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Detection of foreign elements in organic compounds

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FOREIGN ELEMENTS:

Other elements rather than carbon and hydrogen present in organic compounds are called foreign elements.

e.g. NH2-C-NH2 ,       ---Cl . here Nitrogen(N), Chlorine(Cl) are foreign elements.

DETECTIONOF FOREIGN ELEMETNS IN ORGANIC COMPOUNDS:

Elements like nitrogen, sulphur, halogens, phosphorous, etc. are not common elements in organic compounds. However they occasionally  appear in the organic compounds. These elements are called hetero elements. There are different methods of detection of hetero elements in the organic compounds. Among them, one of the easiest method is “Lassaigne’s test” which is discussed below:

LASSAIGNE’S TEST:

In the Lassaigne’s test, first of all Lassaigne’s extract or sodium extract is prepared.

·         PREPARATION OF LASSAIGNE’S EXTRACT:

At first, a small piece of sodium metal is taken in a fusion tube (ignition tube). Then the fusion tube is heated so that the sodium metal turns silvery white. Then a pinch of organic compound is added into the tube and gradually heated first and then strongly heated until the tube becomes red hot. The red hot tube is plunged into the porcelain basin with cold water. The content is grinded and then filtered. Thus, obtained filtrate is called sodium extract.

The sodium extract is subjected to further chemical analysis to decide whether the organic compound has foreign element or not.

Ø  What is the purpose of making sodium extract?

Organic compounds are covalent compounds. Common lab reagents are ionic compounds and their aqueous solution are used in the laboratory. Hence, the organic compounds do not inter with common lab reagents.

When organic compounds are fused with sodium metal at high temperature, covalently bonded atoms of organic compounds are converted into  ionic compounds of sodium.

For e.g.

If nitrogen is present in organic compound, Na + C + N àNaCN is formed
If halogens (X) are present then, Na + X
à NaX is formed where X=(fluorine, chlorine,  iodine, bromine)

If sulphur is present then Na + S à Na2S is formed.

If both nitrogen and sulphur are present, Na + C + N + S àNaCNS is formed.

Ø  Why sodium extract is alkaline in nature?

The sodium metal used for making sodium extract is usually in excess and all of the metal is not utilized in the reaction. When fusion tube is plunged into the porcelain basin the excess sodium metal reacts vigorously with water to form strong base sodium hydroxide (NaOH). Thus sodium extract is alkaline in nature.


Na + H2O à NaOH + H2     + Energy

 

§  DETECTION OF NITROGEN:

About 1 ml of sodium extract is taken In a test tube and made alkaline by adding dilute sodium hydroxide (NaOH) if it is not alkaline.

Then freshly prepared ferrous sulphate(FeSO4) solution is added to it, it is boiled, cooled and a few drops of ferric chloride is added. Finally a few drops dil. Hydrochloric acid (HCl) is also added. Formation of prussian green or blue color indicates the presence of nitrogen in the organic compound

REACTIONS INVOLVED:

FeSO4 + 2NaOH àFe(OH)2 + Na2SO4

6NaCN + Fe(OH)2  à Na4[Fe(CN)6] + 2NaOH

3Na4[Fe(CN)6] + 4FeCl3  à Fe4[Fe(CN)6]3 + 12 NaCl

        (Prussian green)

The role of HCl is to dissolve the dirty green precipitate of ferrous hydroxide(Fe(OH)2 ) which otherwise marks the color.

 

§  DETECTION OF SULPHUR

If sulphur is present in the organic compound then sodium sulphide is formed.
Na+S
à Na2S

Two different tests are done using sodium extract to confirm sulphur:

a.      Sodium Nitroprusside test:

Few drops of Sodium nitroprusside solution are added into the sodium extract. Appearance of violet colour indicates the presence in the given compound.
Na2S + Na2[Fe(CN)5NO]
à Na4[Fe(CN)5NOS]

          Sodium nitroprusside         sodium thionitroprusside

b.      Sodium Acetate test:

Given organic compound in the form of sodium extract is treated with lead acetate. The formation of black ppt indicates the presence of sulphur.

Na2S + (CH3COO)2Pb à PbS + CH3COONa

 

v DETECTION OF BOTH NITROGEN AND SULPHUR:

If nitrogen and sulphur are both present in the organic compound, sodium thiocynate(NaCNS) is formed in the sodium extract which reacts with ferric chloride(FeCl3) solution to give blood red solution.
Na + C + N + S
à NaCNS
3NaCNS + FeCl3
à Fe(CNS)3 + 3NaCl

                             Ferric thiocynate 
                                      Blood red solution

If sodium metal is in excess and fusion is incomplete, in such cases NaCNS further reacts with excess Na.
NaCNS + Na
à NaCN + Na2S
Here, instead of blood red colour test for nitrogen and sulphur are done separately as mentioned above.

§  TEST FOR HALOGENS:

If halogens are present in the organic compound then sodium halide is formed.
Na + X
à NaX
     (X = Fluorine, Chlorine, Bromine and Iodine)

 

The halide is detected by Silver nitrate (Ag(NO)3) test

For this a little sodium extract is taken in a test tube and few drops of nitric acid is added and boiled and cooled and finally few drops of aqueous silver nitrate is added:

a.      If chlorine is present in the organic compound, then white precipitate(ppt) is obtained which is soluble in NH4OH(ammonium hydroxide) and reappears on adding HNO3 (Nitric acid)
 Cl- + AgNO3 à AgCl + NO3-
AgCl + 2NH4OH
à [Ag(NH3)2]Cl +2H20
                             (di-amine silver chloride)
[Ag(NH3)2]Cl + HNO3  
à AgCl + 2 NH4OH

b.      If bromine is present in the organic compound then pale yellow precipitate(ppt) is obtained which is sparingly soluble in ammonium hydroxide (NH4OH).
Br- + AgNO3 à AgBr + NO3-
AgBr + 2NH4OH
à [Ag(NH3)2]Br + 2H20
                            (di-amine silver bromide)
                                              (sparingly soluble)

c.       If iodine is present in the organic compound then yellow ppt is obtained which is insoluble in (NH4OH):

I- + AgNO3 à AgI + NO3-
AgI + NH4OH
à No reaction

#Why is sodium extract boiled with HNO3 before the test of halogens?

If Nitrogen is present in the organic compound NaCN is produced in the sodium extract which also imparts white ppt with aq. AgNO3
CN- + AgNO3
à AgCN + NO3-

Similarly if sulphur is present in the organic compound then Na2S is formed in the sodium extract which imparts black ppt with aq. AgNO3
S-- --+2 AgNO3
à Ag2S +2  NO3

 

Also if sodium extract is alkaline, dirty white ppt is obtained:
OH- + AgNO3
à AgOH + NO3-

When sodium extract is boiled with HNO3 , CN- , S - -, OHare removed as HCN, H2S, H2O respectively


HNO3 + CN--  
à HCN + NO3
2 HNO3 + S-- -- 
à H2S +2 NO3

HNO3 + OH—  à H2O + NO3

 

Thus, the purpose of heating sodium extract with HNO3 is to remove CN- , S--, OHetc.