Problem 109 - A Complete Resource Book in Chemistry for JEE Main

Question

The compound formed in the positive test for nitrogen with the Lassaigne solution of an organic compound is [2004] (a) \(\mathrm{Fe}(\mathrm{CN})_{3}\) (b) \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\) (c) \(\mathrm{Na}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{3} \mathrm{NOS}\right]\) (d) \(\mathrm{Na}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\)

Step-by-Step Solution

Answer

The compound formed is \( \text{Fe}_4 [\text{Fe}( ext{CN})_6]_3 \).

Step 1: Understanding the Lassaigne's Test

The Lassaigne's test is used to detect nitrogen in organic compounds. In this test, the organic compound is fused with sodium metal to create sodium cyanide (\( \text{NaCN} \)). This cyanide then reacts with the iron solution to produce a specific complex.

Step 2: Reaction of Sodium Cyanide

In the Lassaigne's test, sodium cyanide formed reacts with ferrous sulfate and traces of ferric ions (from ferric chloride) to form Prussian blue, which is an indicator of the presence of nitrogen. This Prussian blue compound is represented by an iron-cyanide complex.

Step 3: Identifying the Prussian Blue Complex

The Prussian blue complex formed is known as ferric ferrocyanide, (\( \text{Fe}_4 [ ext{Fe}( ext{CN})_6]_3 \)). This matches the formula given in option (b). This compound is identified by its characteristic blue color.

Step 4: Conclusion on Compound Identification

Given that Prussian blue is formed in the positive test for nitrogen in Lassaigne's test, option (b), which is \( \text{Fe}_4 [\text{Fe}( ext{CN})_6]_3 \), correctly represents the compound formed in this process.

Key Concepts

Lassaigne's Test

Lassaigne's Test is a qualitative analysis test specifically designed to detect nitrogen, sulfur, and halogens in organic compounds. The technique involves converting these elements into easily identifiable forms through a fusion process. When performing this test to identify nitrogen, the organic compound is fused with a small piece of sodium metal. This is typically done by heating the mixture strongly in a fusion tube.
Upon heating, sodium reacts with the nitrogen present in the organic compound to form sodium cyanide (\( \text{NaCN} \)). This conversion allows for the nitrogen to be detected further down the line by reacting the formed cyanide with specific reagents. The Lassaigne's Test is quick and straightforward, making it a valuable tool in organic chemistry for identifying elements in complex molecules without needing extensive equipment.

• Firstly, sodium helps extract the elements from the organic matrix.
• The formed \( \text{NaCN} \) makes nitrogen detection feasible through additional reactions.

By using this method, chemists can determine if the compound in question contains nitrogen through the formation of a colored complex, which signals a successful test.

Prussian Blue Complex

The Prussian Blue Complex is an essential part of confirming nitrogen presence through Lassaigne's Test. After sodium cyanide is formed in the initial fusion step, a reaction with ferrous sulfate (\( \text{Fe}^2+ \)) and traces of ferric ions (\( \text{Fe}^3+ \)) takes place. This leads to the formation of Prussian blue, a dense blue pigment.
Prussian blue is not just any iron-cyanide compound, but specifically ferric ferrocyanide. This particular reaction is important because the vivid blue color acts as a visual indicator that nitrogen is indeed present in the sample being tested. It's a confirmation step, leveraging the interaction of chemical species to manifest a noticeable color change.

• Ferrous sulfate reacts with \( \text{NaCN} \) to form \( \text{Fe(CN)}_6^{4-} \).
• Ferric ions then convert \( \text{Fe(CN)}_6^{4-} \) into the insoluble Prussian blue complex.

Thus, Prussian blue formation effectively signals the positive detection of nitrogen in the tested compound through Lassaigne's Test.

Ferric Ferrocyanide

Ferric ferrocyanide is chemically represented as \( \text{Fe}_4 [\text{Fe}(\text{CN})_6]_3 \), and it's the blue compound associated with the positive result for nitrogen detection in Lassaigne's test. It's a type of coordination complex that forms due to the reaction between cyanide ions and iron ions, resulting in this characteristic blue substance.
In the context of nitrogen detection, its presence indicates not only the formation of \( \text{NaCN} \) but also a successful reaction pathway involving iron salts. The chemical nature of ferric ferrocyanide lends itself to being a stable compound, resistant to decomposition under ordinary conditions, hereby making its blue hue an effective sign post for nitrogen presence.

• Prussian blue is essentially made up of this coordination complex.
• The robust blue color confirms the detection of nitrogen when Lassaigne's Test is applied.

By gauging the presence of ferric ferrocyanide, chemists can conclude the original organic sample contained nitrogen, efficiently utilizing colorimetric changes for straightforward analysis.