Question 71: In Lassaigne’s extract of an organic compound, both nitrogen and sulphur are present, which gives blood red colour with \mathrm{Fe}^{3+} due to the formation of –
(1) \left[\mathrm{Fe}(\mathrm{CN})_{5} \mathrm{NOS}\right]^{4-}
(2) [\mathrm{Fe}(\mathrm{SCN})]^{2+}
(3) \mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3} \cdot \mathrm{xH}_{2} \mathrm{O}
(4) NaSCN
Answer: Option (2)
Explanation:
When an organic compound containing both nitrogen and sulphur is fused with sodium in Lassaigne’s test, sodium thiocyanate \mathrm{NaSCN} is formed in the extract.
On adding \mathrm{Fe}^{3+} ions to this Lassaigne’s extract,
thiocyanate ions \mathrm{SCN}^{-} react with ferric ions to form ferric thiocyanate complex.
This complex is [\mathrm{Fe}(\mathrm{SCN})]^{2+},
which is responsible for the characteristic blood red colour.
Therefore, the blood red colour observed is due to the formation of ferric thiocyanate complex,
and the correct option is (2).