Problem 36 - A Complete Resource Book in Chemistry for JEE Main

Question

Lassaigne's test for the detection of nitrogen will fail in case of (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NHNH}_{2} \cdot 2 \mathrm{HCl}\) (b) \(\mathrm{H}_{2} \mathrm{~N} \cdot \mathrm{NH}_{2} \cdot 2 \mathrm{HCl}\) (c) \(\mathrm{NH}_{2} \mathrm{CONH}_{2}\) (d) \(\mathrm{H}_{2} \mathrm{NCONHNH}_{2} \cdot \mathrm{HCl}\)

Step-by-Step Solution

Answer

Lassaigne's test will fail for (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NHNH}_{2} \cdot 2 \mathrm{HCl}\).

Step 1: Understand Lassaigne's Test

Lassaigne's test is used to detect the presence of nitrogen in organic compounds. It involves converting the nitrogen into sodium cyanide by fusing the organic compound with sodium metal. The sodium cyanide formed is then detected using iron(II) sulfate and sulfuric acid, which forms Prussian blue, indicating nitrogen's presence.

Step 2: Determine Conditions for Failing Test

For Lassaigne's test to fail, any nitrogen in the sample must not be converted into simple inorganic forms like NaCN. This happens if the nitrogen is part of a configuration resistant to breakdown by sodium during fusion.

Step 3: Analyze Each Compound

Review each provided compound to determine its suitability for Lassaigne's test:- (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NHNH}_{2} \cdot 2 \mathrm{HCl}\) contains nitrogen in azine linkages.- (b) \(\mathrm{H}_{2} \mathrm{~N} \cdot \mathrm{NH}_{2} \cdot 2 \mathrm{HCl}\) contains nitrogen in a form that the test can usually convert to NaCN.- (c) \(\mathrm{NH}_{2} \mathrm{CONH}_{2}\) has nitrogen in straightforward amide groups.- (d) \(\mathrm{H}_{2} \mathrm{NCONHNH}_{2} \cdot \mathrm{HCl}\) has a nitrogen configuration similar to an azo compound.

Step 4: Identify Resistant Nitrogen Groups

In azide or azo compounds, and other complex nitrogen linkages, the nitrogen might not convert adequately into sodium cyanide due to resistance in breakdown by sodium. Compounds with complex linkage groups like in (a) will not readily react to form NaCN.

Step 5: Conclusion

Among the options, (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NHNH}_{2} \cdot 2 \mathrm{HCl}\) is the compound where nitrogen would not be converted into sodium cyanide effectively, resulting in failure of the Lassaigne test to detect nitrogen.

Key Concepts

Nitrogen Detection

Lassaigne's test is a classical method for detecting nitrogen in organic compounds. It employs a straightforward reaction mechanism that involves fusing the sample with sodium metal. This approach converts the nitrogen present in the compound into sodium cyanide (NaCN), a simple and detectable form. The detection is confirmed further by observing a characteristic blue color.
This blue is due to the formation of Prussian blue when NaCN reacts with specific reagents. However, not all nitrogen configurations will convert to sodium cyanide, making the test ineffective for certain forms of nitrogen.
Primarily, if the nitrogen in a compound is in a complex configuration, like an azine or azo linkage, it might resist conversion to an inorganic form like sodium cyanide.

Organic Compounds

Organic compounds are diverse in structure and function. They range from simple hydrocarbons to complex biomolecules. These compounds contain elements such as carbon, hydrogen, oxygen, and importantly, nitrogen.
Lassaigne's test specifically targets the nitrogen present in organic compounds. The test is widely applied because nitrogen is a crucial element in many biological and synthetic materials. Yet, the test's effectiveness depends largely on the chemical configuration of the nitrogen.
In compounds with nitrogen that is bound in an easily cleavable form, such as simple amides, the test is quite successful. However, in structures where nitrogen is in complex linkages, the test tends to fail, as in cases with azine or azo linkages.

Sodium Cyanide Formation

The formation of sodium cyanide is central to Lassaigne's test. When an organic compound is fused with sodium, the nitrogen is converted into sodium cyanide. This compound is then subjected to further reactions to confirm the presence of nitrogen.
This conversion is efficient for nitrogen in simple chemical linkages. For instance, amides readily release nitrogen to form NaCN. But, in compounds with intricate nitrogen linkages, such as azines, the conversion process is hindered. These resistant structures prevent the nitrogen atoms from breaking free and reacting with sodium.

• The efficient conversion relies on uncomplicated nitrogen configurations.
• Inhibitory configurations can lead to false negatives in nitrogen detection.

Understanding the chemical structure, therefore, is essential before performing the Lassaigne's test.

Prussian Blue Reaction

The Prussian blue reaction is a key indicator in detecting nitrogen through Lassaigne's test. When sodium cyanide, produced from fused organic compounds, is reacted with iron(II) sulfate and then acidified with sulfuric acid, a distinctive Prussian blue pigment forms. This blue color is a clear signal of the presence of nitrogen.
However, if the nitrogen in the compound does not convert to an easily detectable form, like sodium cyanide, the distinctive blue color will not appear.
Thus, despite the simplicity of the Prussian blue reaction, its success hinges on effective prior conversion of nitrogen. The absence of blue can indicate either a lack of nitrogen in easily cognizable forms or simply that the compound resists the conversion processes. This makes accurate preliminary analysis crucial for success with Lassaigne's test.