NEETChemistryGeneral Organic Chemistry
An organic compound containing both nitrogen and sulphur is fused with a large excess of sodium metal. The resulting Lassaigne's extract is then treated with neutral FeCl ₃ solution. What is the expected observation and the chemical reason for it?
Options
- AA blood-red colour is observed because sodium thiocyanate is highly stable and forms a complex with Fe ³⁺ .
- BA blood-red colour is observed because excess sodium accelerates the formation of the [ Fe ( SCN )]²⁺ complex.
- CNo blood-red colour is observed because excess sodium decomposes sodium thiocyanate into sodium cyanide and so
- DA Prussian blue colour is observed immediately because excess sodium directly converts the thiocyanate ion int
Correct answer
C. No blood-red colour is observed because excess sodium decomposes sodium thiocyanate into sodium cyanide and so
Step-by-step solution
When an organic compound containing both nitrogen and sulphur is fused with sodium metal, sodium thiocyanate ( NaSCN ) is initially formed. However, if the fusion is carried out with a large excess of sodium metal, the sodium thiocyanate decomposes to yield sodium cyanide and sodium sulphide: NaSCN + 2 Na NaCN + Na ₂ S Because the thiocyanate ion ( SCN ⁻ ) is destroyed, the extract will not give the characteristic blood-red colour of the [ Fe ( SCN )]²⁺ complex when treated with neutral FeCl ₃ . Instead, the cyanid