This is the notes for Detection of foreign element in organic compounds.
Qualitative Analysis of Organic Compounds (Detection of foreign elements in the organic compound)
Organic compounds are mainly composed of elements like C, H, O, N, Halogens, S and rarely P and the metals. The elements other than C and H present in the organic compounds are called foreign elements or hetero-elements.
Out of these elements, C and H are invariably present in almost all organic compounds. So, the detection of C and H is generally not done while the presence of oxygen is indirectly inferred from the estimation of the constituent elements.
Lassaigne’s Test (Detection of N, S and halogens)
Lassaigne developed a very simple and effective method for the detection of some foreign elements in the organic compounds.
Preparation of sodium extract:
A small piece of dry sodium is slightly heated in a dry fusion tube so that it melts and shines. The fusion tube is cooled and a pinch of organic compound is added. Then, it is slowly heated so the compound reacts with sodium metal. It is now strongly heated till the tube becomes red. Then the red tube is plunged into about 15 ml of distilled water taken in a porcelain basin. The solution is boiled and filtered. The filtrate is known as sodium extract or Lassaigne’s extract. N, S and halogens can be detected from the prepared sodium extract.
Detection of Nitrogen:
Sodium extract is taken and a little of NaOH solution is added if the solution is not alkaline. Then, freshly prepared ferrous sulphate solution is added, boiled, cooled, acidified with dilute HCl and then ferric chloride solution is added. The formation of ferric ferrocyanide which has a Prussian blue or green coloration confirms the presence of nitrogen in the organic compound.
Na + C + N → NaCN (sodium cyanide)
2Na + 2H2O → 2NaOH + H2
FeSO4 + 2NaOH → Fe(OH)2↓(green ppt)+ Na2
SO4
Fe(OH)2 + 6NaCN → Na4 [Fe(CN)6 ] (sodium ferrocyanide) + 2NaOH
3Na4 [Fe(CN)6 ] + 4FeCl3 → Fe4 [Fe(CN)6 ]3 (Ferric ferrocyanide) + 12NaCl
(Prussian blue or green)
Detection of both nitrogen and sulphur
Na + C + N + S → NaCNS (sodium sulphocyanide)
3NaCNS + FeCl3 → Fe(CNS)3(Ferric sulphocyanide) + 3NaCl
(blood red)
Detection of sulphur
I. When few drops of lead acetate solution is added to sodium extract in the presence of dilute acetic acid, a black ppt of lead sulphide is obtained which confirms the presence of sulphur in the organic compound.
2Na + S → Na2 S (sodium sulphide)
Na2 S + (CH3COO)2 Pb ----Acetic acid→ PbS↓+ 2CH3COONa
lead acetate lead sulphide (black ppt)
II. When few drops of sodium nitroprusside solution is added to sodium extract, a deep violet coloration of sodium sulphonitroprusside is obtained which confirms the presence of sulphur in the organic compound.
2Na + S → Na2 S (sodium sulphide)
Na2 S + Na2 [Fe(CN)5NO+ → Na4 [Fe(CN)5NOS]
sodium nitroprusside sodium sulphonitroprusside (violet)
Detection of halogens
The sodium extract is taken and boiled with a few drops of concentrated nitric acid to remove CN- and S 2- ions if present, then cooled and added silver nitrate solution in excess.
Na + X → NaX [X= Cl, Br, I]
NaX + AgNO3 → AgX↓+ NaNO3
i. White ppt. of AgCl soluble in NH4OH indicates the presence of chloride ion.
ii. Light yellow ppt. of AgBr difficult to dissolve in NH4OH indicates the presence of bromide ion.
iii. Pale yellow ppt. of AgI insoluble even in strong ammonia solution (NH4OH) indicates the presence of iodide ion.
Questions for you
Q.1. Why is sodium extract usually alkaline in nature?
Q.2. Why is it necessary to prepare sodium extract for the detection of foreign elements in the organic compound?
Q.3. Why is it necessary to boil sodium extract with conc. HNO3 before reacting with AgNO3 solution during the detection of halogens in the organic compound?
Hope this will help you a lot.
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