Question

The compound gives blood red colouration when its Lassaigne's extract is treated with alkali and ferric chloride?

• A. Benzamide
• B. Phenyl hydrozine
• C. Diphenyl sulphide
• D. Thiourea

Answer 1

The correct option is D Thiourea

Thiourea $S=C(N{H}_2{)}_2$ gives blood red colouration when its Lassaigne's extract is treated with alkali and ferric chloride.
When both N and S are present in a compound, during preparation of Lassaigne's extract, NaCNS is formed.
$Na+C+N+S\to NaCNS$
NaCNS reacts with $FeC{l}_3$ to form red coloured $Fe(CNS{)}_3$ (ferric sulphocyanide)
$3NaCNS+FeC{l}_3\to Fe(CNS{)}_3+3NaCl$