hydrogen is nearly always present. On heating the organic compound with dry cupric oxide, carbon is oxidized to CO2 and hydrogen to H2O. CO2 is detected by lime water which turns milky while H2O is detected by anhydrous CuSo4 (white) which turns it blue. This method is known as copper oxide test.
If the substance under examination is a volatile liquid or gas, the vapours are passed over heated copper oxide kept in combustion tube and the gaseous products are tested as above.
Lassaigne method
This is used to detect nitrogen, halogen and sulphur. Organic compounds are fused with dry sodium in a fusion-tube and fused mass after extraction with H2O is boiled and filtered. Filtrate called sodium extract (S.E.) is used to detect elements (other than C and H) and the tests are given in the table.
· Organic compounds being covalents normally do not have ionisable groups, hence direct test is not possible.
- Fusion with Na forms soluble salt (like NaCl, NaCN etc.) which can be easily detected.
· This test fails in case of diazo elements.
· Sometimes when the amount of nitrogen present is small, the prussian blue is present in colloidal form and the solution looks green.
Table : Lassaigne method (Detection of elements
Table : Other methods for detection of elements
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Element
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Test
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Nitrogen
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Soda lime test : A pinch of an organic compound is heated strongly with soda lime (NaOH + CaO) in a test tube. If ammonia gas acts, it shows nitrogen. CH3CONH2 + NaOH →CH3COONa + NH3. This test is, however, not reliable since certain compounds like nitro, azo etc do not evolve NH3 when heated with soda lime.
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Sulphur
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Oxidation test : Sulphur can also be tested by oxidation test. The organic compound is fused with fusion mixture (a mixture of sodium carbonate and potassium nitrate). The sulphur, if present in the organic compound, is oxidised to sodium sulphate. Na2CO3 + S + 3O → Na2SO4 + CO2. The fused mass is dissolved in water and the solution is acidified with hydrochloric acid. Barium chloride solution is then included. The formation of a white precipitate shows the presence of sulphur. Na2SO4 + BaCl2 → BaSO4 + 2NaCl.
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Halogens
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Beilstein's test (copper wire test) : A clean copper wire is heated in the Bunsen flame till it does not impart any green colour to the flame. The heated side is dipped in the organic compound and heated again. The existence of a green or bluish green flame due to the formation of volatile cupric halides indicates the presence of some halogen in the organic element. Though this test is very sensitive yet it does not confirm the presence of halogens in an organic compound since certain organic compounds like urea, thiourea, pyridine, organic acids etc. which do not contain halogens give this test due to the formation of volatile cupric cyanide. It does not tell as to which halogen is present.
Special test for bromine and iodine (layer test) : Boil a portion of the Lassaigne's extract with nitric acid. Add a few drops of CS2 and then add chlorine water slowly with constant shaking.
An orange colouration in CS2 layer confirms the presence of bromine where as a violet colouration in the layer confirms the presence of iodine.
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Phosphorus
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Phosphorus is detected by fusing the organic compound with sodium peroxide when phosphorus is converted into sodium phosphate. 2P + 5Na2O2 → 2Na3PO4 + 2Na2O. The fused mass is extracted with H2O, boiled with conc. HNO3 and then ammonium molybdate is included. Existence of yellow ppt. or colouration due to the formation of ammonium phosphomolybdate indicates the presence of phosphorus.
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Oxygen
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There is no satisfactory qualitative method for the detection of oxygen. However, its presence can be inferred indirectly.
(i) If the organic compound is heated alone in a dry test tube in the presence of nitrogen, the formation of water drops on cooler parts of the tube may indicate the presence of oxygen.
(ii) The presence of oxygen can be inferred by testing the presence of functional groups known to contain oxygen, e.g., hydroxyl (-OH), aldehydic (-CHO), carboxyl (-COOH) groups, etc.
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