Problem 6 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Kjeldahl's method cannot be used to estimate nitrogen for which of the following compounds? (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{C} \square \mathrm{N}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NO}_{2}\) (d) NC(N)=O

Step-by-Step Solution

Answer

Kjeldahl's method cannot be used for \\(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{C} \equiv \mathrm{N}\\) and \\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NO}_{2}\\).

Step 1: Understand Kjeldahl's Method

Kjeldahl's method is used to estimate the nitrogen content in organic compounds. It is suitable for compounds where nitrogen is present as amino (-NH2), imine (-NH), or amide (CONH) groups.

Step 2: Analyze Option (a)

For \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\), aniline, the nitrogen is present as an amino group (-NH2), making it suitable for Kjeldahl's method.

Step 3: Analyze Option (b)

In \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{C} \equiv \mathrm{N}\), propionitrile, nitrogen is in the form of a nitrile group (\(\equiv \mathrm{N}\)). Kjeldahl's method is not suitable for compounds with nitrile nitrogen.

Step 4: Analyze Option (c)

For \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NO}_{2}\), nitrobenzene, nitrogen is part of a nitro group (-NO2), which Kjeldahl's method does not measure.

Step 5: Analyze Option (d)

In formamide, represented by \(\mathrm{NC(N)=O}\), nitrogen is part of an amide group, which is suitable for Kjeldahl's method.

Step 6: Identify the Compound Not Suitable for Kjeldahl's Method

Among the options, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NO}_{2}\) and \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{C} \equiv \mathrm{N}\) contain nitro and nitrile groups, respectively, and are not suitable for Kjeldahl's method.

Key Concepts

Nitrogen Estimation

Nitrogen estimation is a fundamental process in organic chemistry, especially important in determining the nitrogen content of various compounds. Kjeldahl's method is renowned for this purpose, specifically in analyzing organic substances where nitrogen exists in certain bond forms. In this approach, the sample is first digested with sulfuric acid in the presence of a catalyst, converting nitrogen into ammonium sulfate. The resulting mixture is then neutralized with a base, liberating ammonia, which is distilled and titrated to estimate the nitrogen content.

This method excels with compounds having amino (-NH2), imino (-NH), or amido (CONH) groups. However, it falls short with nitrogen present in nitrile or nitro groups, as these forms do not convert readily into ammonia during digestion. Alternatively, other methods like the Dumas combustion technique can be used where Kjeldahl fails.

Organic Chemistry

Understanding organic chemistry is like unveiling the magical world of carbon compounds. It's the study of carbon-based compounds, known for their versatility, reactivity, and structural diversity. Nitrogen, a key player in organic chemistry, is fundamental in amino acids, proteins, and many synthetic materials.

In organic compounds, nitrogen forms various functional groups such as amino, nitrile, imino, and nitro. The nature of these groups greatly impacts the chemical's properties and reactivity.

• Amino groups typically form in amines and amino acids, essential for life processes.
• Nitrile groups make compounds suitable for plastic and fiber production.
• Nitro groups feature in explosives and dyes, providing vibrant colors and significant energy release.

Understanding these groups' characteristics helps chemists predict reaction pathways and optimize synthetic designs.

Nitrile Group

The nitrile group is characterized by a carbon triple-bonded to nitrogen, noted as \(-C \equiv N\). It's famous for its contribution to several industrial applications. Nitriles are prevalent in plastics, nylon fibers, and synthetic resins, known for their stability and strength.

However, the nitrile bond's robustness presents challenges in nitrogen estimation like in Kjeldahl's method. This method cannot efficiently convert the nitrogen in the nitrile group to ammonia.

This difficulty arises because the strong triple bond resists the breakdown into ammonia during acid digestion. As a result, specialized techniques or additional processing steps may be necessary for accurate nitrogen measurement in nitrile-containing compounds.

Nitro Group

The nitro group, represented as \(-NO_2\), is vital in organic chemistry due to its role in energy-dense materials like explosives and propellants. Additionally, nitro groups serve as intermediates in synthesizing aromatic amines and other derivatives.

Despite their significance, nitro groups pose issues in nitrogen estimation methods like Kjeldahl's. This method doesn't easily account for the nitrogen in this form, as the nitro group does not release nitrogen to ammonia efficiently during the digestion process.

For accurate determination, methods beyond Kjeldahl's, such as alternative reduction or combustion analysis, might be supplemented. Understanding the limitations and capabilities of different analytical techniques is crucial to successful nitrogen estimation in nitro-containing compounds.