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Question

Which of the following compounds will give red colour on Lassaigne test ?

A
NaCNS
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B
NH2CSNH2 (thiourea)
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C
NH2CONH2 ( urea)
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D
pNH2C6H4SO3H ( p-aminobenzene sulphonicacid)
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Solution

The correct options are
A NH2CSNH2 (thiourea)
B NaCNS
D pNH2C6H4SO3H ( p-aminobenzene sulphonicacid)
When nitrogen and sulphur both are present in the organic compound, they combine during fusion to form sodium thiocyanate (sulphocyanide), This when heated with FeSO4orFeCl3, produces a red blood colouration due to ferric thiocyanate (or sulphocyanide). Thus, thiourea, and p-aminobenzene sulphonic acid give red colour on Lassaigne test.

NaCN+S8ΔNaSCNSodiumthiocyanate(sodiumsulphocyanide)

Fe3++3NaSCNFe(SCN)3ferricthiocyanate(ferricsulphocyanide)(bloodredcolouration)
Compound given in option C doesn't contain sulphur. So, it will not give the
Lassaigne test.

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