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Detection of elements in organic compounds
Introduction and Methods for Detection of organic compounds
1) Most of the organic compounds contain Carbon and Hydrogen and these can be detected easily in the laboratory.Some organic compounds also contain Nitrogen,Oxygen, Sulphur and Halogens.Oxygen cannot be detected by a direct method.However Nitrogen, Sulphur and halogens can be detected practically in the laboratory.
2) Organic compounds are covalent and do not ionize in solution.
3) Therefore, organic compounds should be converted first into inorganic substances and then tests are performed to detect different elements.
(A) Detection Of Carbon :
1) Mix the given organic substance with dry cupric oxide.
2) Heat the mixture in a test tube fitted with a delivery tube.
3) Pass the gas through the lime water taken in another test tube.
If lime water turns milky, it confirms the presence of Carbon in the given organic compounds.
(B) Detection of Hydrogen :
(1) Heat the mixture of organic compound and cupric oxide in a test tube.
(2) Pass the issuing gases through a test tube containing anhydrous copper sulphate.(Prepare anhydrous copper sulphate by strongly heating blue hydrated Copper Culphate in a china dish until become white).
(3) The change of colour from white to blue confirms the presence of hydrogen in the given compound.
(C) Detection of Nitrogen, Sulphur and Halogens :
Lassaigne`s solution or sodium fusion extract should be prepared to detect Nitrogen,Sulphur and Halogens in the given organic compounds.
Preparation of Lassaigne`s solution or sodium extract :
1) Take a small piece of sodium metal in a fusion tube.
2) Heat it gently to melting.Remove it from the flame.
3) Put a small quantity of the given organic compound over melted sodium.
4) Heat it first gently and then strongly till the tube becomes red hot.
5) Now put it into distilled water (10 ml) taken in a china dish while still red-hot.
6) The fusion tube will break. Dissolve the contents by stirring with a glass rod.
7) The solution is boiled and filtered. THe filtrateis called Lassaigne`s solution or sodium extract.
(D) Test for Nitrogen :
1) Take 2ml of Lassaigne`s solution in a test tube and add few drops of NaOH solution to it.
2) Now add few ml of freshly preapared Ferrous Sulphate solution to it.
3) Warm,Cool and then add few drops of FeCl3 Solution.
4) Finally acidify it with dilute Sulphuric Acid or concentrated Hydrochloric acid.
5) Appearance of prussian blue or green precipitate or solution indicates the presence of nitrogen.
(E) Test for Sulphur :
(1) Acidify about 2 ml of Lassaigne`s solution with few drops of acetic acid.
(2) Add to it lead acetate solution. Black precipitate of lead sulphide confirms the presence of Sulphur.
(F) Test for Halogens :
Take Lassaigne`s solution in a china dish , add few drops of concentrated HNO3 and boil it to remove any CN and S ions in the form of HCN and H2S. Now add AgNo3 solution.
1) Curdy white ppt soluble in NH4OH indicates the presence of Chloride ion.
2) Pale yellow ppt partially soluble in NH4OH indicates the presence of Bromide ion.
3) Deep Yellow ppt, insoluble in NH4OH indicates the presence of Iodine ion.