Foreign Element Detection

 

Fundamental Principles of Organic Chemistry

13.2 Qualitative analysis of organic compounds (detection of N, S and halogens by Lassaigne's test)

Detection of foreign elements

As we know, hydrocarbons and their derivatives are called organic compounds. It means carbon and hydrogen are the essential elements in organic compounds. Many organic compounds include oxygen as well. Besides, a few elements like nitrogen, sulphur, halogens, phosphorus etc. may be present in organic compounds. These elements are foreign elements.

Generally, foreign elements are detected via the formation of sodium extract, also called Lassaigne’s extract. The whole process is called sodium extract or Lassaigne’s extract test. This is done because organic compounds are covalent compounds. And reactions involving covalent bonds are slow and complex. It is difficult to test for the presence of foreign elements as such. So these are first changed into ionic form by strongly heating them with reactive sodium metal, whose presence could be easily tested.

Preparation of Sodium extract or Lassaigne’s extract:

Sodium metal is generally stored in kerosene. A small piece of sodium metal is taken and dried in the fold of blotting paper. Then it is kept in a fusion tube or ignition tube and heated gently so that the sodium metal just melts into a shiny globule. Then, after cooling it slightly, the sample of the organic compound, in which the presence of foreign elements is to be detected, is added in an amount nearly equal to the size of sodium. Then it is heated, first gently, then strongly, until red-hot and plunged in a porcelain basin containing water. The fusion tube and content are crushed and warmed, then filtered. The filtrate is called sodium extract or Lassaigne’s extract, and it is used to detect foreign elements.

The sodium extract is generally alkaline due to the presence of NaOH formed by the reaction of water with excess sodium.

H₂O + Na                NaOH + H₂ + heat

The following reactions occur during the preparation of Sodium extract, if N, S, both N and S or halogens are present in the organic compound, respectively.

1.       Detection of Nitrogen

For the detection of nitrogen, the sodium extract must be alkaline. If it is not, it should be made alkaline by adding a small amount of NaOH solution. About 1 ml of the sodium extract is taken in a test tube and a few drops of freshly prepared FeSO₄ solution are added. Then it is boiled where dirty green ppt. is generally formed. After cooling in tap water, a few drops of FeCl₃ are added and acidified with dilute HCl. The formation of Prussian blue or green colouration indicates the presence of nitrogen in the given organic compound. The following reactions are involved.

During this test, the formation of blood red colouration indicates the presence of both nitrogen and sulphur in the organic compound. (Test for both sulphur and nitrogen)

Detection of Sulphur:

The presence of sulphur is detected in two ways.

a)      Lead acetate test: The sodium extract is acidified with acetic acid and then treated with lead acetate. The formation of black ppt. confirms the presence of sulphur.

b)      Sodium nitroprusside test: To a small portion of the sodium extract solution, nitroprusside solution is added. The appearance of violet colouration confirms the presence of sulphur in the organic compound.

Detection of halogen:

To detect halogens, the first 1 ml of the sodium extract is boiled with dilute nitric acid to remove cyanide or sulphide if present.

Because if not removed, these ions interfere with the result of the silver nitrate test as

After boiling, it is cooled, and then a few drops of AgNO₃ solution are added. The formation of white ppt. soluble in ammonia solution (ammonium hydroxide) and reappearing in dilute HNO₃ indicates the presence of chlorine.

The formation of pale yellow ppt. slightly soluble in ammonium hydroxide or aq. NH₃ indicates the presence of bromine.

The formation of yellow ppt. insoluble in ammonium hydroxide or NH₃ solution indicates the presence of iodine.

1)      For the detection of foreign elements (N, halogens and S) in organic compounds, why is sodium fusion carried out?                                                                                         2

2)      Why is an organic compound fused with sodium metal before testing for foreign elements?    1

3)      Explain why sodium extract is alkaline in nature.                   1

4)      Why is an aqueous solution of sodium extract alkaline?              2

5)      What is the composition of sodium extract if the organic compound contains nitrogen?  1

6)      Give Lassaigne’s test for nitrogen.                                         4

7)      Write a short note on the detection of nitrogen in organic compounds.     5

8)      Write a short note on Lassaigne’s test for nitrogen.                     5

9)      How would you detect the presence of sulphur in the organic compound? 2

10)  How would you detect the presence of nitrogen and sulphur together in the organic compound? 2

11)  Write a short note on the detection of sulphur and halogens in organic compounds.        4

12)  Why is Lassaigne’s extract boiled with concentrated nitric acid while testing for the halogens?   2

13)  Write a short note on the detection of foreign elements in the organic compounds.          5

 

1.       An element least likely to be found in an organic compound is

a.       Hydrogen      b. Carbon               c. Nitrogen            d. Oxygen

2.       Lassaigne's test is not used for the detection of

a.       Halogens        b. Sulphur              c. Nitrogen            d. Phosphorus

3.       During the detection of the organic compound, sodium extract is prepared to

a.       Dissolve it in water.                     C. Convert it into ionic form.

b.       Make it more reactive.               D. Make the reaction slow.

4.       In which of the following compounds, nitrogen cannot be detected by the use of Lassaigne’s test:

a.       NH₂NH₂

b.       C₆H₅NH₂

c.        CH₃NO₃

d.       NH₂CONH₂

5.       The metal used in Lassaigne's test for hetero elements is

a.       Sodium           b. Potassium          c. Calcium              d. Zinc

6.       Lassaigne's test involves

a.       Oxidation of foreign elements

b.       Reduction of foreign elements

c.        Formation of ionic compounds of foreign elements

d.       Rearrangement of foreign elements

7.       In sodium extract solution, the nitrogen of the organic compound is converted into

a.       Sodalime                                        c. Sodium nitrate

b.       Sodium nitrite                              d. Sodium cyanide

8.       Which of the following compounds is present in the sodium extract if the organic compound contains both N and S?

a.       NaCN              b. Na₂S                   c. NaCNS                d. NaCl

9.       In the detection of foreign elements, the appearance of blood red colour after adding FeCl₃ solution to Lassaigne's extract  acidified with HCl shows the presence of

a.       Na and S        b. N and Cl             c. S and N              d. S and Cl

10.    The formation of violet/purple colour on the addition of sodium nitroprusside in sodium extract shows

a.       Presence of nitrogen

b.       Presence of nitrogen and sulphur

c.        Presence of sulphur

d.       Presence of halogens

11.    A sodium nitroprusside test is carried out for the detection of

a.       Halogens        b. Nitrogen            c. Sulphur              d. Carbon

12.    If sulphur is present in the given organic compound, which colour is obtained by adding lead acetate to its sodium extract acidified with acetic acid?

a.       Black               b. White                 c. Purple                d. Violet

 

*** This is not a complete note. It is just to guide you. Please study the prescribed textbooks. ***