Name: Structure of Benzene
Uploaded: 2019-07-19
Duration: 3 min 53 s
Description: Benzene is the most commonly known aromatic compound and is a significant part of gasoline. In this video, we will study the structure and properties of benz...

Question

Write the equations involved in the detection of Nitrogen, Halogens and sulphur in organic compounds.

Accepted Answer

Lassaigne's Test: In Lassaigne's test, the organic compound is mixed with sodium metal and fused strongly. The fused mass is extracted with water by plunging the red hot ignition tube in distilled water and the contents are boiled for 5 minutes and filtered. The filtrate is called Sodium extract or Lassaigne's extract. If nitrogen is present in the organic compound, it reacts with sodium and carbon forming sodium cyanide.

$Na + C + N \to NaCN$

If sulphur is present in the organic compound, it reacts with sodium forming sodium sulphide.

$2 Na + S \to {Na}_{2} S$

If halogens are present in the organic compound, they form sodium halides.

$2 Na + X \to2 NaX (X = Cl, Br or I)$

Test for Nitrogen: A little of the Lassaigne's extract is made alkaline with $NaOH$ solution and then freshly prepared ${FeSO}_{4}$ solution is added. To it $2 - 3$ drops of ${FeCl}_{3}$ solution is added, cooled and acidified with $conc . HCl$ .

A prussian blue or green precipitate or colouration is observed.

${FeSO}_{4} + 2 NaOH \to Fe (OH)_{2} + {Na}_{2} {SO}_{4}$

$6 NaCN + Fe (OH)_{2} \to {Na}_{4} [Fe (CN)_{6}] + 2 NaOH Sodium ferrocyanide$

Test for Halogens: A little of the Lassaigne's extract is acidified with nitric acid and treated with ${AgNO}_{3}$ solution.

If white precipitate soluble in ${NH}_{4} OH$ is formed, then the halide is ${Cl}^{-}$ . Hence, chlorine is present.

If pale yellow precipitate partially soluble in ${NH}_{4} OH$ is formed, then the halide is ${Br}^{-}$ . Hence, bromine is present. If yellow precipitate insoluble in ${NH}_{4} OH$ is formed, then the halide is $I^{-}$ . Hence, iodine is present.

${AgNO}_{3} + NaX \to AgX ↓ + {NaNO}_{3}$

Test for Sulphur: To a little of the Lassaigne's extract freshly prepared sodium nitroprusside solution is added. Deep purple colouration is observed.

${Na}_{2} S + {Na}_{2} [Fe (CN)_{5} NO] \to {Na}_{4} [Fe (CN)_{5} NOS] sodium nitroprusside purple coloured complex$

Write the equations involved in the detection of Nitrogen, Halogens and sulphur in organic compounds.

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