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 (a) \(\mathrm{Fe}(\mathrm{CN})\), (b) \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\) (c) \(\mathrm{Na}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{5} \mathrm{NOS}\right]\) (d) \(\mathrm{Na}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\)

Step-by-Step Solution

Answer

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

Step 1: Understanding the Question

The problem asks us to identify the compound formed in the positive test for nitrogen when using the Lassaigne solution method. This method is used to convert covalently bonded atoms in organic compounds into their ionic forms.

Step 2: Identifying Lassaigne's Test Compounds

Lassaigne's test involves converting nitrogen in organic compounds into sodium cyanide. When sodium cyanide is reacted with ferrous sulfate and ferric ions in an acid medium, it forms a Prussian blue colored complex.

Step 3: Recognizing the Prussian Blue Complex

The Prussian blue compound, which forms during Lassaigne's test for nitrogen, is chemically represented as \[ \mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3} \] This is the indicator of a positive nitrogen test.

Step 4: Comparing with Given Options

Compare the recognized Prussian blue complex from Lassaigne's test, \[ \mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3} \], with the provided options to find a match.

Key Concepts

Prussian Blue Complex

In Lassaigne's Test, one of the most vivid indicators of a positive nitrogen presence is the formation of the Prussian Blue complex. This complex is notoriously known for its deep blue color, which arises when a mixture of sodium cyanide from the organic compound reacts in succession with ferrous sulfate and ferric ions in the presence of acid. The chemical representation of this compound is \[ \mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3} \].
This complex forms when iron (II) ions, from the ferrous sulfate, react with the cyanide ion to form iron (III) hexacyanoferrate, which collectively is recognized as the Prussian Blue complex.

• This helps in identifying the presence of nitrogen because the deep blue color is a striking product of these reactions.
• Its formation not only indicates the presence of cyanide ions, but it indirectly confirms that nitrogen was present in the original organic compound.

When students are tasked with Lassaigne's Test, recognizing the formation of the Prussian Blue complex is essential for a correct interpretation of test results.

Sodium Cyanide Formation

The initial step in detecting organic nitrogen involves transforming the nitrogen atoms in the compound into cyanide ions, specifically sodium cyanide. This transformation occurs during the preparation of the Lassaigne solution.
To achieve this, the organic compound is fused with metallic sodium, converting any potential nitrogenous residues into accessible sodium cyanide. The chemical equation representing this conversion is:

• \[ \text{R-N} + 2\text{Na} \rightarrow \text{NaCN} + [\text{byproducts}] \]
• Where \( \text{R-N} \) is the nitrogen-containing residue in the organic compound.

The production of sodium cyanide is a vital step as it acts as the reactive cyanide component that later participates in the formation of Prussian Blue.
Understanding how sodium cyanide is formed and its role in subsequent reactions is critical for those studying organic chemistry, particularly when analyzing the results of Lassaigne's Test.

Organic Nitrogen Detection

Detecting nitrogen in organic compounds involves a fascinating process known as Lassaigne's Test. This method uniquely converts nitrogen in an organic compound into its ionic form, specifically as cyanide ions.
Nitrogen detection is often challenging because of its covalent nature in organic compounds. However, by performing Lassaigne's Test, the nitrogen forms simple salts that can participate in distinctive reactions. The process can be summarized through the following points:

• The organic compound is first heated with sodium metal, allowing nitrogen atoms to form sodium cyanide.
• When treated with ferrous sulfate, the cyanide ions form ferrocyanide ions, which react further to produce the Prussian Blue complex.
• The appearance of this complex indicates the presence of nitrogen in the tested compound.

This reliable method is particularly useful in systematically identifying nitrogen content in various organic substances. Recognizing these steps helps students understand how this detection method transforms more complex organic structures into simple testable ionic forms.