Question

In the Lassaigne’s test for nitrogen in an organic compound, the Prussian blue colour is obtained due to the formation of:

Answer 1

In the Lassaigne’s test for nitrogen in an organic compound, the Prussian blue colour is obtained due to the formation of a coordination complex during the reaction.
The coordination complex has iron in +2 and +3 oxidation state and the ambidentate ligand is involved in the complex.

Complete step by step answer:
- Let’s see basic information about Lassaigne’s test.
- The Lassaigne’s test is a general test for the detection of elements like halogens, nitrogen and sulphur in the organic compound.

- It is done by fusing the organic compound containing nitrogen and carbon in the presence of excess sodium metal to form an ionic compound, sodium cyanide (NaCN). We can give reaction as
$Na\text{ + }C+N\xrightarrow{\Delta }NaCN$

- Now, $NaCN$is treated with ferrous sulphate and this forms ferrous cyanide along with a side product, sodium sulphate.
$NaCN\text{ }+\text{ }FeS{{O}_{4}}\to \text{ }Fe{{(CN)}_{2}}\text{ }+\text{ }N{{a}_{2}}S{{O}_{4}}$

- Then, ferrous cyanide reacts further with an excess of $NaCN$to form sodium ferrocyanide.
$Fe{{(CN)}_{2}}\text{ }+\text{ }NaCN(excess)\to \text{ }N{{a}_{4}}[Fe{{(CN)}_{6}}]$

- Now, as we have also added dilute sulphuric acid in the solution, it will oxidise iron from its +2 oxidation state to +3 oxidation state. So, there will be a presence of $F{{e}^{3+}}$ ions in the solution.

- Here, Sodium ferrocyanide reacts with those ferric ions to form ferric ferrocyanide. This complex is the reason behind the Prussian blue colour. The reaction can be given as
$N{{a}_{4}}[Fe{{(CN)}_{6}}]\text{ }+\text{ }F{{e}^{3+}}\to \text{ }\underset{\text{Prussian Blue colour}}{\mathop{F{{e}_{4}}{{[Fe{{(CN)}_{6}}]}_{3}}}}\,$

- This complex is also known as ‘3-2 complex’ as the iron outside the coordination sphere is in +3 oxidation state and that inside the coordination sphere is in +2 oxidation state.

Note: Remember that only complex $F{{e}_{4}}{{[Fe{{(CN)}_{6}}]}_{3}}$ is responsible for prussian blue colour formation. There are other complexes as well but no other complex formed is responsible for this colouration. Do not get confused between Ferric and Ferrous as they show iron in +3 and +2 oxidation state.