Problem 11 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Which of the following reactions occur during the detection of nitrogen in organic substances by Lassaigne's test? (a) \(\mathrm{Na}+\mathrm{C}+\mathrm{N} \longrightarrow \mathrm{NaCN}\) (b) \(\mathrm{FeSO}_{4}+6 \mathrm{NaCN} \longrightarrow \mathrm{Na}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]+\mathrm{Na}_{2} \mathrm{SO}_{4}\) (c) \(3 \mathrm{Na}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]+2 \mathrm{Fe}_{2}\left(\mathrm{SO}_{4}\right)_{3} \longrightarrow \mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}+6 \mathrm{Na}_{2} \mathrm{SO}_{4}\) (d) None of these

Step-by-Step Solution

Answer

Reactions (a) occur during nitrogen detection in Lassaigne's test.

Step 1: Understanding Lassaigne's Test

Lassaigne's test is used to detect the presence of nitrogen, sulfur, and halogens in organic compounds. In this test, the organic compound is fused with sodium. If nitrogen is present, sodium cyanide (NaCN) is formed, which can be further reacted to confirm the presence of nitrogen.

Step 2: Analyzing Reaction (a)

Reaction (a) is \( \mathrm{Na} + \mathrm{C} + \mathrm{N} \longrightarrow \mathrm{NaCN} \). This reaction represents the formation of sodium cyanide when sodium reacts with carbon and nitrogen, which is a key step in Lassaigne's test to detect nitrogen.

Step 3: Analyzing Reaction (b)

Reaction (b) is \( \mathrm{FeSO}_{4} + 6 \mathrm{NaCN} \longrightarrow \mathrm{Na}_{4} \left[ \mathrm{Fe} (\mathrm{CN})_{6} \right] + \mathrm{Na}_{2} \mathrm{SO}_{4} \). This step involves the formation of sodium ferrocyanide, a complex compound, but it is not directly related to the initial detection of nitrogen, rather a subsequent confirmation.

Step 4: Analyzing Reaction (c)

Reaction (c) is \( 3 \mathrm{Na}_{4} \left[ \mathrm{Fe} (\mathrm{CN})_{6} \right] + 2 \mathrm{Fe}_{2} (\mathrm{SO}_{4})_{3} \longrightarrow \mathrm{Fe}_{4} \left[ \mathrm{Fe} (\mathrm{CN})_{6} \right]_{3} + 6 \mathrm{Na}_{2} \mathrm{SO}_{4} \). This is a further continuation of the complex compound formation and is used for further confirmation, not direct detection.

Step 5: Selecting the Correct Reaction

Based on the analysis, reaction (a) directly involves the formation of \( \mathrm{NaCN} \), which indicates nitrogen's presence in the organic compound during Lassaigne's test. Thus, reaction (a) is the correct and direct step used in the detection.

Key Concepts

Detection of Nitrogen

Detecting nitrogen in organic compounds is crucial in organic chemistry because it helps in understanding the compound's structural composition. Lassaigne's test is specifically designed for this purpose. During this test, the organic compound is fused with sodium metal. This leads to the formation of sodium cyanide if nitrogen is present.

In the presence of carbon and nitrogen, sodium reacts to form sodium cyanide, which is indicated by the chemical equation: - Reaction: \( \mathrm{Na} + \mathrm{C} + \mathrm{N} \longrightarrow \mathrm{NaCN} \)

This reaction represents the direct detection of nitrogen. The production of \( \mathrm{NaCN} \) serves as proof of nitrogen in the compound. Later steps in Lassaigne's test focus more on confirming the presence of this nitrogen further.

Organic Chemistry

Organic chemistry deals with the study of carbon-containing compounds, and it includes the analysis of elements like nitrogen, which are often present in organic compounds. The structure and behavior of these compounds can be complex.

Lassaigne's test is an essential method in organic chemistry, as it helps identify heteroatoms like nitrogen within these organic structures. This test contributes significantly to the field by providing a simple and reliable method to confirm the presence of nitrogen, which can be crucial for understanding organic reactions and designing further experiments.

Knowing about nitrogen's presence and behavior in organic compounds helps chemists predict how these compounds will react under various conditions.

Sodium Cyanide Formation

One of the key aspects of Lassaigne's test is the formation of sodium cyanide (\( \mathrm{NaCN} \)). Sodium cyanide serves as a crucial indicator of the presence of nitrogen within an organic compound. When the compound is fused with sodium, a reaction takes place which involves carbon, nitrogen, and sodium.

The reaction results in: - \( \mathrm{Na} + \mathrm{C} + \mathrm{N} \longrightarrow \mathrm{NaCN} \)

This formation is key because \( \mathrm{NaCN} \) is an intermediate product that readily interacts with other reagents to further validate the presence of nitrogen. This is just the beginning of a series of reactions aimed at thoroughly confirming nitrogen's presence in the tested material.

Confirmatory Tests

Once sodium cyanide is formed, confirmatory tests take place, enhancing the reliability of nitrogen detection in Lassaigne's test. These tests involve transforming the initial sodium cyanide into more complex versions to confirm nitrogen's presence.

For instance, a sequence of reactions can further process \( \mathrm{NaCN} \) into different ferrocyanide complexes through interactions with iron salts: - Reaction: \( \mathrm{FeSO}_{4} + 6 \mathrm{NaCN} \longrightarrow \mathrm{Na}_{4} \left[ \mathrm{Fe} (\mathrm{CN})_{6} \right] + \mathrm{Na}_{2} \mathrm{SO}_{4} \)

This confirms the detection initially made by \( \mathrm{NaCN} \). Though this isn't directly involved in the detection, it acts as a supplementary step to ensure the reaction's validity.

Such confirmatory steps are an integral part of Lassaigne's test, guaranteeing that the detection of nitrogen is justified and trustworthy.