PYQ NEET- Organic Chemistry Some Basic Principles L-2

Question: 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-#

A) $\mathrm{NaSCN}$

B) $\left[\mathrm{Fe}(\mathrm{CN})_5 \mathrm{NOS}\right]^{4-}$

C) $[\mathrm{Fe}(\mathrm{SCN})]^{2+}$

D) $\mathrm{Fe}_4\left[\mathrm{Fe}\left(\mathrm{CN}_6\right)\right]_3 \cdot \mathrm{xH}_2 \mathrm{O}$

Answer: $[\mathrm{Fe}(\mathrm{SCN})]^{2+}$#

Solution:#

In case nitrogen and sulphur both are present in an organic compound, sodium thiocyanate is formed, it give blood red colour and no prussian blue since there are no free cyanide lons $$ \begin{aligned} & \mathrm{Na}+\mathrm{C}+\mathrm{N}+\mathrm{S} \rightarrow \mathrm{NaSCN} \ & \mathrm{Fe}^{+3}+\mathrm{SCN}^{\ominus} \longrightarrow[\mathrm{Fe}(\mathrm{SCN})]^{2+} \text { Blood red } \end{aligned} $$