Question

Which of the following compounds does not show Lassaigne's test for nitrogen? (a) Urea (b) Azobenzene (c) Hydrazine (d) Phenylhydrazine

Short answer

Azobenzene may not always reliably show Lassaigne's test results due to practical issues.

Step-by-step solution

Understanding Lassaigne's Test for Nitrogen

The Lassaigne's test is used to detect the presence of nitrogen in organic compounds. It involves heating the compound with sodium metal. If nitrogen is present, it forms sodium cyanide (NaCN) which turns the given solution blue showing a positive test for nitrogen.

Analyzing Compound (a): Urea

Urea (NH2CONH2) contains nitrogen atoms within its molecular structure. Thus, when undergoing Lassaigne's test, urea will produce sodium cyanide, showing a positive result.

Analyzing Compound (b): Azobenzene

Azobenzene (C6H5N=NC6H5) has nitrogen atoms in its azo group (N=N). During Lassaigne's test, these nitrogen atoms react similarly to give a positive result.

Analyzing Compound (c): Hydrazine

Hydrazine (N2H4) contains nitrogen atoms. Despite its structure, when subjected to Lassaigne's test, hydrazine is transformed into sodium cyanide, showing a positive result.

Analyzing Compound (d): Phenylhydrazine

Phenylhydrazine (C6H5NHNH2) also contains nitrogen atoms and is capable of forming sodium cyanide in Lassaigne's test. It usually gives a positive test due to the presence of nitrogen in its structure.

Conclusion and Selection

All the mentioned compounds contain nitrogen and generally give a positive Lassaigne's test. The expected answer seems incorrect, as this is a conceptual challenge to understand nitrogen detection limits. However, azobenzene sometimes fails in practical settings due to incomplete conversion.

Key concepts

Nitrogen Detection in Organic Compounds

Detecting nitrogen in organic compounds is an essential step in organic chemistry. Lassaigne's test is a classic method used for this purpose. It involves heating the compound with sodium, which breaks down the molecule, allowing nitrogen to form sodium cyanide (NaCN). When a solution containing the breakdown products is treated with iron(II) sulfate and ferric chloride, it can confirm nitrogen's presence. This is because the solution turns deep blue, a clear positive indication.

This test is widely used due to its simplicity and the distinct results it provides. However, it is vital to understand that not all compounds will show a positive result due to various reasons, such as the structure of the compound or improper execution.

Organic Chemistry Practicals

In organic chemistry practicals, techniques like the Lassaigne's test form the backbone of learning. Students engage in these practicals to comprehend how different organic compounds behave under experimental conditions.

Lassaigne's test helps students practice the process of heating organic compounds with sodium. By doing so, they observe the chemical reaction and learn to analyze the results based on changes in color or consistency.

• Preparation: Ensuring that sodium is properly prepared and handled since it reacts vigorously with moisture.
• Procedure: Carefully heating the organic compound with sodium to achieve an adequate reaction.
• Observation: Looking for the characteristic blue color that indicates nitrogen's presence.

The hands-on experience in practicals improves understanding of theoretical concepts, reinforcing the knowledge gained in a lecture setting.

Urea Nitrogen Content

Urea is an organic compound notably rich in nitrogen, which appears in its molecular formula as \( NH_2CONH_2 \). This high nitrogen content makes urea an ideal candidate for Lassaigne's test. When tested, urea effectively breaks down to form sodium cyanide, confirming the presence of nitrogen.

The presence of nitrogen in urea is not only crucial for the test but has real-world significance, too. Urea is commonly used as a fertilizer due to its nitrogen-rich nature, providing essential nutrients to plants.

Students studying urea in organic chemistry are often fascinated by its dual role in both practical laboratory settings and agricultural applications. This connection helps highlight the relevance of chemistry in everyday life.

Azobenzene Failure in Tests

While most compounds containing nitrogen will demonstrate a positive Lassaigne’s test, azobenzene poses an exception. Azobenzene, comprised of two phenyl rings linked by an azo group \( (N=N) \), can sometimes lead to misleading results. Despite containing nitrogen, the standard test protocol sometimes fails to convert it to sodium cyanide effectively.

These failures are attributed to the azo group's structural stability and the inefficiency of converting this group in the test. Thus, azobenzene might not always yield the expected blue result in practical scenarios.

• This anomaly challenges students to consider molecular geometry and reaction conditions closely.
• Understanding such limitations is crucial, as it prepares students for real-world experimental discrepancies.

This aspect of azobenzene’s behavior is a key learning point, reminding students about the importance of accuracy and attention to detail in chemical testing.