NCERT Class XI Chemistry Organic Chemistry Some basic principles and Techniques Solutions
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Question : 21
Total: 40
Discuss the chemistry of Lassaigne’s test.
Solution:
The elements nitrogen, sulphur and halogens are tested in an organic compound by Lassaigne’s test. The organic compound (N, S or halogens) is fused with sodium metal as to convert these elements into ionisable inorganic substances, i.e., nitrogen into sodium cyanide, sulphur into sodium sulphide and halogens into sodium halides.
2Na + S
N a 2 S
Na + C + N
Na + X
Once the ions are formed, the inorganic tests can be applied to them and the compound can be analysed.
(i) Test for Nitrogen : The sodium fusion extract is boiled with iron(II) sulphate and then acidified with concentrated sulphuric acid. The formation of Prussian blue colour confirms the presence of nitrogen.
6 C N − + F e 2 + [ F e ( C N ) 6 ] 4 −
3 [ F e ( C N ) 6 ] 4 − + 4 F e 3 +
(ii) Test for Sulphur : The sodium fusion extract is acidified with acetic acid and lead acetate is added to it. A black precipitate of lead sulphide indicates the presence of sulphur.
S 2 − + P b 2 +
↓
(iii) Test for Halogens : The sodium fusion extract is acidified with nitric acid and then treated with silver nitrate. A white precipitate, soluble in ammonium hydroxide shows the presence of chlorine, a yellowish precipitate, sparingly soluble in ammonium hydroxide shows the presence of bromine and a yellow precipitate, insoluble in ammonium hydroxide shows the presence of iodine.
X − + A g +
X represents a halogen – Cl, Br or I.
(iv) Test for Phosphorus : The compound is heated with an oxidising agent (sodium peroxide). The phosphorus present in the compound is oxidised to phosphate. The solution is boiled with nitric acid and then treated with ammonium molybdate. A yellow colouration or precipitate indicates the presence of phosphorus.
N a 3 P O 4 + 3 H N O 3 H 3 P O 4 + 3 N a N O 3 H 3 P O 4 +
+ 21 H N O 3
+ 21 N H 4 N O 3 + 12 H 2 O
2Na + S
Na + C + N
Na + X
Once the ions are formed, the inorganic tests can be applied to them and the compound can be analysed.
(i) Test for Nitrogen : The sodium fusion extract is boiled with iron(II) sulphate and then acidified with concentrated sulphuric acid. The formation of Prussian blue colour confirms the presence of nitrogen.
(ii) Test for Sulphur : The sodium fusion extract is acidified with acetic acid and lead acetate is added to it. A black precipitate of lead sulphide indicates the presence of sulphur.
(iii) Test for Halogens : The sodium fusion extract is acidified with nitric acid and then treated with silver nitrate. A white precipitate, soluble in ammonium hydroxide shows the presence of chlorine, a yellowish precipitate, sparingly soluble in ammonium hydroxide shows the presence of bromine and a yellow precipitate, insoluble in ammonium hydroxide shows the presence of iodine.
X represents a halogen – Cl, Br or I.
(iv) Test for Phosphorus : The compound is heated with an oxidising agent (sodium peroxide). The phosphorus present in the compound is oxidised to phosphate. The solution is boiled with nitric acid and then treated with ammonium molybdate. A yellow colouration or precipitate indicates the presence of phosphorus.
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