Problem 93 - A Complete Resource Book in Chemistry for JEE Main

Question

Select the correct statements regarding Lassaigne's test? (a) \(\mathrm{Li}\) and \(\mathrm{K}\) are not used in this test (b) \(\mathrm{NH}_{4} \mathrm{Cl}, \mathrm{NH}_{2}-\mathrm{NH}_{2}\) Can't be detected by this test. (c) Urea, Acetamide can not give this test. (d) Benzamide, Azobenzene can give this test.

Step-by-Step Solution

Answer

The correct statements are (a) and (b).

Step 1: Understand Lassaigne's Test

Lassaigne's test is a qualitative analysis procedure used to detect the presence of nitrogen, sulfur, and halogens in an organic compound. It involves the fusion of the compound with sodium metal to convert these elements into water-soluble ionic forms, which can be detected by further chemical reaction tests.

Step 2: Evaluate Statement (a)

Statement (a) claims that lithium (Li) and potassium (K) are not used in Lassaigne's test. This is correct because sodium (Na) is traditionally used for the fusion process, as it effectively reacts with compounds to produce sodium salts of the detected elements, whereas Li and K are not typically used.

Step 3: Evaluate Statement (b)

Statement (b) claims that ammonium chloride (NH4Cl) and hydrazine (NH2-NH2) cannot be detected by Lassaigne's test. This is correct because such compounds contain nitrogen in a form that is not converted into the water-soluble ionic form detectable in Lassaigne's test, which is generally applied to detect N as cyanide.

Step 4: Evaluate Statement (c)

Statement (c) claims that urea and acetamide cannot give a positive Lassaigne's test. This is incorrect, as both urea and acetamide contain nitrogen in a form that can be converted into soluble cyanide ions detectable by this test after fusion with sodium.

Step 5: Evaluate Statement (d)

Statement (d) claims that benzamide and azobenzene can give a positive Lassaigne's test. Benzamide contains nitrogen which can participate in forming soluble ions after fusion, hence giving a positive test. However, azobenzene typically does not give a positive result as its nitrogen is not easily converted into a soluble form. Thus, this statement is partially correct.

Key Concepts

Qualitative Analysis

In chemistry, qualitative analysis is the process of determining the components or elements of a substance without focusing on their quantity or concentration. This method is crucial for identifying unknown substances, especially in organic compounds.
For instance, Lassaigne's Test is a type of qualitative analysis specifically designed to detect the presence of nitrogen, sulfur, and halogens in organic compounds. It provides essential information about the elemental composition of a compound, which is useful in both research and industrial applications.

• Allows identification of basic building blocks in compounds.
• Uses various reaction tests to indicate presence through color change or precipitate formation.
• Essential for scientists to classify substances before further analysis.

With qualitative analysis, chemists can make initial determinations, which are crucial for subsequent quantitative methods.

Nitrogen Detection

Nitrogen detection in organic compounds often employs Lassaigne's Test to confirm its presence. The test transforms the nitrogen in organic compounds into a water-soluble form, typically as a cyanide ion.
Upon successful detection and transformation, the cyanide can be further identified using specific reagents such as ferrous sulfate and ferric chloride, which produce Prussian blue as an indicator of nitrogen presence.
This process involves multiple steps aimed at ensuring accurate detection:

• First, fusion of the organic compound with sodium (avoiding lithium and potassium).
• The resulting sodium cyanide solution is then subjected to further tests for confirmation.
• Colorimetric tests help in confirming nitrogen, evidenced by a blue precipitate.

Overall, accurate nitrogen detection is vital for understanding the chemical behavior and reactivity of organic compounds in varied applications.

Organic Compounds

Organic compounds, composed primarily of carbon, along with hydrogen, nitrogen, oxygen, and other elements, form the basis of life and many synthetic materials. Within this vast class of molecules, nitrogen-containing compounds are particularly significant for their roles in life processes and industrial applications.

• Examples include amino acids, urea, and amines, each playing distinct biological roles.
• The elemental analysis of organic compounds helps in the identification and functional classification.
• Lassaigne's Test is often one of the primary qualitative tests performed to determine elemental compositions.

Recognizing the presence of nitrogen, sulfur, and halogens changes the compound's properties and reactions.
By determining these elements accurately, chemists can infer potential uses and behaviors of compounds.

Sodium Fusion Process

The sodium fusion process, commonly known as the Lassaigne's sodium fusion test, is a critical step in qualitative analysis for detecting elements like nitrogen, sulfur, and halogens in organic compounds.
This process involves heating the organic sample with sodium metal to convert elements into detectable forms.

• Sodium reacts with organic substances, breaking them down into simpler, ionic forms.
• The resultant solution, when treated with specific reagents, reveals elements via color changes or precipitates.
• Versatile in detecting multiple elements in a single analysis setup.

By employing sodium in the fusion, chemists ensure the transformation into forms suitable for a wide range of qualitative tests. Ensuring proper conduction and preparation in this process is critical for obtaining reliable results and conclusions.