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

N a C N S

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N H_{2} C S N H_{2}

(thiourea)

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N H_{2} C O N H_{2}

( urea)

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p - N H_{2} C_{6} H_{4} S O_{3} H

( p-aminobenzene sulphonicacid)

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Solution

The correct options are
A $N H_{2} C S N H_{2}$ (thiourea)
B $N a C N S$
D $p - N H_{2} C_{6} H_{4} S O_{3} H$ ( 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 $F e S O_{4} o r F e C l_{3}$ , produces a red blood colouration due to ferric thiocyanate (or sulphocyanide). Thus, thiourea, and p-aminobenzene sulphonic acid give red colour on Lassaigne test.

$N a C N + S_{8} Δ \to N a S C N S o d i u m t h i o c y a n a t e (s o d i u m s u l p h o c y a n i d e)$

$F e^{3 +} + 3 N a S C N \to F e (S C N)_{3} f e r r i c t h i o c y a n a t e (f e r r i c s u l p h o c y a n i d e) (b l o o d r e d c o l o u r a t i o n)$
Compound given in option C doesn't contain sulphur. So, it will not give the Lassaigne test.

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