Name: Detection of Carbon and Hydrogen
Uploaded: 2019-07-19
Duration: 57 s
Description: Carbon and hydrogen are very important elements of nature. Our body mostly consists of these. Let's see how we can estimate the carbon and hydrogen.

Question

If an organic compound contains both nitrogen and sulphur, nitroprusside test on sodium extract in Lassaigne's test fails to produce violet colour, why? How these compounds are analysed for the elements?

Accepted Answer

$1 .$ Test for Sulphur

If sulphur is present in the organic compound,

sulphide ions are readily identified using sodium nitroprusside.

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

${Na}_{2} S + \underset{Sodium nitroprusside}{{Na}_{2} [Fe (CN)_{5} NO]} \to \underset{violet colour}{{Na}_{4} [Fe (CN)_{5} NOS]}$

$2 .$ Test for Nitrogen

The carbon and nitrogen present in the organic compound on fusion with sodium metal give sodium cyanide ( $NaCN$ ) soluble in water.

$Na + C + N \to NaCN$

This is converted into sodium ferrocyanide by the addition of sufficient quantity of ferrous sulphate.

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

Ferric ions generated during the process react with ferrocyanide to form prussian blue precipitate of ferric ferrocyanide.

${Na}_{4} [Fe (CN)_{6}] + {Fe}^{3 +} \to \underset{Ferric ferrocyanide}{{Fe}_{4} [Fe (CN)_{6}]_{3}}$

$3$ . Test for both Nitrogen and Sulphur

If both nitrogen and sulphur are present in an organic compound, sodium fusion will convert it into sodium thiocyanate.

$Na + C + N + S \to NaSCN$

Sodium thiocyanate reacts with ${Fe}^{3 +}$ to form blood colour complex $[Fe (SCN)]^{2 +}$ .

${Fe}^{3 +} + 2 Na \to [Fe (SCN)]^{2 +}$

Nitroprusside test on sodium extract in Lassaigne's test fails to produce violet colour.

Hence, here, in this type of cases, both sulphur and nitrogen are to be identified in separate tests.

$NaSCN + 2 Na \to NaCN + {Na}_{2} S$ .

If an organic compound contains both nitrogen and sulphur, nitroprusside test on sodium extract in Lassaigne's test fails to produce violet colour, why? How these compounds are analysed for the elements?

Important Questions on Purification of Organic Compounds (Qualitative and Quantitative)

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