Question

During detection of nitrogen in Lassaigne's test which compound will fail to form prussian blue precipitate. (A) \(\mathrm{Ph}-\mathrm{NH}-\mathrm{NH}_{3}^{+} \mathrm{\Gamma}\) (B) \(\mathrm{NH}_{2}-\mathrm{NH}_{3}^{+} \mathrm{Cl}^{-}\) (C) \(\mathrm{NH}_{2}-\mathrm{C}-\mathrm{NH}-\mathrm{NH}_{2}\) (D)All

Short answer

The compound that will fail to form a Prussian blue precipitate during the Lassaigne's test for nitrogen detection is option (B) \(\mathrm{NH}_{2}-\mathrm{NH}_{3}^{+} \mathrm{Cl}^{-}\), as it will not release cyanide ions during the test.

Step-by-step solution

Understanding Lassaigne's test for Nitrogen

First, let's briefly understand the Lassaigne's test for Nitrogen. In this test, the organic compound is fused with sodium metal, which leads to the formation of cyanide (CN-) ions if nitrogen is present in the compound. The cyanide ions then react with ferric ions (Fe3+) to form the ferric hexacyanoferrate (II) complex, which is Prussian blue in color. This formation of the Prussian blue precipitate confirms the presence of nitrogen in the compound. Step 2: Observing the structure of each compound

Observing the structure of each compound

Now let's analyze the given compounds in each option: (A) \(\mathrm{Ph}-\mathrm{NH}-\mathrm{NH}_{3}^{+} \mathrm{\Gamma}\) (B) \(\mathrm{NH}_{2}-\mathrm{NH}_{3}^{+} \mathrm{Cl}^{-}\) (C) \(\mathrm{NH}_{2}-\mathrm{C}-\mathrm{NH}-\mathrm{NH}_{2}\) The nitrogen is present in all the given compounds, but we need to find the compound that won't release CN- ions during the Lassaigne's test. Step 3: Determining the ability to release CN- ions

Determining the ability to release CN- ions

From the given compounds, we look for compounds that cannot release CN- ions during the Lassaigne's test: (A) In the first compound, nitrogen is part of the amide group, which can release CN- ions during the Lassaigne's test. (B) In the second compound, nitrogen is part of the ammonium group, which will not release CN- ions. (C) In the third compound, nitrogen is part of the guanidine group, which can release CN- ions during the Lassaigne's test. Step 4: Conclusion

Conclusion

Based on our analysis, option (B) (\(\mathrm{NH}_{2}-\mathrm{NH}_{3}^{+} \mathrm{Cl}^{-}\)) is the compound that will fail to form a Prussian blue precipitate during the Lassaigne's test for nitrogen detection, as it will not release cyanide ions during the test. Therefore, the correct answer is (B).

Key concepts

Prussian Blue Precipitate

The Prussian blue precipitate is a deep blue colored compound that plays a critical role in the detection of nitrogen in organic compounds through Lassaigne's test. This precipitate, known formally as ferric ferrocyanide or ferric hexacyanoferrate (II), forms when ferric ions (Fe^{3+}) react with cyanide ions (CN^{-}) in a solution. If you are performing the Lassaigne's test and observe a blue coloration, it indicates that nitrogen was present in the original compound and has successfully formed cyanide ions which reacted to produce this characteristic blue complex.

To make the detection process clearer for students, consider that Prussian blue formation is akin to completing a puzzle; only when the correct piece (cyanide ion) fits will the puzzle (detection of nitrogen) be successfully completed. Without the cyanide ion, we can't complete our picture. Thus, the non-formation of Prussian blue precipitate implies that the essential piece, cyanide ion, was absent, leading to a negative result for nitrogen in the compound being tested.

Cyanide Ion Formation

Understanding the cyanide ion formation is vital for interpreting the Lassaigne's test correctly. When an organic compound contains nitrogen and is heated with sodium metal, a highly reactive sodium cyanide (NaCN) is formed if nitrogen is present. This reaction is pivotal because the CN^{-} ion is responsible for forming the Prussian blue precipitate upon further reaction with ferric ions. However, not all nitrogen-containing compounds can form cyanide ions. Compounds with nitrogen in certain configurations, such as ammonium ions (NH_{4}^{+}) or some azides, do not generate cyanide ions upon fusion with sodium.

In simpler terms, think of the nitrogen in the organic compound like a locked box. The process of fusing it with sodium is like using a key. If the box (nitrogen configuration) is the correct one, opening it (fusing with sodium) will reveal the cyanide ion. However, just as some boxes can't be opened with certain keys, not all forms of nitrogen in compounds will yield the desired cyanide ion.

Nitrogen Detection in Organic Compounds

The detection of nitrogen in organic compounds using the Lassaigne's test is a fundamental procedure in qualitative organic analysis. It's a simple yet effective test where the organic compound suspected to contain nitrogen is fused with sodium metal. This fusion serves to convert nitrogen in organic compounds to a form that can react with other chemicals, namely cyanide ions. These cyanide ions are then further treated to yield the telltale Prussian blue precipitate if nitrogen is indeed present.

Introducing this concept to students can be made easier by comparing it to a scientific treasure hunt. We have clues (the organic compound), and we are seeking treasure (proof of nitrogen). The fusion with sodium is our map and tools, leading us to the possibility of finding cyanide ions, which are like landmarks. Following these landmarks to the end of the hunt gives us the prized Prussian blue precipitate, confirming that nitrogen is our found treasure. It's important to note that a negative result doesn't mean nitrogen is absent; it could also mean that the compound doesn't break down to form cyanide ions in these conditions.