Sometimes a red colour is not produced in the Lassaigne's test even if both nitrogen and sulphur are present in the organic compound. Explain.

Sometimes a red colour is not produced in the Lassaigne's test even if

1.	Sometimes a red colour is not produced in the Lassaigne's test even if both nitrogen and sulphur are present in the organic compound. Explain.
Answer» Solution :In principle, if the organic compound contains both N and S, sodium thiocyanate should be formed in Lassaigne's test and this should give BLOOD red colouration with `FeCl_(3)` `{:(Na+,underset("(From organic compound)")(C+N+S),overset("Fusion")rarr,underset("Sod. thiocyanate")(NACNS),),(,3NaCNS+FeCl_(3),rarr,underset(underset("(Blood red colouration)")("FERRIC thiocyanate"))(Fe(SCN)_(3)),+ 3NaCl):}` However, if blood red colouration is not obtained, it does not necessariy mean that S is absent. This is because in presence of excess of sodium METAL, sodium thiocynanate initially formed, decomposes to FORM sodium cyanide and sodium sulphide. `2Na + NaCNS overset(Delta)rarr NaCN + Na_(2)S` As a result, blood red colouration is not obtained.