Qualitative analysis of organic compounds.

The detection of various elements present in an organic compound is called qualitative analysis. Carbon and hydrogen are present in almost all the organic compounds. Other commonly present elements in organic compounds are oxygen, nitrogen, halogens, sulphur and sometimes phosphorus.

The presence of various elements present in any organic compound can be detected as follows.

Detection of Carbon and Hydrogen

Principle

Carbon and hydrogen are detected by heating the organic compound with cupric oxide (CuO) strongly, where carbon is oxidized to carbon dioxide and hydrogen to water. Carbon dioxide is tested by lime water test, whereas water is tested by anhydrous copper sulphate test.

Fig:16.14 Detection of carbon and hydrogen in an organic compound

Reactions

Process

The given organic compound is mixed with dry copper oxide (CuO) and heated in a hard glass tube. The products of the reaction are passed over (white) anhydrous copper sulphate and then bubbled through lime water. If copper sulphate turns blue due to the formation of CuSO₄.5H₂O (by water vapor) then the compound contains hydrogen. If lime water is turned milky by CO₂, then the compound contains carbon.

Detection of nitrogen, sulphur and halogens

Nitrogen, sulphur and halogens in any organic compound are detected by 'Lassaigne's test'.

Theory

Elements like nitrogen, sulphur and halogens are bonded covalently in the organic compounds. In order to detect them, these have to be converted into their ionic forms. This is done by fusing the organic compound with sodium metal. The ionic compounds formed during the fusion are extracted in aqueous solution, and can be detected by simple chemical tests. The aqueous solution obtained by extracting the fused mass in water is called sodium extract or Lassaigne's extract.

Preparation of Lassaigne's Extract (or Sodium Extract)

A small piece of sodium is heated gently in an ignition tube till the sodium melts. About 50 to 60 mg of the organic compound is added to this and the tube heated strongly for 2-3 minutes to fuse the material inside it. After cooling, the tube is carefully broken in a china dish containing about 20 to 30 mL of distilled water. The fused material along with the pieces of ignition tube is crushed with the help of a glass rod and the contents of the china dish are boiled for a few minutes. The sodium salts formed in the above reactions (i.e. NaCN, Na₂S, NaX or NaSCN) dissolve. Excess of sodium reacts with water to give sodium hydroxide. This alkaline solution is called Lassaigne's extract or sodium extract. The solution is then filtered to remove the insoluble materials and the filtrate is used for making the tests for nitrogen, sulphur and halogens.

Reactions

An organic compound containing C, H, N, S and halogens when fused with sodium metal gives the following reactions.

(NaSCN) is formed during fusion, which in the presence of excess sodium forms sodium cyanide and sodium sulphide.

Detection of nitrogen

A small quantity of the sodium extract is taken in a test tube. It is made alkaline by adding 2-3 drops of sodium hydroxide (NaOH) solution. 1 mL of freshly prepared solution of ferrous sulphate is added to this solution. The mixture of the two solutions is boiled and then acidified with dilute sulphuric acid. The appearance of prussian blue or green colouration of the precipitate confirms the presence of nitrogen in the given organic compound.

The carbon and nitrogen present in the organic compound on fusion with sodium metal give sodium cyanide (NaCN) soluble in water. So, the sodium extract contains sodium cyanide which, on reaction with ferrous sulphate, gives sodium ferrocyanide. Some of the ferrous salt is oxidised to the ferric salt on heating and this reacts with sodium ferrocyanide to form ferric ferrocyanide.

Note: When nitrogen and sulphur both are present in any organic compound, sodium thiocyanate is formed during fusion. When extracted with water sodium thiocyanate goes into the sodium extract and gives 'blood red coloration' with ferric ions due to the formation of ferric thiocyanate.

Detection of sulphur

The presence of sulphur in any organic compound is detected by using sodium extract as follows:

Lead acetate test

A small portion of sodium extract is acidified with acetic acid and lead acetate solution is added to it. A black precipitate of lead sulphide indicates the presence of sulphur.

Sodium nitroprusside test

To a small quantity of sodium extract taken in a test tube, 2 to 3 drops of sodium nitroprusside are added to the solution. A violet color indicates the presence of sulphur. This color fades away slowly on standing.