Problem 136 - A Complete Resource Book in Chemistry for JEE Main

Question

An organic compound \(\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O}\) does not give a precipitate with 2,4 -dinitrophenylhydrazine and does not react with metallic sodium. It could be (a) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CHO}\) (b) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2} \mathrm{OH}\) (c) \(\mathrm{CH}_{3}-\mathrm{CO}-\mathrm{CH}_{3}\) (d) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{O}-\mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

The compound could be \( \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{O}-\mathrm{CH}_{3} \) (option d).

Step 1: Analyzing the Reaction with 2,4-Dinitrophenylhydrazine

2,4-Dinitrophenylhydrazine reacts with aldehydes and ketones to form a precipitate. Since the compound does not give a precipitate with 2,4-dinitrophenylhydrazine, it is neither an aldehyde nor a ketone. This rules out options (a) \( \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CHO} \) and (c) \( \mathrm{CH}_{3}-\mathrm{CO}-\mathrm{CH}_{3} \), as these include aldehyde and ketone groups, respectively.

Step 2: Analyzing the Reaction with Metallic Sodium

Alcohols react with metallic sodium to release hydrogen gas, whereas ethers do not react. Since the compound does not react with metallic sodium, it cannot be an alcohol. This rules out option (b) \( \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2} \mathrm{OH} \) as it contains a hydroxyl group \( (-\mathrm{OH}) \).

Step 3: Identifying the Remaining Option

After ruling out options (a), (b), and (c), the remaining compound is (d) \( \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{O}-\mathrm{CH}_{3} \), an ether. Ethers do not react with 2,4-dinitrophenylhydrazine and do not react with metallic sodium, matching the given conditions.

Key Concepts

Functional Groups

Functional groups play a crucial role in organic chemistry because they define the characteristics and chemical reactivity of molecules. In this exercise, we distinguish the functional groups present in different organic compounds, aiding in determining the correct identity of the compound based on its reactions or lack thereof.
Let's take a look at the functional groups mentioned:

• Aldehyde (2 ext{CH}_{3}- ext{CH}_{2}- ext{CHO}2): Aldehydes possess a carbonyl group (C=O"). They typically react with 2,4-Dinitrophenylhydrazine to form a precipitate.
• Ketone (C ext{CH}_{3}- ext{CO}- ext{CH}_{3}C): Like aldehydes, ketones also contain a carbonyl group, reacting similarly to 2,4-Dinitrophenylhydrazine.
• Alcohol ( ext{CH}_{2}= ext{CH}- ext{CH}_{2} ext{OH}): Alcohols feature a hydroxyl group (-OH"). They react with metallic sodium by releasing hydrogen gas.
• Ether ( ext{CH}_{2}= ext{CH}- ext{O}- ext{CH}_{3}): Ethers have an oxygen atom bonded to two carbon atoms. They don't react with 2,4-Dinitrophenylhydrazine or metallic sodium, making them an important point of distinction in this scenario.

By understanding the presence of these functional groups, one can extrapolate how an organic compound might react under different conditions.

Reaction Mechanisms

Reaction mechanisms are the step-by-step processes by which chemical reactions occur. Understanding these mechanisms is key to predicting how substances behave under certain conditions.
In the exercise, we used reaction mechanisms to eliminate possible answers:

• 2,4-Dinitrophenylhydrazine reacts specifically with aldehydes and ketones. They form a stable precipitate due to the formation of a hydrazone derivative. Our compound did not form this precipitate, meaning that it lacks the carbonyl group needed for the reaction to proceed.


• Metallic sodium is often used to test for alcohols. When sodium is added to an alcohol, it reacts with the hydroxyl group to release hydrogen gas. Our compound did not release hydrogen gas, which indicates the absence of an alcohol group.

Understanding these mechanisms not only helps to solve the exercise but gives insight into how these substances can be used or modified in practical applications.

Chemical Analysis

Chemical analysis involves the process of testing compounds to determine their chemical composition and ascertain their molecular identity. This exercise relied on two specific tests to analyze the compound:

• 2,4-Dinitrophenylhydrazine Test: A chemical test used to identify aldehydes and ketones. If a compound contains these functional groups, it will typically form a yellow or orange precipitate. The lack of this precipitate in our exercise meant that neither an aldehyde nor a ketone was present in the compound.


• Reaction with Metallic Sodium: This test helps to identify alcohols, as they react with sodium to produce hydrogen gas. The absence of a reaction indicated that the compound did not possess a hydroxyl group.

By applying these tests, we narrowed down the options to confirm that the compound was most likely an ether, demonstrating how chemical analysis is a methodical approach to identifying unknown organic molecules.