Question

Three students, Manish, Ramesh and Rajni were determining the extra elements present in an organic compound given by their teacher. They prepared the Lassaignes 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 Lassigne’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

Lassaigne test:  

This test is used to detect the presence in an organic compound of nitrogen, sulphur, halogens, and phosphorus. These elements are present in an organic compound in a covalent form. They are converted to the ion form by fusing the sodium metal compound.

                                   $Na + C + N \rightarrow NaCN$

                                    $2Na + S \rightarrow { Na }_{ 2 }S$

                                $Na + X \rightarrow  NaX (X = Cl, Br, I)$

Cyanide, sulphide, and halide formed by sodium are extracted by boiling in the distilled water from the fused mass. The resulting extract is called Lassaigne extract. This extract of Lassaigne is then tested for nitrogen, sulphur, halogens, and phosphorus.

Test for nitrogen: $Lassaignes's\quad extract + Ferrous\quad sulphate + Conc. Sulphuric\quad acid$

The sodium fusion extract is boiled with iron (II) sulphate in the Lassaigne nitrogen test in an organic compound and then acidified with sulphuric acid. Sodium cyanide first reacts with sulphate of iron (II) and forms sodium hexacyanoferrate (II).

Then, when sulphuric acid is heated, some iron (II) becomes oxidized to form iron (III) hexacyanoferrate (II) which color is Prussian blue. The chemical reactions are as follows,

$6C{ N }^{ - } + { Fe }^{ 2+ } \rightarrow [Fe(CN{ ) }_{ 6 }{ ] }^{ 4- }\\ 3[Fe(CN{ ) }_{ 6 } + 4{ Fe }^{ 3+ } \rightarrow { Fe }_{ 4 }[Fe(CN{ ) }_{ 6 }{ ] }_{ 3 }.x{ H }_{ 2 }O$

[Prussian blue colour]