Question

In the Lassaigne's test for nitrogen in an organic compound, the Prussian blue colour is obtained due to the formation of: (a) ${\mathrm{Na}}_4[\mathrm{Fe}(\mathrm{CN}{)}_6]$ (b) ${\mathrm{Fe}}_4{[\mathrm{Fe}(\mathrm{CN}{)}_6]}_3$ (c) ${\mathrm{Fe}}_2[\mathrm{Fe}(\mathrm{CN}{)}_6]$ (d) ${\mathrm{Fe}}_3{[\mathrm{Fe}(\mathrm{CN}{)}_6]}_4$

Short answer

(b) ${\mathrm{Fe}}_4{[\mathrm{Fe}(\mathrm{CN}{)}_6]}_3$ is responsible for Prussian blue.

Step-by-step solution

Understand the Context of the Test

Lassaigne's test is used to detect the presence of nitrogen, sulfur, and halogens in an organic compound. When testing for nitrogen, the compound is fused with sodium, converting the nitrogen into sodium cyanide ($\text{NaCN}$).

Reaction with Ferrous Sulfate

In the reaction, sodium cyanide formed reacts with ferrous sulfate, leading to the production of sodium hexacyanoferrate(II) according to:$${\text{6 NaCN + FeSO}}_4\to {\text{Na}}_4[{\text{Fe(CN)}}_6]+\text{other products}$$

Formation of Prussian Blue

The sodium hexacyanoferrate(II) product is further oxidized by ferric ions (${\text{Fe}}^{3+}$) to form the compound responsible for the Prussian blue color, which is essentially ferric ferrocyanide:$${\text{Fe}}_4[{\text{Fe(CN)}}_6{]}_3$$

Identify the Correct Answer

The formation of Prussian blue indicates the presence of nitrogen, and it involves the formation of ferric ferrocyanide. Among the given options, ${\text{Fe}}_4[{\text{Fe(CN)}}_6{]}_3$ corresponds to this compound.

Key concepts

Nitrogen Detection

Detecting nitrogen in organic compounds can sometimes seem daunting, but Lassaigne's test simplifies this process. This test involves fusing the organic compound with sodium, leading to the conversion of nitrogen in the compound into sodium cyanide (NaCN).
This happens because sodium reacts directly with the compounds in the presence of heat, efficiently turning nitrogen into a detectable form.
The beauty of this method is that it is not only direct but also reliable, making it a cornerstone of qualitative analysis in organic chemistry.

• Lassaigne's test provides a clear method for detecting nitrogen.
• The nitrogen is converted into sodium cyanide during the test.
• The simplicity and reliability make it widely used in labs.

Prussian Blue Formation

Prussian blue is a beautifully intense blue compound that forms when certain chemical reactions occur. During Lassaigne's test, Prussian blue appears as a consequence of the interaction between different chemical entities.
Initially, sodium cyanide reacts with ferrous sulfate (FeSO₄), leading to the creation of sodium hexacyanoferrate(II) ($${\text{Na}}_4[{\text{Fe(CN)}}_6]$$).
What makes Prussian blue appear is the subsequent reaction where sodium hexacyanoferrate(II) is oxidized by ferric ions (Fe³⁺), yielding ferric ferrocyanide:$${\text{Fe}}_4[{\text{Fe(CN)}}_6{]}_3$$, resulting in the striking Prussian blue color.

• Prussian blue is formed through a remarkable series of reactions.
• It involves oxidation of sodium hexacyanoferrate(II).
• The process is indicative of nitrogen's presence in an organic compound.

Ferric Ferrocyanide

Ferric ferrocyanide is a complex pigment that contributes to the dazzling color known as Prussian blue. Formed after sodium hexacyanoferrate(II) reacts with ferric ions (Fe³⁺), it is composed of a mixture of iron and cyanide groups.
This compound ${\text{Fe}}_4[{\text{Fe(CN)}}_6{]}_3$ is not just a beautiful pigment, but also a key indicator in the detection of nitrogen in Lassaigne's test.
Through this reaction, scientists can confirm the presence of nitrogen because the blue color is unmistakable, providing a visual cue.

• Ferric ferrocyanide gives Prussian blue its distinctive hue.
• It's a crucial step in confirming nitrogen presence.
• The reaction illustrates intricate chemistry behind simple tests.