Three students, Manish, Ramesh and Rajini 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 solide `FeSO_(4)` 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 colour.

Explanation

In Lassaigne.s test, `SCN^(-)` ions are formed due to the presence of sulphur and nitrogen both. These the presence of sulphur and nitrogen both. These `Fe^(3+)` ions.
`CH_3 underset(underset(CH_3)|) CH CH_2 CH_3`
The possible carbocations are :
`CH_3 Fe^(3+) + 3Na SCN to underset(underset("(Red colouration)")"Ferric thiocyanate")(Fe (SCN)_3) + 3 Na^(+)`
This happens when fusion is not carried out in the excess of sodium. However, if excess of sodium metal is used, the sodium thiocyanate initially formed decomposes to form sodium cyanide and sodium sulphide as :
`Na SCN + 2Na to Na CN + Na_2 S`
Sodium cyanide formed gives prussian blue colour due to the formation of ferric ferrocyanide or iron (III) hexacyanoferrate (II), `Fe_(4) [ Fe (CN)_6]_3`.