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) \(\mathrm{Na}_{4}[\mathrm{Fe}(\mathrm{CN}), \mathrm{NOS}]\) (d) \(\mathrm{Na}_{1}[\mathrm{Fe}(\mathrm{CN})]\)

Step-by-Step Solution

Answer

The compound formed is \( \mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3} \), option (b).

Step 1: Understanding Lassaigne's Test for Nitrogen

The Lassaigne test is used to detect elements like nitrogen, sulfur, and halogens in an organic compound. When an organic compound contains nitrogen, it forms sodium cyanide (NaCN) upon fusion with sodium. This is the basis for the test.

Step 2: Formation of Ferricyanide Complex

In a Lassaigne test for nitrogen, the cyanide ion ext{(CN}^- ext{)} formed reacts with ext{Fe}^{3+} ions in the test to form the ferricyanide complex ext{Fe(CN)}_3. However, the correct complex indicating the presence of nitrogen is a prussian blue complex involving multiple ext{Fe} ions.

Step 3: Identifying the Correct Compound

The prussian blue color, which identifies the presence of nitrogen, is due to the formation of ext{Fe}_4[ ext{Fe(CN)}_6]_3. This complex is a deep blue color and is characteristic of a positive test for nitrogen.

Key Concepts

Nitrogen Detection

Detecting nitrogen in organic compounds can be crucial, and the Lassaigne's Test is a classic method for this purpose. The test focuses on converting elements within the compound into inorganic forms that can be easily analyzed.
When nitrogen is present in an organic compound, it is converted into sodium cyanide (\( \text{NaCN} \)) by fusing the compound with sodium metal. This step is essential because it transforms nitrogen into a detectable form.
The sodium cyanide is later involved in further reactions that confirm the presence of nitrogen by producing distinct colors, such as the deep blue of the Prussian Blue complex. By understanding these steps, students can appreciate the logic and chemistry underlying the Lassaigne's Test.

Prussian Blue Complex

The Prussian Blue complex is the key indicator of nitrogen in the Lassaigne test. Its deep blue color is not only vivid but also historically significant in detecting nitrogen.
This complex is scientifically represented as\[\text{Fe}_4[\text{Fe(CN)}_6]_3\],and is formed when sodium cyanide reacts with ferric ions (\( \text{Fe}^{3+} \)).
The complex is structured with iron ions coordinated to cyanide groups, creating a stable and strikingly colored complex.
In practice, when testing for nitrogen, this distinct color serves as the confirmation of its presence, offering a straightforward and visual affirmation to chemists and students alike.

Sodium Cyanide Formation

The formation of sodium cyanide (\( \text{NaCN} \)) is a crucial initial step in the Lassaigne test for nitrogen. When the organic compound containing nitrogen is mixed with sodium and heated, the high temperature drives the reaction, resulting in the release of sodium cyanide.
This product is vital as it further reacts with iron ions to produce the distinguishable Prussian Blue complex.

• The fusion process breaks down the organic matrix, allowing nitrogen atoms to bond with sodium.
• This inorganic transformation makes the compound suitable for further testing.

Recognizing the importance of this reaction helps students understand how elements are transformed from complex organic systems into simple analytical compounds.

Ferricyanide Complex

In the context of Lassaigne's Test, the formation of a ferricyanide complex involves the interaction of cyanide ions, derived from sodium cyanide, with ferric ions (\( \text{Fe}^{3+} \)).
Initially, one might consider that ferricyanide itself is directly indicative of nitrogen. However, the true confirmation happens when Prussian Blue forms, involving further bonding beyond the ferricyanide ion.
Understanding the distinction between these complexes aids in grasping the sequential nature of chemical reactions. It is important to note the stepwise creation of intermediates such as ferricyanide, leading up to the final visibly detectable Prussian Blue, which confirms nitrogen’s presence in the tested compound.