Problem 64 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements is wrong? (a) in the estimation of carbon, an organic compound is heated with \(\mathrm{CaO}\) in a combustion tube. (b) using Lassaigne's test nitrogen and sulphur present in an organic compound can be tested. (c) using Beilstein's test the presence of halogen in a compound can be tested (d) Lassaigne's tests fails to identify nitrogen in diazo compound.

Step-by-Step Solution

Answer

Statement (a) is wrong.

Step 1: Analyze the first statement

The first statement suggests that in the estimation of carbon, an organic compound is heated with \( \text{CaO} \) in a combustion tube. This is incorrect because the estimation of carbon typically involves oxidation with \( \text{CuO} \) or another oxidizing agent, not \( \text{CaO} \). \( \text{CaO} \) is often used to absorb \( \text{CO}_2 \). Therefore, statement (a) is wrong.

Step 2: Analyze the second statement

The second statement explains that Lassaigne's test can be used to detect the presence of nitrogen and sulfur in an organic compound. This is true, as Lassaigne's test involves converting these elements into ionic forms that can be detected using specific tests for cyanides or sulfides.

Step 3: Analyze the third statement

The third statement claims that using the Beilstein test, we can test for the presence of halogens in a compound. This is correct, as the Beilstein test detects halogens by producing a colored flame when the compound is burned.

Step 4: Analyze the fourth statement

The fourth statement mentions that Lassaigne's test fails to identify nitrogen in diazo compounds. This is accurate because the unique structure of diazo compounds can interfere with the formation of the typical ionic species used for detection in Lassaigne's test.

Step 5: Conclusion

After analyzing all the statements, statement (a) is the incorrect one as the estimation of carbon does not involve heating with \( \text{CaO} \). It is instead generally conducted with \( \text{CuO} \) for efficient oxidization.

Key Concepts

Lassaigne's Test

Lassaigne's Test is a classic method used in qualitative analysis to detect certain elements like nitrogen, sulfur, and halogens in organic compounds. The test is named after its inventor, Jean Louis Lassaigne. This test involves fusing a small sample of the compound with sodium metal, which helps to convert the elements into soluble salts.

The essential steps include:

• Heating the organic compound strongly with a small piece of sodium metal in a fusion tube.
• The product of this fusion is dissolved in distilled water to obtain a sodium extract, commonly referred to as Lassaigne's extract.
• From this extract, nitrogen is identified by the formation of Prussian blue precipitate by adding iron sulfate, while sulfur is detected through the formation of a black precipitate with lead acetate or the production of a reddish-brown ring with sodium nitroprusside.

Lassaigne's Test is sensitive and widely used, but it has its limitations. For example, it might fail to detect nitrogen in compounds like azo or nitro compounds because these structures do not easily convert to the ionic forms needed for detection. Overall, it is a reliable method for identifying other compounds, but chemists should be aware of its boundaries.

Carbon Estimation

Carbon estimation is an important process in the analysis of organic compounds. It involves determining the amount of carbon within a substance. This process can be complex, typically requiring the complete combustion of the compound to transform the carbon content into carbon dioxide, which is then measured.

The traditional method involves:

• Combusting the sample in the presence of a strong oxidizing agent like copper oxide (CuO) instead of calcium oxide (CaO). The carbon in the sample is oxidized to carbon dioxide (CO₂).
• The generated CO₂ is then passed through a series of absorbers, often containing strong bases like sodium hydroxide, to capture and quantify the gas.
• By measuring the amount of CO₂ produced, the carbon content of the original sample can be calculated.

This method is preferred for its accuracy but can be labor-intensive. Understanding the steps and equipment used for combustion and absorption is critical for effective carbon estimation. This method is foundational in organic chemistry, offering insights into the elemental composition of compounds.

Beilstein Test

The Beilstein Test is a quick and straightforward qualitative test for detecting halogens in organic compounds. Named after the chemist Friedrich Konrad Beilstein, this test is known for its simplicity and efficiency.

Here's how the test works:

• A copper wire is cleaned by heating it in a flame until it no longer produces any color (usually a greenish hue indicates impurities).
• Once clean, the wire is dipped in a small amount of the organic compound.
• The wire is then introduced back into the flame. If the compound contains halogens (chlorine, bromine, iodine), a characteristic green or blue-green flame will appear.

This color is caused by the formation of volatile copper halides. Despite its simplicity, the Beilstein Test is quite sensitive and can detect even small amounts of halogens.

However, there are some limitations, as it does not quantify the amount of halogen present, nor can it distinguish between different types of halogens. Yet, as a preliminary test, it is highly beneficial for quickly confirming the presence of halogens before more specific analytical techniques are applied.