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.

Short answer

Statement (a) is wrong; \(\mathrm{CaO}\) isn't used for carbon estimation.

Step-by-step solution

Understand Each Statement

We are given four statements about different chemical tests related to organic compounds. Our task is to determine which statement is incorrect by analyzing the scientific context and methods described in each.

Evaluate Statement (a)

Statement (a) claims that an organic compound is heated with \(\mathrm{CaO}\) in a combustion tube for the estimation of carbon. This is incorrect because organic carbon estimation is typically done using a different technique, such as the Dumas method, where the organic compound is combusted to convert carbon into \(\mathrm{CO_2}\). Heating with \(\mathrm{CaO}\) does not accurately facilitate this estimation.

Evaluate Statement (b)

Statement (b) states that Lassaigne's test can be used to detect nitrogen and sulfur in organic compounds. This statement is correct, as Lassaigne's test involves the conversion of elements present in the compound into water-soluble inorganic forms that can be tested by further chemical reactions.

Evaluate Statement (c)

Statement (c) refers to Beilstein's test for halogen presence in compounds. This is correct, as Beilstein's test is a flame test where a copper wire is used to detect halogens by observing a characteristic green flame.

Evaluate Statement (d)

Statement (d) claims that Lassaigne's test fails to detect nitrogen in diazo compounds. This statement is correct because diazo compounds can undergo decomposition that interferes with the normal detection of nitrogen in Lassaigne's test.

Key concepts

Carbon Estimation

Estimating the carbon content within an organic compound is a crucial step in various chemical analyses. Unlike the erroneous suggestion of heating the compound with calcium oxide (CaO), researchers commonly use techniques like the Dumas Method for precise carbon estimation. This method revolves around combusting the organic compound in the presence of oxygen. As the compound burns, carbon within it reacts to form carbon dioxide (CO_2). This CO_2 is then precisely measured to deduce the amount of carbon originally present in the compound. The accuracy of this method hinges on the complete combustion of the carbon source and proper calibration of the measurement system, often involving gas collection and mass spectrometry or infrared spectroscopy. For students, it's essential to understand that accurate carbon measurement is pivotal for calculating the empirical formula and understanding the elemental composition of organic substances.

Lassaigne's Test

Lassaigne's Test is a foundational procedure in organic chemistry for determining the presence of specific elements like nitrogen, sulfur, and halogens in a compound. The test begins by heating the organic sample with sodium metal. This process converts the elements into their respective water-soluble inorganic forms. Consequently: - Nitrogen forms sodium cyanide (NaCN). - Sulfur turns into sodium sulfide (Na2S). - Halogens form sodium halides (NaX, where X could be any halogen). After conversion, these ionic forms can be detected using further chemical reactions. For example, the presence of nitrogen is confirmed by the Prussian blue color formation in a subsequent reaction, while sulfur creates a black lead sulfide precipitate. It's noteworthy that Lassaigne’s test can sometimes fail to detect nitrogen in certain compounds like diazo compounds due to decomposition during the process, which releases nitrogen gas.

Beilstein's Test

Beilstein's Test is a straightforward method for detecting halogens in organic compounds. To perform this test, a copper wire is initially cleaned to avoid contamination and then dipped into the test compound. The copper wire is subsequently held in a flame. If halogens are present, they react with the copper to form copper halides, which produce a characteristic green flame. This green flame indicates the presence of halogens such as chlorine, bromine, or iodine in the sample. While Beilstein's Test is simple and quick, it primarily serves as a preliminary test for halogens. It does not specify which halogen is present; hence, further chemical analyses are necessary if specific halogen identification is required.

Diazo Compounds

Diazo compounds are an intriguing class of organic molecules characterized by their nitrogen-nitrogen double bond, denoted by the -N2 functional group. These compounds are often used in dyes, pigments, and as intermediates in organic synthesis due to their vibrant colors and reactive nature. One significant aspect of diazo compounds is their instability, especially under conditions that would normally allow tests like Lassaigne's to operate effectively. In Lassaigne’s test, diazo compounds can decompose, releasing nitrogen gas and thus hindering the accurate detection of nitrogen through conventional methods. Understanding the behavior of diazo compounds is essential for chemists who aim to manipulate or detect various chemical species in complex organic mixtures effectively.