Problem 94 - A Complete Resource Book in Chemistry for JEE Main

Question

In the detection of ' \(\mathrm{N}\) ' by Lassaigne's test which of the following compounds are formed during this test? (a) \(\mathrm{Fe}(\mathrm{OH})_{2}\) (b) \(\mathrm{Na}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) (c) \(\mathrm{FeCl}_{3}\) (d) \(\left.\mathrm{Fe}_{4}[\mathrm{Fe}(\mathrm{CN})]_{3}\right]\)

Step-by-Step Solution

Answer

Compound (d) ( Fe_4[Fe(CN)_6]_3 ) is formed in Lassaigne's test for nitrogen detection.

Step 1: Understand the Lassaigne's Test

Lassaigne's Test is used to detect the presence of nitrogen in organic compounds. It involves fusing the organic compound with sodium metal, which converts nitrogen (if present) into sodium cyanide ( NaCN).

Step 2: Formation of Sodium Cyanide

When the organic compound containing nitrogen is fused with sodium, the nitrogen is converted to sodium cyanide (NaCN).

Step 3: Reaction with Ferric Chloride

In the subsequent reactions, sodium cyanide ( NaCN) reacts with a solution of ferrous sulfate ( FeSO_4), ultimately forming sodium hexacyanoferrate(II) ( Na_4[Fe(CN)_6] ).

Step 4: Formation of Prussian Blue

The sodium hexacyanoferrate(II) ( Na_4[Fe(CN)_6] ) generated reacts with ferric chloride ( FeCl_3 ) to form Prussian Blue ( Fe_4[Fe(CN)_6]_3 ), a confirmation of nitrogen presence.

Key Concepts

Detection of Nitrogen

Lassaigne's Test is a simple yet fascinating method used to detect the presence of nitrogen in organic compounds. This test is essential in organic chemistry as it helps to determine whether an unknown compound consists of nitrogen atoms. The procedure involves a clever chemical transformation that provides a visual indication of nitrogen's presence.
The first step in the test requires heating or fusing the organic compound with a small piece of sodium metal. This fusion is critical because the high temperature and reactive nature of sodium allow the compound to break down while interacting with the metal. During this process, any nitrogen within the compound forms sodium cyanide (\( \text{NaCN} \) ), a significant result indicating the presence of nitrogen.
The formation of sodium cyanide sets the stage for further chemical reactions, leading to the ultimate visual confirmation of nitrogen presence in the form of Prussian Blue.

Sodium Cyanide Formation

The formation of sodium cyanide is a crucial step in Lassaigne's Test, serving as a bridge between the initial assay and the ultimate visual confirmation. When the organic compound is heated with sodium, any nitrogen present is transformed into sodium cyanide (\( \text{NaCN} \)). This compound is pivotal because it participates in the following reactions necessary to complete the test.
Understanding this stage helps clarify how nitrogen, an otherwise invisible element within an organic structure, can be detected. Sodium acts as a powerful catalyst, causing the nitrogen to bind with itself and the sodium to form sodium cyanide.
This newly formed \( \text{NaCN} \) is then dissolved and reacts with ferrous sulfate (\( \text{FeSO}_4 \)) in the presence of water and air, which oxidizes the iron, progressing the reaction to the next crucial stage.

Prussian Blue

Prussian Blue is the iconic, visually striking result of Lassaigne's test for nitrogen. This deep blue compound forms through a fascinating multi-step reaction that begins with sodium cyanide (\( \text{NaCN} \)) and ferrous sulfate.
Initially, the \( \text{NaCN} \) reacts with the ferrous sulfate in solution to form sodium hexacyanoferrate(II) (\( \text{Na}_4[\text{Fe(CN)}_6] \)). This intermediate compound is crucial because when it reacts further with ferric chloride (\( \text{FeCl}_3 \)), Prussian Blue is formed.
This blue coloration is more than just a confirmation of nitrogen presence; it's a hallmark of successful Lassaigne’s test. Its formation indicates the proper transformation of the compound throughout the series of reactions and ultimately assures the presence of bound nitrogen in the original organic sample. This demonstrable color change simplifies nitrogen detection, making it an accessible method in chemical analysis.