Detection of Foreign Element

Introduction to detection of foreign element: All organic compounds contain carbon. There are no organic compounds without carbon. Hydrogen is usually present. Very limited organic compounds are present except hydrogen. eg. CCl4. Some organic compounds contain other elements like oxygen, nitrogen, sulphur, halogen, phosphorous and some metals.
C- always present
H- almost always present
O- generally present
N, S, Halogen- occasionally present
P and some metals- rarely present

The occasionally present organic compounds like nitrogen, sulphur and halogen that forms basic constituent of organic compounds are called foreign element or heteroelement.

Organic compounds are covalent compounds and generally insoluble in water that carries out the molecular reaction and is very slow. So, given organic compound is fused with highly reactive sodium metal to give water-soluble ionic compound that carries out an ionic reaction which is fast.

Detection of foreign element in organic compound
Preparation of sodium extract solution

A small piece of fresh-cut sodium metal is taken from the kerosene oil where it has been preserved. It was soaked with filter paper and followed by gentle heating to make sodium metal spherical and shiny. After cooling, a pinch of organic compound is added to the fusion tube and is strongly heated till the lower portion of the fusion tube becomes red hot. Then it is punched in about 15 ml of distilled water and crushed. Finally, it is subjected to filtration to get filtrate which contains heteroelement (foreign element). Such obtained solution is called sodium extract solution.
This test is introduced by J. L. Lassaigne, so it is also called Lassaigne’s test.

-If nitrogen is present in the organic compound
Na + C + N → NaCN

-If sulphur is present in the organic compound
Na + S → Na2S

-If nitrogen and sulphur both are present in the organic compound
Na + C + N + S → NaCNS

-If halogen is present in the organic compound
Na + X → NaX (X=Cl, Br, I)

1. Detection of nitrogen

About 2 ml of sodium extract solution is taken in a clean and dry test tube and made alkaline by adding dilute NaOH. Few drops of freshly prepared FeSO4 solution is added to it. It is boiled, cooled and few drops of FeCl3 solution is added. Finally, few drops of dilute HCl is added. The formation of Prussian blue or green colour indicates the presence of nitrogen.

Na + C + N \rightarrow NaCN\\ FeSO_{4} + 2NaOH \rightarrow \underset{Dirty\ green\ ppt}{Fe(OH)_{2}} + Na_{2}SO_{4}\\ Fe(OH)_{2} + 2NaCN \rightarrow Fe(CN)_{2} + 2NaOH\\ Fe(CN)_{2} + 4NaCN \rightarrow \underset{sodium\ ferro\ cyanide}{Na_{4}[Fe(CN)_{6}]}\\ 3Na_{4}[Fe(CN)_{6}] + 4FeCl_{3} \rightarrow \underset {\underset {(Prussian\ blue\ or\ green\ color)}{ferric\ ferrocyanide}}{Fe_{4}[Fe(CN)_{6}]_{3}} + 12NaCl
2. Detection of sulphur

Two different tests are carried out using sodium extract solution.

i. Lead acetate test: About 2 ml of sodium extract solution is taken in a clean and dry test tube that was acidified with 2 drops of acetic acid. Then, few drops of lead acetate solution are added to it. The formation of a black precipitate indicates the presence of sulphur.

Na + S \rightarrow Na_{2}S\\ Na_{2}S + (CH_{3}COO)_{2}Pb \rightarrow \underset {Black ppt.}{PbS} + 2CH_{3}COONa

(ii) Sodium nitroprusside test: About 2 ml of sodium extract solution is taken in a clean and dry test tube and few drops of sodium nitroprusside solution is added to it. Formation of violet colour indicates the presence of sulphur.

Na + S \rightarrow Na_{2}S\\ Na_{2}S + \underset {\underset {(Colourless\ solution)}{Sodium\ nitroprusside}}{Na_{2}[Fe(CN)_{5}NO]} \rightarrow \underset {\underset {(violet\ solution)}{Sodium\ thionitroprusside}}{Na_{4}[Fe(CN)_{5}NOS]}
3. Detection of nitrogen and sulphur together

About 2 ml of sodium extract solution is taken in a clean and dry test tube and few drops of FeCl3 solution is added to it. The formation of blood-red colour indicates the presence of nitrogen and sulphur together.

Na + C + N + S \rightarrow NaCNS\\ 3NaCNS + FeCl_{3} \rightarrow \underset {\underset {(blood\ red\ colour)}{Ferric\ thiocyanate}}{Fe(CNS)_{3}} + 3NaCl
4. Detection of halogen

About 2 ml of sodium extract solution is taken in a clean and dry test tube and few drops of concentrated nitric acid are added to it to remove CN and S2- ion if present, then cooled and few drops of silver nitrate solution is added.

NaCN + 2HNO_{3} \rightarrow 2NaNO_{3} + HCN\\ Na_{2}S + 2HNO_{3} \rightarrow 2NaNO_{3} + H_{2}S

(i) Formation of a white precipitate indicates the presence of chlorine which dissolves in ammonia solution and reappears with dilute nitric acid.

NaCl + AgNO_{3} \rightarrow \underset {white\ ppt}{AgCl} + NaNO_{3}\\ AgCl + 2NH_{4}OH \rightarrow \underset {Diamine\ silver\ chloride}{[Ag(NH_{3})_{2}]Cl} + 2H_{2}O\\ \left [ Ag(NH_{3})_{2}] \right ]Cl + 2HNO_{3} \rightarrow \underset {white\ ppt}{AgCl} + 2NH_{4}NO_{3}

(ii) Formation of a pale yellow precipitate indicates the presence of bromine which is sparingly soluble in ammonia solution.

NaBr + AgNO_{3} \rightarrow \underset {pale\ yellow\ ppt}{AgBr} + NaNO_{3}

(iii) Formation of yellow precipitate indicates the presence of iodine which is insoluble in ammonia solution.

NaI + AgNO_{3} \rightarrow \underset {yellow\ ppt}{AgI} + NaNO_{3}
Some Important Questions
  1. What is foreign element? Why sodium is used for its detection in organic compounds?
  2. How nitrogen is detected by Lassaigne’s test?
  3. Write two reactions of detection of sulphur by Lassaigne’s test?

References:
Mishra, AD, et al. Pioneer Chemistry. Dreamland Publication.
Mishra, AD et al. Pioneer Practical Chemistry. Dreamland Publication
Wagley, P. et al. Comprehensive Chemistry. Heritage Publisher & Distributors Pvt. Ltd.

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