Question

Three students, Manish, Ramesh and Rajni were determining the extra elements present in an organic compound given by their teacher. They prepared the Lassaigne’s extract (L.E.) independently by the fusion of the compound with sodium metal. Then they added solid FeSO₄ and dilute sulphuric acid to a part of Lassaigne’s extract. Manish and Rajni obtained prussian blue colour but Ramesh got red colour. Ramesh repeated the test with the same Lassaigne’s extract, but again got red colour only. They were surprised and went to their teacher and told him about their observation. Teacher asked them to think over the reason for this. Can you help them by giving the reason for this observation. Also, write the chemical equations to explain the formation of compounds of different colours.

Answer 1

Whenever an organic compound contains both N and S, then on fusion with Na metal compound gives NaSCN or NaCN and Na₂S depending on the quantity of Na metal. If Na metal is less, then only NaSCN is formed. In that case L.E on treating with FeSO₄ and H2SO₄ gives red colour due to the formation of ferric thiocyanide Fe(SCN)₃. In case of NaCN, L.E on treating with FeSO₄ and H₂SO₄ gives Prussian blue colour. 

Chemical reactions: \[\ce{\underset{(Oxidation)}{Fe^{2+} -> Fe^{3+}}}\]  

\[\ce{\underset{Red colour}{Fe^{3+} + 3NaSCN -> Fe(SCN)3 + 3Na+}}\]

If Na is in excess then: 

\[\ce{NaSCN + 2Na ->[Δ] NaCN + Na2S}\]

\[\ce{6CN- + Fe^{2+} -> [Fe(CN)6]^{4-}}\]

\[\ce{3[Fe(CN)6]^{4-} + 4Fe^{3+} ->[{x}H2O] \underset{Prussian blue}{Fe4[Fe(CN)6]3.{x}H2O}}\]